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Title: A Popular History of Astronomy During the Nineteenth Century
Fourth Edition
Author: Agnes M. (Agnes Mary) Clerke
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A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY
* * * * *
BY THE SAME AUTHOR
PROBLEMS IN ASTROPHYSICS.
Demy 8vo., cloth. Containing over 100
Illustrations. Price 20s. net.
THE SYSTEM OF THE STARS.
Second Edition. Thoroughly revised and
largely rewritten. Containing numerous
and new Illustrations. Demy 8vo., cloth.
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* * * * *
[Illustration: THE GREAT NEBULA IN ORION, 1883
_See p. 408_]
A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY
by
AGNES M. CLERKE
[Illustration: JUPITER 1879
SATURN 1885]
London
Adam and Charles Black
1908
First Edition, Post 8vo., published 1885
Second Edition, Post 8vo., published 1887
Third Edition, Demy 8vo., published 1893
Fourth Edition, Demy 8vo., published 1902
Fourth Edition, Post 8vo., reprinted February, 1908
PREFACE TO THE FOURTH EDITION
Since the third edition of the present work issued from the press, the
nineteenth century has run its course and finished its record. A new era
has dawned, not by chronological prescription alone, but to the vital
sense of humanity. Novel thoughts are rife; fresh impulses stir the
nations; the soughing of the wind of progress strikes every ear. "The
old order changeth" more and more swiftly as mental activity becomes
intensified. Already many of the scientific doctrines implicitly
accepted fifteen years ago begin to wear a superannuated aspect.
Dalton's atoms are in process of disintegration; Kirchhoff's theorem
visibly needs to be modified; Clerk Maxwell's medium no longer figures
as an indispensable factotum; "absolute zero" is known to be situated on
an asymptote to the curve of cold. Ideas, in short, have all at once
become plastic, and none more completely so than those relating to
astronomy. The physics of the heavenly bodies, indeed, finds its best
opportunities in unlooked-for disclosures; for it deals with
transcendental conditions, and what is strange to terrestrial experience
may serve admirably to expound what is normal in the skies. In celestial
science especially, facts that appear subversive are often the most
illuminative, and the prospect of its advance widens and brightens with
each divagation enforced or permitted from the strait paths of rigid
theory.
This readiness for innovation has undoubtedly its dangers and drawbacks.
To the historian, above all, it presents frequent occasions of
embarrassment. The writing of history is a strongly selective operation,
the outcome being valuable just in so far as the choice what to reject
and what to include has been judicious; and the task is no light one of
discriminating between barren speculations and ideas pregnant with
coming truth. To the possession of such prescience of the future as
would be needed to do this effectually I can lay no claim; but diligence
and sobriety of thought are ordinarily within reach, and these I shall
have exercised to good purpose if I have succeeded in rendering the
fourth edition of _A Popular History of Astronomy during the Nineteenth
Century_ not wholly unworthy of a place in the scientific literature of
the twentieth century.
My thanks are due to Sir David Gill for the use of his photograph of the
great comet of 1901, which I have added to my list of illustrations, and
to the Council of the Royal Astronomical Society for the loan of glass
positives needed for the reproduction of those included in the third
edition.
London, _July_, 1902.
PREFACE TO THE FIRST EDITION
The progress of astronomy during the last hundred years has been rapid
and extraordinary. In its distinctive features, moreover, the nature of
that progress has been such as to lend itself with facility to
untechnical treatment. To this circumstance the present volume owes its
origin. It embodies an attempt to enable the ordinary reader to follow,
with intelligent interest, the course of modern astronomical inquiries,
and to realize (so far as it can at present be realized) the full effect
of the comprehensive change in the whole aspect, purposes, and methods
of celestial science introduced by the momentous discovery of spectrum
analysis.
Since Professor Grant's invaluable work on the _History of Physical
Astronomy_ was published, a third of a century has elapsed. During the
interval a so-called "new astronomy" has grown up by the side of the
old. One effect of its advent has been to render the science of the
heavenly bodies more popular, both in its needs and in its nature, than
formerly. More popular in its needs, since its progress now primarily
depends upon the interest in, and consequent efforts towards its
advancement of the general public; more popular in its nature, because
the kind of knowledge it now chiefly tends to accumulate is more easily
intelligible--less remote from ordinary experience--than that evolved by
the aid of the calculus from materials collected by the use of the
transit-instrument and chronograph.
It has thus become practicable to describe in simple language the most
essential parts of recent astronomical discoveries, and, being
practicable, it could not be otherwise than desirable to do so. The
service to astronomy itself would be not inconsiderable of enlisting
wider sympathies on its behalf, while to help one single mind towards a
fuller understanding of the manifold works which have in all ages
irresistibly spoken to man of the glory of God might well be an object
of no ignoble ambition.
The present volume does not profess to be a complete or exhaustive
history of astronomy during the period covered by it. Its design is to
present a view of the progress of celestial science, on its most
characteristic side, since the time of Herschel. Abstruse mathematical
theories, unless in some of their more striking results, are excluded
from consideration. These, during the eighteenth century, constituted
the sum and substance of astronomy, and their fundamental importance can
never be diminished, and should never be ignored. But as the outcome of
the enormous development given to the powers of the telescope in recent
times, together with the swift advance of physical science, and the
inclusion, by means of the spectroscope, of the heavenly bodies within
the domain of its inquiries, much knowledge has been acquired regarding
the nature and condition of those bodies, forming, it might be said, a
science apart, and disembarrassed from immediate dependence upon
intricate, and, except to the initiated, unintelligible formulæ. This
kind of knowledge forms the main subject of the book now offered to the
public.
There are many reasons for preferring a history to a formal treatise on
astronomy. In a treatise, _what_ we know is set forth. A history tells
us, in addition, _how_ we came to know it. It thus places facts before
us in the natural order of their ascertainment, and narrates instead of
enumerating. The story to be told leaves the marvels of imagination far
behind, and requires no embellishment from literary art or high-flown
phrases. Its best ornament is unvarnished truthfulness, and this, at
least, may confidently be claimed to be bestowed upon it in the ensuing
pages.
In them unity of treatment is sought to be combined with a due regard to
chronological sequence by grouping in separate chapters the various
events relating to the several departments of descriptive astronomy. The
whole is divided into two parts, the line between which is roughly drawn
at the middle of the present century. Herschel's inquiries into the
construction of the heavens strike the keynote of the first part; the
discoveries of sun-spot and magnetic periodicity and of spectrum
analysis determine the character of the second. Where the nature of the
subject required it, however, this arrangement has been disregarded.
Clearness and consistency should obviously take precedence of method.
Thus, in treating of the telescopic scrutiny of the various planets, the
whole of the related facts have been collected into an uninterrupted
narrative. A division elsewhere natural and helpful would here have been
purely artificial, and therefore confusing.
The interests of students have been consulted by a full and authentic
system of references to the sources of information relied upon.
Materials have been derived, as a rule with very few exceptions, from
the original authorities. The system adopted has been to take as little
as possible at second-hand. Much pains have been taken to trace the
origin of ideas, often obscurely enunciated long before they came to
resound through the scientific world, and to give to each individual
discoverer, strictly and impartially, his due. Prominence has also been
assigned to the biographical element, as underlying and determining the
whole course of human endeavour. The advance of knowledge may be called
a vital process. The lives of men are absorbed into and assimilated by
it. Inquiries into the kind and mode of the surrender in each separate
case must always possess a strong interest, whether for study or for
example.
The acknowledgments of the writer are due to Professor Edward S. Holden,
director of the Washburn Observatory, Wisconsin, and to Dr. Copeland,
chief astronomer of Lord Crawford's Observatory at Dunecht, for many
valuable communications.
London, _September_, 1885.
CONTENTS
_INTRODUCTION_
page
Three Kinds of Astronomy--Progress of the Science during the
Eighteenth Century--Popularity and Rapid Advance during the Nineteenth
Century 1
PART I
_PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE NINETEENTH CENTURY_
CHAPTER I
FOUNDATION OF SIDEREAL ASTRONOMY
State of Knowledge regarding the Stars in the Eighteenth Century--
Career of Sir William Herschel--Constitution of the Stellar System--
Double Stars--Herschel's Discovery of their Revolutions--
His Method of Star-gauging--Discoveries of Nebulæ--Theory of their
Condensation into Stars--Summary of Results 9
CHAPTER II
PROGRESS OF SIDEREAL ASTRONOMY
Exact Astronomy in Germany--Career of Bessel--His _Fundamenta
Astronomiæ_--Career of Fraunhofer--Parallaxes of Fixed
Stars--Translation of the Solar System--Astronomy of the
Invisible--Struve's Researches in Double Stars--Sir John Herschel's
Exploration of the Heavens--Fifty Years' Progress 27
CHAPTER III
PROGRESS OF KNOWLEDGE REGARDING THE SUN
Early Views as to the Nature of Sun-spots--Wilson's Observations and
Reasonings--Sir William Herschel's Theory of the Solar
Constitution--Sir John Herschel's Trade-Wind Hypothesis--Baily's
Beads--Total Solar Eclipse of 1842--Corona and Prominences--Eclipse of
1851 52
CHAPTER IV
PLANETARY DISCOVERIES
Bode's Law--Search for a Missing Planet--Its Discovery by Piazzi--
Further Discoveries of Minor Planets--Unexplained Disturbance of
Uranus--Discovery of Neptune--Its Satellite--An Eighth Saturnian
Moon--Saturn's Dusky Ring--The Uranian System 71
CHAPTER V
COMETS
Predicted Return of Halley's Comet--Career of Olbers--Acceleration of
Encke's Comet--Biela's Comet--Its Duplication--Faye's Comet--Comet of
1811--Electrical Theory of Cometary Emanations--The Earth in a Comet's
Tail--Second Return of Halley's Comet--Great Comet of 1843--Results to
Knowledge 88
CHAPTER VI
INSTRUMENTAL ADVANCES
Two Principles of Telescopic Construction--Early Reflectors--Three
Varieties--Herschel's Specula--High Magnifying Powers--Invention of
the Achromatic Lens--Guinand's Optical Glass--The Great Rosse
Reflector--Its Disclosures--Mounting of Telescopes--Astronomical
Circles--Personal Equation 108
PART II
_RECENT PROGRESS OF ASTRONOMY_
CHAPTER I
FOUNDATION OF ASTRONOMICAL PHYSICS
Schwabe's Discovery of a Decennial Sun-spot Period--Coincidence with
Period of Magnetic Disturbance--Sun-spots and Weather--Spectrum
Analysis--Preliminary Inquiries--Fraunhofer Lines--Kirchhoff's
Principle--Anticipations--Elementary Principles of Spectrum
Analysis--Unity of Nature 125
CHAPTER II
SOLAR OBSERVATIONS AND THEORIES
Black Openings in Spots--Carrington's Observations--Rotation of the
Sun--Kirchhoff's Theory of the Solar Constitution--Faye's Views--Solar
Photography--Kew Observations--Spectroscopic Method--Cyclonic Theory
of Sun-spots--Volcanic Hypothesis--A Solar Outburst--Sun-spot
Periodicity--Planetary Influence--Structure of the Photosphere 143
CHAPTER III
RECENT SOLAR ECLIPSES
Expeditions to Spain--Great Indian Eclipse--New Method of Viewing
Prominences--Total Eclipse Visible in North America--Spectrum of the
Corona--Eclipse of 1870--Young's Reversing Layer--Eclipse of
1871--Corona of 1878--Varying Coronal Types--Egyptian
Eclipse--Daylight Coronal Photography--Observations at Caroline
Island--Photographs of Corona in 1886 and 1889--Eclipses of 1896,
1898, 1900, and 1901--Mechanical Theory of Corona--Electro-Magnetic
Theories--Nature of Corona 166
CHAPTER IV
SOLAR SPECTROSCOPY
Chemistry of Prominences--Study of their Forms--Two
Classes--Photographs and Spectrographs of Prominences--Their
Distribution--Structure of the Chromosphere--Spectroscopic Measurement
of Radial Movements--Spectroscopic Determination of Solar
Rotation--Velocities of Transport in the Sun--Lockyer's Theory of
Dissociation--Solar Constituents--Oxygen Absorption in Solar
Spectrum 194
CHAPTER V
TEMPERATURE OF THE SUN
Thermal Power of the Sun--Radiation and Temperature--Estimates of
Solar Temperature--Rosetti's and Wilson's Results--Zöllner's Method
--Langley's Experiment at Pittsburg--The Sun's Atmosphere--Langley's
Bolometric Researches--Selective Absorption by our Air--The Solar
Constant 216
CHAPTER VI
THE SUN'S DISTANCE
Difficulty of the Problem--Oppositions of Mars--Transits of
Venus--Lunar Disturbance--Velocity of Light--Transit of
1874--Inconclusive Result--Opposition of Mars in 1877--Measurements of
Minor Planets--Transit of 1882--Newcomb's Determination of the
Velocity of Light--Combined Result 227
CHAPTER VII
PLANETS AND SATELLITES
Schröter's Life and Work--Luminous Appearances during Transits of
Mercury--Mountains of Mercury--Intra-Mercurian Planets--Schiaparelli's
Results for the Rotation of Mercury and Venus--Illusory
Satellite--Mountains and Atmosphere of Venus--Ashen Light--Solidity of
the Earth--Variation of Latitude--Secular Changes of Climate--Figure
of the Globe--Study of the Moon's Surface--Lunar Atmosphere--New
Craters--Thermal Energy of Moonlight--Tidal Friction 243
CHAPTER VIII
PLANETS AND SATELLITES--(_continued_)
Analogy between Mars and the Earth--Martian Snowcaps, Seas, and
Continents--Climate and Atmosphere--Schiaparelli's Canals--Discovery
of Two Martian Satellites--Photographic Detection of Minor
Planets--Orbit of Eros--Distribution of the Minor Planets--Their
Collective Mass and Estimated Diameters--Condition of Jupiter--His
Spectrum--Transits of his Satellites--Discovery of a Fifth
Satellite--The Great Red Spot--Constitution of Saturn's Rings--Period
of Rotation of the Planet--Variability of Japetus--Equatorial Markings
on Uranus--His Spectrum--Rotation of Neptune--Trans-Neptunian
Planets 274
CHAPTER IX
THEORIES OF PLANETARY EVOLUTION
Origin of the World according to Kant--Laplace's Nebular
Hypothesis--Maintenance of the Sun's Heat--Meteoric
Hypothesis--Radiation as an Effect of Contraction--Regenerative
Theory--Faye's Scheme of Planetary Development--Origin of the
Moon--Effects of Tidal Friction 308
CHAPTER X
RECENT COMETS
Donati's Comet--The Earth again Involved in a Comet's Tail--Comets of
the August and November Meteors--Star Showers--Comets and
Meteors--Biela's Comet and the Andromedes--Holmes's Comet--Deflection
of the Leonids--Orbits of Meteorites--Meteors with Stationary
Radiants--Spectroscopic Analysis of Cometary Light--Comet of
1901--Coggia's Comet 323
CHAPTER XI
RECENT COMETS--(_continued_)
Forms of Comets' Tails--Electrical Repulsion--Brédikhine's Three
Types--Great Southern Comet--Supposed Previous Appearances--Tebbutt's
Comet and the Comet of 1807--Successful Photographs--Schaeberle's
Comet--Comet Wells--Sodium Blaze in Spectrum--Great Comet of
1882--Transit across the Sun--Relation to Comets of 1843 and
1880--Cometary Systems--Spectral Changes in Comet of 1882--Brooks's
Comet of 1889--Swift's Comet of 1892--Origin of Comets 345
CHAPTER XII
STARS AND NEBULÆ
Stellar Chemistry--Four Orders of Stars--Their Relative Ages--Gaseous
Stars--Spectroscopic Star-Catalogues--Stellar Chemistry--Hydrogen
Spectrum in Stars--The Draper Catalogue--Velocities of Stars in Line
of Sight--Spectroscopic Binaries--Eclipses of Algol--Catalogues of
Variables--New Stars--Outbursts in Nebulæ--Nova Aurigæ--Nova
Persei--Gaseous Nebulæ--Variable Nebulæ--Movements of Nebulæ--Stellar
and Nebular Photography--Nebulæ in the Pleiades--Photographic
Star-charting--Stellar Parallax--Double Stars--Stellar
Photometry--Status of Nebulæ--Photographs and Drawings of the Milky
Way--Star Drift 372
CHAPTER XIII
METHODS OF RESEARCH
Development of Telescopic Power--Silvered Glass Reflectors--Giant
Refractors--Comparison with Reflectors--The Yerkes
Telescope--Atmospheric Disturbance--The Lick Observatory--Mechanical
Difficulties--The Equatoreal _Coudé_--The Photographic
Camera--Retrospect and Conclusion 428
APPENDIX
Chronology, 1774-1893--Chemical Elements in the Sun (Rowland,
1891)--Epochs of Sun-spot Maximum and Minimum from 1610 to
1901--Movements of Sun and Stars--List of Great Telescopes--List of
Observatories employed in the Construction of the Photographic Chart
and Catalogue of the Heavens 443
INDEX 471
LIST OF ILLUSTRATIONS
Photograph of the Great Nebula in Orion, 1883 _Frontispiece_
Photographs of Jupiter, 1879, and of Saturn, 1885 _Vignette_
Plate I. Photographs of the Solar Chromosphere
and Prominences _To face p. 198_
Plate II. Photograph of the Great Comet of May,
1901 (Taken at the Royal Observatory, Cape
of Good Hope) " 343
Plate III. The Great Comet of September, 1882
(Photographed at the Cape of Good Hope) " 359
Plate IV. Photographs of Swift's Comet, 1892 " 368
Plate V. Photographic and Visual Spectrum of
Nova Aurigæ " 396
Plate VI. Photograph of the Milky Way in Sagittarius " 424
HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY
_INTRODUCTION_
We can distinguish three kinds of astronomy, each with a different
origin and history, but all mutually dependent, and composing, in their
fundamental unity, one science. First in order of time came the art of
observing the returns, and measuring the places, of the heavenly bodies.
This was the sole astronomy of the Chinese and Chaldeans; but to it the
vigorous Greek mind added a highly complex geometrical plan of their
movements, for which Copernicus substituted a more harmonious system,
without as yet any idea of a compelling cause. The planets revolved in
circles because it was their nature to do so, just as laudanum sets to
sleep because it possesses a _virtus dormitiva_. This first and oldest
branch is known as "observational," or "practical astronomy." Its
business is to note facts as accurately as possible; and it is
essentially unconcerned with schemes for connecting those facts in a
manner satisfactory to the reason.
The second kind of astronomy was founded by Newton. Its nature is best
indicated by the term "gravitational"; but it is also called
"theoretical astronomy."[1] It is based on the idea of cause; and the
whole of its elaborate structure is reared according to the dictates of
a single law, simple in itself, but the tangled web of whose
consequences can be unravelled only by the subtle agency of an elaborate
calculus.
The third and last division of celestial science may properly be termed
"physical and descriptive astronomy." It seeks to know what the heavenly
bodies are in themselves, leaving the How? and the Wherefore? of their
movements to be otherwise answered. Now, such inquiries became possible
only through the invention of the telescope, so that Galileo was, in
point of fact, their originator. But Herschel first gave them a
prominence which the whole progress of science during the nineteenth
century served to confirm and render more exclusive. Inquisitions begun
with the telescope have been extended and made effective in unhoped-for
directions by the aid of the spectroscope and photographic camera; and a
large part of our attention in the present volume will be occupied with
the brilliant results thus achieved.
The unexpected development of this new physical-celestial science is the
leading fact in recent astronomical history. It was out of the regular
course of events. In the degree in which it has actually occurred it
could certainly not have been foreseen. It was a seizing of the prize by
a competitor who had hardly been thought qualified to enter the lists.
Orthodox astronomers of the old school looked with a certain contempt
upon observers who spent their nights in scrutinising the faces of the
moon and planets rather than in timing their transits, or devoted
daylight energies, not to reductions and computations, but to counting
and measuring spots on the sun. They were regarded as irregular
practitioners, to be tolerated perhaps, but certainly not encouraged.
The advance of astronomy in the eighteenth century ran in general an
even and logical course. The age succeeding Newton's had for its special
task to demonstrate the universal validity, and trace the complex
results, of the law of gravitation. The accomplishment of that task
occupied just one hundred years. It was virtually brought to a close
when Laplace explained to the French Academy, November 19, 1787, the
cause of the moon's accelerated motion. As a mere machine, the solar
system, so far as it was then known, was found to be complete and
intelligible in all its parts; and in the _Mécanique Céleste_ its
mechanical perfections were displayed under a form of majestic unity
which fitly commemorated the successive triumphs of analytical genius
over problems amongst the most arduous ever dealt with by the mind of
man.
Theory, however, demands a practical test. All its data are derived from
observation; and their insecurity becomes less tolerable as it advances
nearer to perfection. Observation, on the other hand, is the pitiless
critic of theory; it detects weak points, and provokes reforms which may
be the beginnings of discovery. Thus, theory and observation mutually
act and react, each alternately taking the lead in the endless race of
improvement.
Now, while in France Lagrange and Laplace were bringing the
gravitational theory of the solar system to completion, work of a very
different kind, yet not less indispensable to the future welfare of
astronomy, was being done in England. The Royal Observatory at Greenwich
is one of the few useful institutions which date their origin from the
reign of Charles II. The leading position which it still occupies in the
science of celestial observation was, for near a century and a half
after its foundation, an exclusive one. Delambre remarked that, had all
other materials of the kind been destroyed, the Greenwich records alone
would suffice for the restoration of astronomy. The establishment was
indeed absolutely without a rival.[2] Systematic observations of sun,
moon, stars, and planets were during the whole of the eighteenth century
made only at Greenwich. Here materials were accumulated for the secure
correction of theory, and here refinements were introduced by which the
exquisite accuracy of modern practice in astronomy was eventually
attained.
The chief promoter of these improvements was James Bradley. Few men have
possessed in an equal degree with him the power of seeing accurately,
and reasoning on what they see. He let nothing pass. The slightest
inconsistency between what appeared and what was to be expected roused
his keenest attention; and he never relaxed his mental grip of a subject
until it had yielded to his persistent inquisition. It was to these
qualities that he owed his discoveries of the aberration of light and
the nutation of the earth's axis. The first was announced in 1729. What
is meant by it is that, owing to the circumstance of light not being
instantaneously transmitted, the heavenly bodies appear shifted from
their true places by an amount depending upon the ratio which the
velocity of light bears to the speed of the earth in its orbit. Because
light travels with enormous rapidity, the shifting is very slight; and
each star returns to its original position at the end of a year.
Bradley's second great discovery was finally ascertained in 1748.
Nutation is a real "nodding" of the terrestrial axis produced by the
dragging of the moon at the terrestrial equatorial protuberance. From it
results an _apparent_ displacement of the stars, each of them describing
a little ellipse about its true or "mean" position, in a period of
nearly nineteen years.
Now, an acquaintance with the fact and the laws of each of these minute
irregularities is vital to the progress of observational astronomy; for
without it the places of the heavenly bodies could never be accurately
known or compared. So that Bradley, by their detection, at once raised
the science to a higher grade of precision. Nor was this the whole of
his work. Appointed Astronomer-Royal in 1742, he executed during the
years 1750-62 a series of observations which formed the real beginning
of exact astronomy. Part of their superiority must, indeed, be
attributed to the co-operation of John Bird, who provided Bradley in
1750 with a measuring instrument of till then unequalled excellence. For
not only was the art of observing in the eighteenth century a peculiarly
English art, but the means of observing were furnished almost
exclusively by British artists. John Dollond, the son of a Spitalfields
weaver, invented the achromatic lens in 1758, removing thereby the chief
obstacle to the development of the powers of refracting telescopes;
James Short, of Edinburgh, was without a rival in the construction of
reflectors; the sectors, quadrants, and circles of Graham, Bird,
Ramsden, and Cary were inimitable by Continental workmanship.
Thus practical and theoretical astronomy advanced on parallel lines in
England and France respectively, the improvement of their several
tools--the telescope and the quadrant on the one side, and the calculus
on the other--keeping pace. The whole future of the science seemed to be
theirs. The cessation of interest through a too speedy attainment of the
perfection towards which each spurred the other, appeared to be the only
danger it held in store for them. When all at once, a rival stood by
their side--not, indeed, menacing their progress, but threatening to
absorb their popularity.
The rise of Herschel was the one conspicuous anomaly in the astronomical
history of the eighteenth century. It proved decisive of the course of
events in the nineteenth. It was unexplained by anything that had gone
before; yet all that came after hinged upon it. It gave a new direction
to effort; it lent a fresh impulse to thought. It opened a channel for
the widespread public interest which was gathering towards astronomical
subjects to flow in.
Much of this interest was due to the occurrence of events calculated to
arrest the attention and excite the wonder of the uninitiated. The
predicted return of Halley's comet in 1759 verified, after an
unprecedented fashion, the computations of astronomers. It deprived such
bodies for ever of their portentous character; it ranked them as
denizens of the solar system. Again, the transits of Venus in 1761 and
1769 were the first occurrences of the kind since the awakening of
science to their consequence. Imposing preparations, journeys to remote
and hardly accessible regions, official expeditions, international
communications, all for the purpose of observing them to the best
advantage, brought their high significance vividly to the public
consciousness; a result aided by the facile pen of Lalande, in rendering
intelligible the means by which these elaborate arrangements were to
issue in an accurate knowledge of the sun's distance. Lastly, Herschel's
discovery of Uranus, March 13, 1781, had the surprising effect of utter
novelty. Since the human race had become acquainted with the company of
the planets, no addition had been made to their number. The event thus
broke with immemorial traditions, and seemed to show astronomy as still
young and full of unlooked-for possibilities.
Further popularity accrued to the science from the sequel of a career so
strikingly opened. Herschel's huge telescopes, his detection by their
means of two Saturnian and as many Uranian moons, his piercing scrutiny
of the sun, picturesque theory of its constitution, and sagacious
indication of the route pursued by it through space; his discovery of
stellar revolving systems, his bold soundings of the universe, his
grandiose ideas, and the elevated yet simple language in which they were
conveyed--formed a combination powerfully effective to those least
susceptible of new impressions. Nor was the evoked enthusiasm limited to
the British Isles. In Germany, Schröter followed--_longo intervallo_--in
Herschel's track. Von Zach set on foot from Gotha that general
communication of ideas which gives life to a forward movement. Bode
wrote much and well for unlearned readers. Lalande, by his popular
lectures and treatises, helped to form an audience which Laplace himself
did not disdain to address in the _Exposition du Système du Monde_.
This great accession of public interest gave the impulse to the
extraordinarily rapid progress of astronomy in the nineteenth century.
Official patronage combined with individual zeal sufficed for the elder
branches of the science. A few well-endowed institutions could
accumulate the materials needed by a few isolated thinkers for the
construction of theories of wonderful beauty and elaboration, yet
precluded, by their abstract nature, from winning general applause. But
the new physical astronomy depends for its prosperity upon the favour of
the multitude whom its striking results are well fitted to attract. It
is, in a special manner, the science of amateurs. It welcomes the most
unpretending co-operation. There is no one "with a true eye and a
faithful hand" but can do good work in watching the heavens. And not
unfrequently, prizes of discovery which the most perfect appliances
failed to grasp, have fallen to the share of ignorant or ill-provided
assiduity.
Observers, accordingly, have multiplied; observatories have been founded
in all parts of the world; associations have been constituted for mutual
help and counsel. A formal astronomical congress met in 1789 at
Gotha--then, under Duke Ernest II. and Von Zach, the focus of German
astronomy--and instituted a combined search for the planet suspected to
revolve undiscovered between the orbits of Mars and Jupiter. The
Astronomical Society of London was established in 1820, and the similar
German institution in 1863. Both have been highly influential in
promoting the interests, local and general, of the science they are
devoted to forward; while functions corresponding to theirs have been
discharged elsewhere by older or less specially constituted bodies, and
new ones of a more popular character are springing up on all sides.
Modern facilities of communication have helped to impress more deeply
upon modern astronomy its associative character. The electric telegraph
gives a certain ubiquity which is invaluable to an observer of the
skies. With the help of a wire, a battery, and a code of signals, he
sees whatever is visible from any portion of our globe, depending,
however, upon other eyes than his own, and so entering as a unit into a
widespread organisation of intelligence. The press, again, has been a
potent agent of co-operation. It has mainly contributed to unite
astronomers all over the world into a body animated by the single aim of
collecting "particulars" in their special branch for what Bacon termed a
History of Nature, eventually to be interpreted according to the
sagacious insight of some one among them gifted above his fellows. The
first really effective astronomical periodical was the _Monatliche
Correspondenz_, started by Von Zach in the year 1800. It was followed in
1822 by the _Astronomische Nachrichten_, later by the _Memoirs_ and
_Monthly Notices_ of the Astronomical Society, and by the host of varied
publications which now, in every civilised country, communicate the
discoveries made in astronomy to divers classes of readers, and so
incalculably quicken the current of its onward flow.
Public favour brings in its train material resources. It is represented
by individual enterprise, and finds expression in an ample liberality.
The first regular observatory in the Southern Hemisphere was founded at
Paramatta by Sir Thomas Makdougall Brisbane in 1821. The Royal
Observatory at the Cape of Good Hope was completed in 1829. Similar
establishments were set to work by the East India Company at Madras,
Bombay, and St. Helena, during the first third of the nineteenth
century. The organisation of astronomy in the United States of America
was due to a strong wave of popular enthusiasm. In 1825 John Quincy
Adams vainly urged upon Congress the foundation of a National
Observatory; but in 1843 the lectures on celestial phenomena of Ormsby
MacKnight Mitchel stirred an impressionable audience to the pitch of
providing him with the means of erecting at Cincinnati the first
astronomical establishment worthy the name in that great country. On the
1st of January, 1882, no less than one hundred and forty-four were
active within its boundaries.
The apparition of the great comet of 1843 gave an additional fillip to
the movement. To the excitement caused by it the Harvard College
Observatory--called the "American Pulkowa"--directly owed its origin;
and the example was not ineffective elsewhere. The United States Naval
Observatory was built in 1844, Lieutenant Maury being its first
Director. Corporations, universities, municipalities, vied with each
other in the creation of such institutions; private subscriptions poured
in; emissaries were sent to Europe to purchase instruments and to
procure instruction in their use. In a few years the young Republic was,
in point of astronomical efficiency, at least on a level with countries
where the science had been fostered since the dawn of civilisation.
A vast widening of the scope of astronomy has accompanied, and in part
occasioned, the great extension of its area of cultivation which our age
has witnessed. In the last century its purview was a comparatively
narrow one. Problems lying beyond the range of the solar system were
almost unheeded, because they seemed inscrutable. Herschel first showed
the sidereal universe as accessible to investigation, and thereby
offered to science new worlds--majestic, manifold, "infinitely infinite"
to our apprehension in number, variety, and extent--for future conquest.
Their gradual appropriation has absorbed, and will long continue to
absorb, the powers which it has served to develop.
But this is not the only direction in which astronomy has enlarged, or
rather has levelled, its boundaries. The unification of the physical
sciences is perhaps the greatest intellectual feat of recent times. The
process has included astronomy; so that, like Bacon, she may now be said
to have "taken all knowledge" (of that kind) "for her province." In
return, she proffers potent aid for its increase. Every comet that
approaches the sun is the scene of experiments in the electrical
illumination of rarefied matter, performed on a huge scale for our
benefit. The sun, stars, and nebulæ form so many celestial laboratories,
where the nature and mutual relations of the chemical "elements" may be
tried by more stringent tests than sublunary conditions afford. The laws
of terrestrial magnetism can be completely investigated only with the
aid of a concurrent study of the face of the sun. The solar spectrum
will perhaps one day, by its recurrent modifications, tell us something
of impending droughts, famines, and cyclones.
Astronomy generalises the results of the other sciences. She exhibits
the laws of Nature working over a wider area, and under more varied
conditions, than ordinary experience presents. Ordinary experience, on
the other hand, has become indispensable to her progress. She takes in
at one view the indefinitely great and the indefinitely little. The
mutual revolutions of the stellar multitude during tracts of time which
seem to lengthen out to eternity as the mind attempts to traverse them,
she does not admit to be beyond her ken; nor is she indifferent to the
constitution of the minutest atom of matter that thrills the ether into
light. How she entered upon this vastly expanded inheritance, and how,
so far, she has dealt with it, is attempted to be set forth in the
ensuing chapters.
FOOTNOTES:
[Footnote 1: The denomination "physical astronomy," first used by
Kepler, and long appropriated to this branch of the science, has of late
been otherwise applied.]
[Footnote 2: _Histoire de l'Astronomie au xviii^e Siècle_, p. 267.]
PART I
PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE NINETEENTH CENTURY
CHAPTER I
_FOUNDATION OF SIDEREAL ASTRONOMY_
Until nearly a hundred years ago the stars were regarded by practical
astronomers mainly as a number of convenient fixed points by which the
motions of the various members of the solar system could be determined
and compared. Their recognised function, in fact, was that of milestones
on the great celestial highway traversed by the planets, as well as on
the byways of space occasionally pursued by comets. Not that curiosity
as to their nature, and even conjecture as to their origin, were at any
period absent. Both were from time to time powerfully stimulated by the
appearance of startling novelties in a region described by philosophers
as "incorruptible," or exempt from change. The catalogue of Hipparchus
probably, and certainly that of Tycho Brahe, some seventeen centuries
later, owed each its origin to the temporary blaze of a new star. The
general aspect of the skies was thus (however imperfectly) recorded from
age to age, and with improved appliances the enumeration was rendered
more and more accurate and complete; but the secrets of the stellar
sphere remained inviolate.
In a qualified though very real sense, Sir William Herschel may be
called the Founder of Sidereal Astronomy. Before his time some curious
facts had been noted, and some ingenious speculations hazarded,
regarding the condition of the stars, but not even the rudiments of
systematic knowledge had been acquired. The facts ascertained can be
summed up in a very few sentences.
Giordano Bruno was the first to set the suns of space in motion; but in
imagination only. His daring surmise was, however, confirmed in 1718,
when Halley announced[3] that Sirius, Aldebaran, Betelgeux, and Arcturus
had unmistakably shifted their quarters in the sky since Ptolemy
assigned their places in his catalogue. A similar conclusion was reached
by J. Cassini in 1738, from a comparison of his own observations with
those made at Cayenne by Richer in 1672; and Tobias Mayer drew up in
1756 a list showing the direction and amount of about fifty-seven proper
motions,[4] founded on star-places determined by Olaus Römer fifty years
previously. Thus the stars were no longer regarded as "fixed," but the
question remained whether the movements perceived were real or only
apparent; and this it was not yet found possible to answer. Already, in
the previous century, the ingenious Robert Hooke had suggested an
"alteration of the very system of the sun,"[5] to account for certain
suspected changes in stellar positions; Bradley in 1748, and Lambert in
1761, pointed out that such apparent displacements (by that time well
ascertained) were in all probability a combined effect of motions both
of sun and stars; and Mayer actually attempted the analysis, but without
result.
On the 13th of August, 1596, David Fabricius, an unprofessional
astronomer in East Friesland, saw in the neck of the Whale a star of the
third magnitude, which by October had disappeared. It was, nevertheless,
visible in 1603, when Bayer marked it in his catalogue with the Greek
letter Omicron, and was watched, in 1638-39, through its phases of
brightening and apparent extinction by a Dutch professor named
Holwarda.[6] From Hevelius this first-known periodical star received the
name of "Mira," or the Wonderful, and Boulliaud in 1667 fixed the length
of its cycle of change at 334 days. It was not a solitary instance. A
star in the Swan was perceived by Janson in 1600 to show fluctuations of
light, and Montanari found in 1669 that Algol in Perseus shared the same
peculiarity to a marked degree. Altogether the class embraced in 1782
half-a-dozen members. When it is added that a few star-couples had been
noted in singularly, but it was supposed accidentally, close
juxtaposition, and that the failure of repeated attempts to measure
stellar parallaxes pointed to distances _at least_ 400,000 times that of
the earth from the sun,[7] the picture of sidereal science, when the
last quarter of the eighteenth century began, is practically complete.
It included three items of information: that the stars have motions,
real or apparent; that they are immeasurably remote; and that a few
shine with a periodically variable light. Nor were these scantily
collected facts ordered into any promise of further development. They
lay at once isolated and confused before the inquirer. They needed to be
both multiplied and marshalled, and it seemed as if centuries of patient
toil must elapse before any reliable conclusions could be derived from
them. The sidereal world was thus the recognised domain of far-reaching
speculations, which remained wholly uncramped by systematic research
until Herschel entered upon his career as an observer of the heavens.
The greatest of modern astronomers was born at Hanover, November 15,
1738. He was the fourth child of Isaac Herschel, a hautboy-player in the
band of the Hanoverian Guard, and was early trained to follow his
father's profession. On the termination, however, of the disastrous
campaign of 1757, his parents removed him from the regiment, there is
reason to believe, in a somewhat unceremonious manner. Technically,
indeed, he incurred the penalties of desertion, remitted--according to
the Duke of Sussex's statement to Sir George Airy--by a formal pardon
handed to him personally by George III. on his presentation in 1782.[8]
At the age of nineteen, then, his military service having lasted four
years, he came to England to seek his fortune. Of the life of struggle
and privation which ensued little is known beyond the circumstances that
in 1760 he was engaged in training the regimental band of the Durham
Militia, and that in 1765 he was appointed organist at Halifax. In the
following year he removed to Bath as oboist in Linley's orchestra, and
in October 1767 was promoted to the post of organist in the Octagon
Chapel. The tide of prosperity now began to flow for him. The most
brilliant and modish society in England was at that time to be met at
Bath, and the young Hanoverian quickly found himself a favourite and the
fashion in it. Engagements multiplied upon him. He became director of
the public concerts; he conducted oratorios, engaged singers, organised
rehearsals, composed anthems, chants, choral services, besides
undertaking private tuitions, at times amounting to thirty-five or even
thirty-eight lessons a week. He in fact personified the musical activity
of a place then eminently and energetically musical.
But these multifarious avocations did not take up the whole of his
thoughts. His education, notwithstanding the poverty of his family, had
not been neglected, and he had always greedily assimilated every kind of
knowledge that came in his way. Now that he was a busy and a prosperous
man, it might have been expected that he would run on in the deep
professional groove laid down for him. On the contrary, his passion for
learning seemed to increase with the diminution of the time available
for its gratification. He studied Italian, Greek, mathematics;
Maclaurin's Fluxions served to "unbend his mind"; Smith's Harmonics and
Optics and Ferguson's Astronomy were the nightly companions of his
pillow. What he read stimulated without satisfying his intellect. He
desired not only to know, but to discover. In 1772 he hired a small
telescope, and through it caught a preliminary glimpse of the rich and
varied fields in which for so many years he was to expatiate.
Henceforward the purpose of his life was fixed: it was to obtain "a
knowledge of the construction of the heavens";[9] and this sublime
ambition he cherished to the end.
A more powerful instrument was the first desideratum; and here his
mechanical genius came to his aid. Having purchased the apparatus of a
Quaker optician, he set about the manufacture of specula with a zeal
which seemed to anticipate the wonders they were to disclose to him. It
was not until fifteen years later that his grinding and polishing
machines were invented, so the work had at that time to be entirely done
by hand. During this tedious and laborious process (which could not be
interrupted without injury, and lasted on one occasion sixteen hours),
his strength was supported by morsels of food put into his mouth by his
sister,[10] and his mind amused by her reading aloud to him the Arabian
Nights, Don Quixote, or other light works. At length, after repeated
failures, he found himself provided with a reflecting telescope--a
5-1/2-foot Gregorian--of his own construction. A copy of his first
observation with it, on the great Nebula in Orion--an object of
continual amazement and assiduous inquiry to him--is preserved by the
Royal Society. It bears the date March 4, 1774.[11]
In the following year he executed his first "review of the heavens,"
memorable chiefly as an evidence of the grand and novel conceptions
which already inspired him, and of the enthusiasm with which he
delivered himself up to their guidance. Overwhelmed with professional
engagements, he still contrived to snatch some moments for the stars;
and between the acts at the theatre was often seen running from the
harpsichord to his telescope, no doubt with that "uncommon precipitancy
which accompanied all his actions."[12] He now rapidly increased the
power and perfection of his telescopes. Mirrors of seven, ten, even
twenty feet focal length, were successively completed, and unprecedented
magnifying powers employed. His energy was unceasing, his perseverance
indomitable. In the course of twenty-one years no less than 430
parabolic specula left his hands. He had entered upon his forty-second
year when he sent his first paper to the _Philosophical Transactions_;
yet during the ensuing thirty-nine years his contributions--many of them
elaborate treatises--numbered sixty-nine, forming a series of
extraordinary importance to the history of astronomy. As a mere explorer
of the heavens his labours were prodigious. He discovered 2,500 nebulæ,
806 double stars, passed the whole firmament in review four several
times, counted the stars in 3,400 "gauge-fields," and executed a
photometric classification of the principal stars, founded on an
elaborate (and the first systematically conducted) investigation of
their relative brightness. He was as careful and patient as he was
rapid; spared no time and omitted no precaution to secure accuracy in
his observations; yet in one night he would examine, singly and
attentively, up to 400 separate objects.
The discovery of Uranus was a mere incident of the scheme he had marked
out for himself--a fruit, gathered as it were by the way. It formed,
nevertheless, the turning-point in his career. From a star-gazing
musician he was at once transformed into an eminent astronomer; he was
relieved from the drudgery of a toilsome profession, and installed as
Royal Astronomer, with a modest salary of £200 a year; funds were
provided for the construction of the forty-foot reflector, from the
great space-penetrating power of which he expected unheard-of
revelations; in fine, his future work was not only rendered possible,
but it was stamped as authoritative.[13] On Whit-Sunday 1782, William
and Caroline Herschel played and sang in public for the last time in St.
Margaret's Chapel, Bath; in August of the same year the household was
moved to Datchet, near Windsor, and on April 3, 1786, to Slough. Here
happiness and honours crowded on the fortunate discoverer. In 1788 he
married Mary, only child of James Baldwin, a merchant of the city of
London, and widow of Mr. John Pitt--a lady whose domestic virtues were
enhanced by the possession of a large jointure. The fruit of their union
was one son, of whose work--the worthy sequel of his father's--we shall
have to speak further on. Herschel was created a Knight of the
Hanoverian Guelphic Order in 1816, and in 1821 he became the first
President of the Royal Astronomical Society, his son being its first
Foreign Secretary. But his health had now for some years been failing,
and on August 25, 1822, he died at Slough, in the eighty-fourth year of
his age, and was buried in Upton churchyard.
His epitaph claims for him the lofty praise of having "burst the
barriers of heaven." Let us see in what sense this is true.
The first to form any definite idea as to the constitution of the
stellar system was Thomas Wright, the son of a carpenter living at
Byer's Green, near Durham. With him originated what has been called the
"Grindstone Theory" of the universe, which regarded the Milky Way as the
projection on the sphere of a stratum or disc of stars (our sun
occupying a position near the centre), similar in magnitude and
distribution to the lucid orbs of the constellations.[14] He was
followed by Kant,[15] who transcended the views of his predecessor by
assigning to nebulæ the position they long continued to occupy, rather
on imaginative than scientific grounds, of "island universes," external
to, and co-equal with, the Galaxy. Johann Heinrich Lambert,[16] a
tailor's apprentice from Mühlhausen, followed, but independently. The
conceptions of this remarkable man were grandiose, his intuitions bold,
his views on some points a singular anticipation of subsequent
discoveries. The sidereal world presented itself to him as a hierarchy
of systems, starting from the planetary scheme, rising to throngs of
suns within the circuit of the Milky Way--the "ecliptic of the stars,"
as he phrased it--expanding to include groups of many Milky Ways; these
again combining to form the unit of a higher order of assemblage, and so
onwards and upwards until the mind reels and sinks before the immensity
of the contemplated creations.
"Thus everything revolves--the earth round the sun; the sun round the
centre of his system; this system round a centre common to it with other
systems; this group, this assemblage of systems, round a centre which is
common to it with other groups of the same kind; and where shall we have
done?"[17]
The stupendous problem thus speculatively attempted, Herschel undertook
to grapple with experimentally. The upshot of this memorable inquiry was
the inclusion, for the first time, within the sphere of human knowledge,
of a connected body of facts, and inferences from facts, regarding the
sidereal universe; in other words, the foundation of what may properly
be called a science of the stars.
Tobias Mayer had illustrated the perspective effects which must ensue in
the stellar sphere from a translation of the solar system, by comparing
them to the separating in front and closing up behind of trees in a
forest to the eye of an advancing spectator;[18] but the appearances
which he thus correctly described he was unable to detect. By a more
searching analysis of a smaller collection of proper motions, Herschel
succeeded in rendering apparent the very consequences foreseen by Mayer.
He showed, for example, that Arcturus and Vega did, in fact, appear to
recede from, and Sirius and Aldebaran to approach, each other by very
minute amounts; and, with a striking effort of divinatory genius, placed
the "apex," or point of direction of the sun's motion, close to the star
Lambda in the constellation Hercules,[19] within a few degrees of
the spot indicated by later and indefinitely more refined methods of
research. He resumed the subject in 1805,[20] but though employing a
more rigorous method, was scarcely so happy in his result. In 1806,[21]
he made a preliminary attempt to ascertain the speed of the sun's
journey, fixing it, by doubtless much too low an estimate, at about
three miles a second. Yet the validity of his general conclusion as to
the line of solar travel, though long doubted, has been triumphantly
confirmed. The question as to the "secular parallax" of the fixed stars
was in effect answered.
With their _annual_ parallax, however, the case was very different. The
search for it had already led Bradley to the important discoveries of
the aberration of light and the nutation of the earth's axis; it was now
about to lead Herschel to a discovery of a different, but even more
elevated character. Yet in neither case was the object primarily sought
attained.
From the very first promulgation of the Copernician theory the seeming
immobility of the stars had been urged as an argument against its truth;
for if the earth really travelled in a vast orbit round the sun, objects
in surrounding space should appear to change their positions, unless
their distances were on a scale which, to the narrow ideas of the
universe then prevailing, seemed altogether extravagant.[22] The
existence of such apparent or "parallactic" displacements was
accordingly regarded as the touchstone of the new views, and their
detection became an object of earnest desire to those interested in
maintaining them. Copernicus himself made the attempt; but with his
"Triquetrum," a jointed wooden rule with the divisions marked in ink,
constructed by himself,[23] he was hardly able to measure angles of ten
minutes, far less fractions of a second. Galileo, a more impassioned
defender of the system, strained his ears, as it were, from Arcetri, in
his blind and sorrowful old age, for news of a discovery which two more
centuries had still to wait for. Hooke believed he had found a parallax
for the bright star in the Head of the Dragon; but was deceived. Bradley
convinced himself that such effects were too minute for his instruments
to measure. Herschel made a fresh attempt by a practically untried
method.
It is a matter of daily experience that two objects situated at
different distances seem to a beholder in motion to move relatively to
each other. This principle Galileo, in the third of his Dialogues on the
Systems of the World,[24] proposed to employ for the determination of
stellar parallax; for two stars, lying apparently close together, but in
reality separated by a great gulf of space, must shift their mutual
positions when observed from opposite points of the earth's orbit; or
rather, the remoter forms a virtually fixed point, to which the
movements of the other can be conveniently referred. By this means
complications were abolished more numerous and perplexing than Galileo
himself was aware of, and the problem was reduced to one of simple
micrometrical measurement. The "double-star method" was also suggested
by James Gregory in 1675, and again by Wallis in 1693;[25] Huygens
first, and afterwards Dr. Long of Cambridge (about 1750), made futile
experiments with it; and it eventually led, in the hands of Bessel, to
the successful determination of the parallax of 61 Cygni.
Its advantages were not lost upon Herschel. His attempt to assign
definite distances to the nearest stars was no isolated effort, but part
of the settled plan upon which his observations were conducted. He
proposed to sound the heavens, and the first requisite was a knowledge
of the length of his sounding-line. Thus it came about that his special
attention was early directed to double stars.
"I resolved," he writes,[26] "to examine every star in the heavens with
the utmost attention and a very high power, that I might collect such
materials for this research as would enable me to fix my observations
upon those that would best answer my end. The subject has already proved
so extensive, and still promises so rich a harvest to those who are
inclined to be diligent in the pursuit, that I cannot help inviting
every lover of astronomy to join with me in observations that must
inevitably lead to new discoveries."
The first result of these inquiries was a classed catalogue of 269
double stars presented to the Royal Society in 1782, followed, after
three years, by an additional list of 434. In both these collections the
distances separating the individuals of each pair were carefully
measured, and (with a few exceptions) the angles made with the
hour-circle by the lines joining their centres (technically called
"angles of position") were determined with the aid of a "revolving-wire
micrometer," specially devised for the purpose. Moreover, an important
novelty was introduced by the observation of the various colours visible
in the star-couples, the singular and vivid contrasts of which were now
for the first time described.
Double stars were at that time supposed to be a purely optical
phenomenon. Their components, it was thought, while in reality
indefinitely remote from each other, were brought into fortuitous
contiguity by the chance of lying nearly in the same line of sight from
the earth. Yet Bradley had noticed a change of 30°, between 1718 and
1759, in the position-angle of the two stars forming Castor, and was
thus within a hair's breadth of the discovery of their physical
connection.[27] While the Rev. John Michell, arguing by the doctrine of
probabilities, wrote as follows in 1767:--"It is highly probable in
particular, and next to a certainty in general, that such double stars
as appear to consist of two or more stars placed very near together, do
really consist of stars placed near together, and under the influence of
some general law."[28] And in 1784:[29] "It is not improbable that a few
years may inform us that some of the great number of double, triple
stars, etc., which have been observed by Mr. Herschel, are systems of
bodies revolving about each other."
This remarkable speculative anticipation had a practical counterpart in
Germany. Father Christian Mayer, a Jesuit astronomer at Mannheim, set
himself, in January 1776, to collect examples of stellar pairs, and
shortly after published the supposed discovery of "satellites" to many
of the principal stars.[30] But his observations were neither exact nor
prolonged enough to lead to useful results in such an inquiry. His
disclosures were derided; his planet-stars treated as results of
hallucination. _On n'a point cru à des choses aussi extraordinaires_,
wrote Lalande[31] within one year of a better-grounded announcement to
the same effect.
Herschel at first shared the general opinion as to the merely optical
connection of double stars. Of this the purpose for which he made his
collection is in itself sufficient evidence, since what may be called
the _differential_ method of parallaxes depends, as we have seen, for
its efficacy upon disparity of distance. It was "much too soon," he
declared in 1782,[32] "to form any theories of small stars revolving
round large ones;" while in the year following,[33] he remarked that the
identical proper motions of the two stars forming, to the naked eye, the
single bright orb of Castor could only be explained as both equally due
to the "systematic parallax" caused by the sun's movement in space.
Plainly showing that the notion of a physical tie, compelling the two
bodies to travel together, had not as yet entered into his speculations.
But he was eminently open to conviction, and had, moreover, by
observations unparalleled in amount as well as in kind, prepared ample
materials for convincing himself and others. In 1802 he was able to
announce the fact of his discovery, and in the two ensuing years, to lay
in detail before the Royal Society proofs, gathered from the labours of
a quarter of a century, of orbital revolution in the case of as many as
fifty double stars, henceforth, he declared, to be held as real binary
combinations, "intimately held together by the bond of mutual
attraction."[34] The fortunate preservation in Dr. Maskelyne's note-book
of a remark made by Bradley about 1759, to the effect that the line
joining the components of Castor was an exact prolongation of that
joining Castor with Pollux, added eighteen years to the time during
which the pair were under scrutiny, and confirmed the evidence of change
afforded by more recent observations. Approximate periods were fixed for
many of the revolving suns--for Castor 342 years; for Gamma Leonis,
1200, Delta Serpentis, 375, Eta Bootis, 1681 years; Eta Lyræ
was noted as a "double-double-star," a change of relative
situation having been detected in each of the two pairs composing the
group; and the occultation was described of one star by another in the
course of their mutual revolutions, as exemplified in 1795 by the
rapidly circulating system of Zeta Herculis.
Thus, by the sagacity and perseverance of a single observer, a firm
basis was at last provided upon which to raise the edifice of sidereal
science. The analogy long presumed to exist between the mighty star of
our system and the bright points of light spangling the firmament was
shown to be no fiction of the imagination, but a physical reality; the
fundamental quality of attractive power was proved to be common to
matter so far as the telescope was capable of exploring, and law,
subordination, and regularity to give testimony of supreme and
intelligent design no less in those limitless regions of space than in
our narrow terrestrial home. The discovery was emphatically (in Arago's
phrase) "one with a future," since it introduced the element of precise
knowledge where more or less probable conjecture had previously held
almost undivided sway; and precise knowledge tends to propagate itself
and advance from point to point.
We have now to speak of Herschel's pioneering work in the skies. To
explore with line and plummet the shining zone of the Milky Way, to
delineate its form, measure its dimensions, and search out the
intricacies of its construction, was the primary task of his life, which
he never lost sight of, and to which all his other investigations were
subordinate. He was absolutely alone in this bold endeavour. Unaided, he
had to devise methods, accumulate materials, and sift out results. Yet
it may safely be asserted that all the knowledge we possess on this
sublime subject was prepared, and the greater part of it anticipated, by
him.
The ingenious method of "star-gauging," and its issue in the delineation
of the sidereal system as an irregular stratum of evenly-scattered suns,
is the best-known part of his work. But it was, in truth, only a first
rude approximation, the principle of which maintained its credit in the
literature of astronomy a full half-century after its abandonment by its
author. This principle was the general equality of star distribution. If
equal portions of space really held equal numbers of stars, it is
obvious that the number of stars visible in any particular direction
would be strictly proportional to the range of the system in that
direction, apparent accumulation being produced by real extent. The
process of "gauging the heavens," accordingly, consisted in counting the
stars in successive telescopic fields, and calculating thence the depths
of space necessary to contain them. The result of 3,400 such operations
was the plan of the Galaxy familiar to every reader of an astronomical
text-book. Widely-varying evidence was, as might have been expected,
derived from an examination of different portions of the sky. Some
fields of view were almost blank, while others (in or near the Milky
Way) blazed with the radiance of many hundred stars compressed into an
area about one-fourth that of the full-moon. In the most crowded parts
116,000 were stated to have been passed in review within a quarter of an
hour. Here the "length of his sounding-line" was estimated by Herschel
at about 497 times the distance of Sirius--in other words, the bounding
orb, or farthest sun of the system in that direction, so far as could be
seen with the 20-foot reflector, was thus inconceivably remote. But
since the distance of Sirius, no less than of every other fixed star,
was as yet an unknown quantity, the dimensions inferred for the Galaxy
were of course purely relative; a knowledge of its form and structure
might (admitting the truth of the fundamental hypothesis) be obtained,
but its real or absolute size remained altogether undetermined.
Even as early as 1785, however, Herschel perceived traces of a tendency
which completely invalidated the supposition of any approach to an
average uniformity of distribution. This was the action of what he
called a "clustering power" in the Milky Way. "Many gathering
clusters"[35] were already discernible to him even while he endeavoured
to obtain a "true _mean_ result" on the assumption that each star in
space was separated from its neighbours as widely as the sun from
Sirius. "It appears," he wrote in 1789, "that the heavens consist of
regions where suns are gathered into separate systems"; and in certain
assemblages he was able to trace "a course or tide of stars setting
towards a centre," denoting, not doubtfully, the presence of attractive
forces.[36] Thirteen years later, he described our sun and his
constellated companions as surrounded by "a magnificent collection of
innumerable stars, called the Milky Way, which must occasion a very
powerful balance of opposite attractions to hold the intermediate stars
at rest. For though our sun, and all the stars we see, may truly be said
to be in the plane of the Milky Way, yet I am now convinced, by a long
inspection and continued examination of it, that the Milky Way itself
consists of stars very differently scattered from those which are
immediately about us." "This immense aggregation," he added, "is by no
means uniform. Its component stars show evident signs of clustering
together into many separate allotments."[37]
The following sentences, written in 1811, contain a definite
retractation of the view frequently attributed to him:--
"I must freely confess," he says, "that by continuing my sweeps of the
heavens my opinion of the arrangement of the stars and their magnitudes,
and of some other particulars, has undergone a gradual change; and
indeed, when the novelty of the subject is considered, we cannot be
surprised that many things formerly taken for granted should on
examination prove to be different from what they were generally but
incautiously supposed to be. For instance, an equal scattering of the
stars may be admitted in certain calculations; but when we examine the
Milky Way, or the closely compressed clusters of stars of which my
catalogues have recorded so many instances, this supposed equality of
scattering must be given up."[38]
Another assumption, the fallacy of which he had not the means of
detecting since become available, was retained by him to the end of his
life. It was that the brightness of a star afforded an approximate
measure of its distance. Upon this principle he founded in 1817 his
method of "limiting apertures,"[39] by which two stars, brought into
view in two precisely similar telescopes, were "equalised" by covering a
certain portion of the object-glass collecting the more brilliant rays.
The distances of the orbs compared were then taken to be in the ratio of
the reduced to the original apertures of the instruments with which they
were examined. If indeed the absolute lustre of each were the same, the
result might be accepted with confidence; but since we have no warrant
for assuming a "standard star" to facilitate our computations, but much
reason to suppose an indefinite range, not only of size but of intrinsic
brilliancy, in the suns of our firmament, conclusions drawn from such a
comparison are entirely worthless.
In another branch of sidereal science besides that of stellar
aggregation, Herschel may justly be styled a pioneer. He was the first
to bestow serious study on the enigmatical objects known as "nebulæ."
The history of the acquaintance of our race with them is comparatively
short. The only one recognised before the invention of the telescope was
that in the girdle of Andromeda, certainly familiar in the middle of the
tenth century to the Persian astronomer Abdurrahman Al-Sûfi; and marked
with dots on Spanish and Dutch constellation-charts of the fourteenth
and fifteenth centuries.[40] Yet so little was it noticed that it might
practically be said--as far as Europe is concerned--to have been
discovered in 1612 by Simon Marius (Mayer of Genzenhausen), who aptly
described its appearance as that of a "candle shining through horn." The
first mention of the great Orion nebula is by a Swiss Jesuit named
Cysatus, who succeeded Father Scheiner in the chair of mathematics at
Ingolstadt. He used it, apparently without any suspicion of its novelty,
as a term of comparison for the comet of December 1618.[41] A novelty,
nevertheless, to astronomers it still remained in 1656, when Huygens
discerned, "as it were, an hiatus in the sky, affording a glimpse of a
more luminous region beyond."[42] Halley in 1716 knew of six nebulæ,
which he believed to be composed of a "lucid medium" diffused through
the ether of space.[43] He appears, however, to have been unacquainted
with some previously noticed by Hevelius. Lacaille brought back with him
from the Cape a list of forty-two--the first-fruits of observation in
Southern skies--arranged in three numerically equal classes;[44] and
Messier (nicknamed by Louis XV. the "ferret of comets"), finding such
objects a source of extreme perplexity in the pursuit of his chosen
game, attempted to eliminate by methodising them, and drew up a
catalogue comprising, in 1781, 103 entries.[45]
These preliminary attempts shrank into insignificance when Herschel
began to "sweep the heavens" with his giant telescopes. In 1786 he
presented to the Royal Society a descriptive catalogue of 1,000 nebulæ
and clusters, followed, three years later, by a second of as many more;
to which he added in 1802 a further gleaning of 500. On the subject of
their nature his views underwent a remarkable change. Finding that his
potent instruments resolved into stars many nebulous patches in which no
signs of such a structure had previously been discernible, he naturally
concluded that "resolvability" was merely a question of distance and
telescopic power. He was (as he said himself) led on by almost
imperceptible degrees from evident clusters, such as the Pleiades, to
spots without a trace of stellar formation, the gradations being so well
connected as to leave no doubt that all these phenomena were equally
stellar. The singular variety of their appearance was thus described by
him:--
"I have seen," he says, "double and treble nebulæ variously arranged;
large ones with small, seeming attendants; narrow, but much extended
lucid nebulæ or bright dashes; some of the shape of a fan, resembling an
electric brush, issuing from a lucid point; others of the cometic shape,
with a seeming nucleus in the centre, or like cloudy stars surrounded
with a nebulous atmosphere; a different sort, again, contain a
nebulosity of the milky kind, like that wonderful, inexplicable
phenomenon about Theta Orionis; while others shine with a fainter,
mottled kind of light, which denotes their being resolvable into
stars."[46]
"These curious objects" he considered to be "no less than whole sidereal
systems,"[47] some of which might "well outvie our Milky Way in
grandeur." He admitted, however, a wide diversity in condition as well
as compass. The system to which our sun belongs he described as "a very
extensive branching congeries of many millions of stars, which probably
owes its origin to many remarkably large as well as pretty closely
scattered small stars, that may have drawn together the rest."[48] But
the continued action of this same "clustering power" would, he supposed,
eventually lead to the breaking-up of the original majestic Galaxy into
two or three hundred separate groups, already visibly gathering. Such
minor nebulæ, due to the "decay" of other "branching nebulæ" similar to
our own, he recognised by the score, lying, as it were, stratified in
certain quarters of the sky. "One of these nebulous beds," he informs
us, "is so rich that in passing through a section of it, in the time of
only thirty-six minutes, I detected no less than thirty-one nebulæ, all
distinctly visible upon a fine blue sky." The stratum of Coma Berenices
he judged to be the nearest to our system of such layers; nor did the
marked aggregation of nebulæ towards both poles of the circle of the
Milky Way escape his notice.
By a continuation of the same process of reasoning, he was enabled (as
he thought) to trace the life-history of nebulæ from a primitive loose
and extended formation, through clusters of gradually increasing
compression, down to the kind named by him "Planetary" because of the
defined and uniform discs which they present. These he regarded as "very
aged, and drawing on towards a period of change or dissolution."[49]
"This method of viewing the heavens," he concluded, "seems to throw them
into a new kind of light. They now are seen to resemble a luxuriant
garden which contains the greatest variety of productions in different
flourishing beds; and one advantage we may at least reap from it is,
that we can, as it were, extend the range of our experience to an
immense duration. For, to continue the simile which I have borrowed from
the vegetable kingdom, is it not almost the same thing whether we live
successively to witness the germination, blooming, foliage, fecundity,
fading, withering, and corruption of a plant, or whether a vast number
of specimens, selected from every stage through which the plant passes
in the course of its existence, be brought at once to our view?"[50]
But already this supposed continuity was broken. After mature
deliberation on the phenomena presented by nebulous stars, Herschel was
induced, in 1791, to modify essentially his original opinion.
"When I pursued these researches," he says, "I was in the situation of a
natural philosopher who follows the various species of animals and
insects from the height of their perfection down to the lowest ebb of
life; when, arriving at the vegetable kingdom, he can scarcely point out
to us the precise boundary where the animal ceases and the plant begins;
and may even go so far as to suspect them not to be essentially
different. But, recollecting himself, he compares, for instance, one of
the human species to a tree, and all doubt upon the subject vanishes
before him. In the same manner we pass through gentle steps from a
coarse cluster of stars, such as the Pleiades ... till we find ourselves
brought to an object such as the nebula in Orion, where we are still
inclined to remain in the once adopted idea of stars exceedingly remote
and inconceivably crowded, as being the occasion of that remarkable
appearance. It seems, therefore, to require a more dissimilar object to
set us right again. A glance like that of the naturalist, who casts his
eye from the perfect animal to the perfect vegetable, is wanting to
remove the veil from the mind of the astronomer. The object I have
mentioned above is the phenomenon that was wanting for this purpose.
View, for instance, the 19th cluster of my 6th class, and afterwards
cast your eye on this cloudy star, and the result will be no less
decisive than that of the naturalist we have alluded to. Our judgment, I
may venture to say, will be, that _the nebulosity about the star is not
of a starry nature_."[51]
The conviction thus arrived at of the existence in space of a widely
diffused "shining fluid" (a conviction long afterwards fully justified
by the spectroscope) led him into a field of endless speculation. What
was its nature? Should it "be compared to the coruscation of the
electric fluid in the aurora borealis? or to the more magnificent cone
of the zodiacal light?" Above all, what was its function in the cosmos?
And on this point he already gave a hint of the direction in which his
mind was moving by the remark that this self-luminous matter seemed
"more fit to produce a star by its condensation, than to depend on the
star for its existence."[52]
This was not a novel idea. Tycho Brahe had tried to explain the blaze of
the star of 1572 as due to a sudden concentration of nebulous material
in the Milky Way, even pointing out the space left dark and void by the
withdrawal of the luminous stuff; and Kepler, theorising on a similar
stellar apparition in 1604, followed nearly in the same track. But under
Herschel's treatment the nebular origin of stars first acquired the
consistency of a formal theory. He meditated upon it long and earnestly,
and in two elaborate treatises, published respectively in 1811 and 1814,
he at length set forth the arguments in its favour. These rested
entirely upon the "principle of continuity." Between the successive
classes of his assortment of developing objects there was, as he said,
"perhaps not so much difference as would be in an annual description of
the human figure, were it given from the birth of a child till he comes
to be a man in his prime."[53] From diffused nebulosity, barely visible
in the most powerful light-gathering instruments, but which he estimated
to cover nearly 152 square degrees of the heavens,[54] to planetary
nebulæ, supposed to be already centrally solid, instances were alleged
of every stage and phase of condensation. The validity of his reasoning,
however, was evidently impaired by his confessed inability to
distinguish between the dim rays of remote clusters and the milky light
of true gaseous nebulæ.
It may be said that such speculations are futile in themselves, and
necessarily barren of results. But they gratify an inherent tendency of
the human mind, and, if pursued in a becoming spirit, should be neither
reproved nor disdained. Herschel's theory still holds the field, the
testimony of recent discoveries with regard to it having proved strongly
confirmatory of its principle, although not of its details. Strangely
enough, it seems to have been propounded in complete independence of
Laplace's nebular hypothesis as to the origin of the solar system.
Indeed, it dated, as we have seen, in its first inception, from 1791,
while the French geometrician's view was not advanced until 1796.
We may now briefly sum up the chief results of Herschel's long years of
"watching the heavens." The apparent motions of the stars had been
disentangled; one portion being clearly shown to be due to a translation
towards a point in the constellation Hercules of the sun and his
attendant planets; while a large balance of displacement was left to be
accounted for by real movements, various in extent and direction, of the
stars themselves. By the action of a central force similar to, if not
identical with, gravity, suns of every degree of size and splendour, and
sometimes brilliantly contrasted in colour, were seen to be held
together in systems, consisting of two, three, four, even six members,
whose revolutions exhibited a wide range of variety both in period and
in orbital form. A new department of physical astronomy was thus
created,[55] and rigid calculation for the first time made possible
within the astral region. The vast problem of the arrangement and
relations of the millions of stars forming the Milky Way was shown to be
capable of experimental treatment, and of at least partial solution,
notwithstanding the variety and complexity seen to prevail, to an extent
previously undreamt of, in the arrangement of that majestic system. The
existence of a luminous fluid, diffused through enormous tracts of
space, and intimately associated with stellar bodies, was virtually
demonstrated, and its place and use in creation attempted to be divined
by a bold but plausible conjecture. Change on a stupendous scale was
inferred or observed to be everywhere in progress. Periodical stars
shone out and again decayed; progressive ebbings or flowings of light
were indicated as probable in many stars under no formal suspicion of
variability; forces were everywhere perceived to be at work, by which
the very structure of the heavens themselves must be slowly but
fundamentally modified. In all directions groups were seen to be formed
or forming; tides and streams of suns to be setting towards powerful
centres of attraction; new systems to be in process of formation, while
effete ones hastened to decay or regeneration when the course appointed
for them by Infinite Wisdom was run. And thus, to quote the words of the
observer who "had looked farther into space than ever human being did
before him,"[56] the state into which the incessant action of the
clustering power has brought the Milky Way at present, is a kind of
chronometer that may be used to measure the time of its past and future
existence; and although we do not know the rate of going of this
mysterious chronometer, it is nevertheless certain that, since the
breaking-up of the parts of the Milky Way affords a proof that it cannot
last for ever, it equally bears witness that its past duration cannot be
admitted to be infinite.[57]
FOOTNOTES:
[Footnote 3: _Phil. Trans._, vol. xxx., p. 737.]
[Footnote 4: Out of eighty stars compared, fifty-seven were found to
have changed their places by more than 10". Lesser discrepancies were at
that time regarded as falling within the limits of observational error.
_Tobiæ Mayeri Op. Inedita_, t. i., pp. 80, 81, and Herschel in _Phil.
Trans._, vol. lxxiii., pp. 275-278.]
[Footnote 5: _Posthumous Works_, p. 701.]
[Footnote 6: Arago in _Annuaire du Bureau des Longitudes_, 1842, p.
313.]
[Footnote 7: Bradley to Halley, _Phil. Trans._, vol. xxxv. (1728), p.
660. His observations were directly applicable to only two stars,
Gamma Draconis and Eta Ursæ Majoris, but some lesser ones
were included in the same result.]
[Footnote 8: Holden, _Sir William Herschel, his Life and Works_, p. 17.]
[Footnote 9: _Phil. Trans._, vol. ci., p. 269.]
[Footnote 10: Caroline Lucretia Herschel, born at Hanover, March 16,
1750, died in the same place, January 9, 1848. She came to England in
1772, and was her brother's devoted assistant, first in his musical
undertakings, and afterwards, down to the end of his life, in his
astronomical labours.]
[Footnote 11: Holden, _op. cit._, p. 39.]
[Footnote 12: _Memoir of Caroline Herschel_, p. 37.]
[Footnote 13: See Holden's _Sir William Herschel_, p. 54.]
[Footnote 14: _An Original Theory or New Hypothesis of the Universe_,
London, 1750. See also De Morgan's summary of his views in
_Philosophical Magazine_, April, 1848.]
[Footnote 15: _Allgemeine Naturgeschichte und Theorie des Himmels_,
1755.]
[Footnote 16: _Cosmologische Briefe_, Augsburg, 1761.]
[Footnote 17: _The System of the World_, p. 125, London, 1800 (a
translation of _Cosmologische Briefe_). Lambert regarded nebulæ as
composed of stars crowded together, but _not_ as external universes. In
the case of the Orion nebula, indeed, he throws out such a conjecture,
but afterwards suggests that it may form a centre for that one of the
subordinate systems composing the Milky Way to which our sun belongs.]
[Footnote 18: _Opera Inedita_, t. i., p. 79.]
[Footnote 19: _Phil. Trans._, vol. lxxiii. (1783), p. 273. Pierre
Prévost's similar investigation, communicated to the Berlin Academy of
Sciences four months later, July 3, 1783, was inserted in the _Memoirs_
of that body for 1781, and thus _seems_ to claim a priority not its due.
Georg Simon Klügel at Halle gave about the same time an analytical
demonstration of Herschel's result. Wolf, _Gesch. der Astronomie_, p.
733.]
[Footnote 20: _Phil. Trans._, vol. xcv., p. 233.]
[Footnote 21: _Ibid._, vol. xcvi., p. 205.]
[Footnote 22: "Ingens bolus devorandus est," Kepler admitted to Herwart
in May, 1603.]
[Footnote 23: Described in "Præfatio Editoris" to _De Revolutionibus_,
p. xix. (ed. 1854).]
[Footnote 24: _Opere_, t. i., p. 415.]
[Footnote 25: _Phil. Trans._, vol. xvii., p. 848.]
[Footnote 26: _Ibid._, vol. lxxii., p. 97.]
[Footnote 27: Doberck, _Observatory_, vol. ii., p. 110.]
[Footnote 28: _Phil. Trans._, vol. lvii., p. 249.]
[Footnote 29: _Ibid._, vol. lxxiv., p. 56.]
[Footnote 30: _Beobachtungen von Fixsterntrabanten_, 1778; and _De Novis
in Coelo Sidereo Phænomenis_, 1779.]
[Footnote 31: _Bibliographie_, p. 569.]
[Footnote 32: _Phil. Trans._, vol. lxxii., p. 162.]
[Footnote 33: _Ibid._, vol. lxxiii., p. 272.]
[Footnote 34: _Ibid._, vol. xciii., p. 340.]
[Footnote 35: _Phil. Trans._, vol. lxxv., p. 255.]
[Footnote 36: _Ibid._, vol. lxxix., pp. 214, 222.]
[Footnote 37: _Ibid._, vol. xcii., pp. 479, 495.]
[Footnote 38: _Phil. Trans._, vol. ci., p. 269.]
[Footnote 39: _Ibid._, vol. cvii., p. 311.]
[Footnote 40: Bullialdus, _De Nebulosâ Stellâ in Cingulo Andromedæ_
(1667); see also G. P. Bond, _Mém. Am. Ac._, vol. iii., p. 75, Holden's
Monograph on the Orion Nebula, _Washington Observations_, vol. xxv.,
1878 (pub. 1882), and Lady Huggins's drawing, _Atlas of Spectra_, p.
119.]
[Footnote 41: _Mathemata Astronomica_, p. 75.]
[Footnote 42: _Systema Saturnium_, p. 9.]
[Footnote 43: _Phil. Trans._, vol. xxix., p. 390.]
[Footnote 44: _Mém. Ac. des Sciences_, 1755.]
[Footnote 45: _Conn. des Temps_, 1784 (pub. 1781), p. 227. A previous
list of forty-five had appeared in _Mém. Ac. des Sciences_, 1771.]
[Footnote 46: _Phil. Trans._, vol. lxxiv., p. 442.]
[Footnote 47: _Ibid._, vol. lxxix., p. 213.]
[Footnote 48: _Ibid._, vol. lxxv., p. 254.]
[Footnote 49: _Ibid._, vol. lxxix., p. 225.]
[Footnote 50: _Phil. Trans._, vol. lxxix., p. 226.]
[Footnote 51: _Ibid._, vol. lxxxi., p. 72.]
[Footnote 52: _Ibid._, p. 85.]
[Footnote 53: _Phil. Trans._, vol. ci., p. 271.]
[Footnote 54: _Ibid._, p. 277.]
[Footnote 55: J. Herschel, _Phil. Trans._, vol. cxvi., part iii., p. 1.]
[Footnote 56: His own words to the poet Campbell cited by Holden, _Life
and Works_, p. 109.]
[Footnote 57: _Phil. Trans._, vol. civ., p. 283.]
CHAPTER II
_PROGRESS OF SIDEREAL ASTRONOMY_
We have now to consider labours of a totally different character from
those of Sir William Herschel. Exploration and discovery do not
constitute the whole business of astronomy; the less adventurous, though
not less arduous, task of gaining a more and more complete mastery over
the problems immemorially presented to her, may, on the contrary, be
said to form her primary duty. A knowledge of the movements of the
heavenly bodies has, from the earliest times, been demanded by the
urgent needs of mankind; and science finds its advantage, as in many
cases it has taken its origin, in condescension to practical claims.
Indeed, to bring such knowledge as near as possible to absolute
precision has been defined by no mean authority[58] as the true end of
astronomy.
Several causes concurred about the beginning of the last century to give
a fresh and powerful impulse to investigations having this end in view.
The rapid progress of theory almost compelled a corresponding advance in
observation; instrumental improvements rendered such an advance
possible; Herschel's discoveries quickened public interest in celestial
inquiries; royal, imperial, and grand-ducal patronage widened the scope
of individual effort. The heart of the new movement was in Germany.
Hitherto the observatory of Flamsteed and Bradley had been the
acknowledged centre of practical astronomy; Greenwich observations were
the standard of reference all over Europe; and the art of observing
prospered in direct proportion to the fidelity with which Greenwich
methods were imitated. Dr. Maskelyne, who held the post of Astronomer
Royal during forty-six years (from 1765 to 1811), was no unworthy
successor to the eminent men who had gone before him. His foundation of
the _Nautical Almanac_ (in 1767) alone constitutes a valid title to
fame; he introduced at the Observatory the important innovation of the
systematic publication of results; and the careful and prolonged series
of observations executed by him formed the basis of the improved
theories, and corrected tables of the celestial movements, which were
rapidly being brought to completion abroad. His catalogue of thirty-six
"fundamental" stars was besides excellent in its way, and most
serviceable. Yet he was devoid of Bradley's instinct for divining the
needs of the future. He was fitted rather to continue a tradition than
to found a school. The old ways were dear to him; and, indefatigable as
he was, a definite purpose was wanting to compel him, by its exigencies,
along the path of progress. Thus, for almost fifty years after Bradley's
death, the acquisition of a small achromatic[59] was the only notable
change made in the instrumental equipment of the Observatory. The
transit, the zenith sector, and the mural quadrant, with which Bradley
had done his incomparable work, retained their places long after they
had become deteriorated by time and obsolete by the progress of
invention; and it was not until the very close of his career that
Maskelyne, compelled by Pond's detection of serious errors, ordered a
Troughton's circle, which he did not live to employ.
Meanwhile, the heavy national disasters with which Germany was
overwhelmed in the early part of the nineteenth century seemed to
stimulate rather than impede the intellectual revival already for some
years in progress there. Astronomy was amongst the first of the sciences
to feel the new impulse. By the efforts of Bode, Olbers, Schröter, and
Von Zach, just and elevated ideas on the subject were propagated,
intelligence was diffused, and a firm ground prepared for common action
in mutual sympathy and disinterested zeal. They received powerful aid
through the foundation, in 1804, by a young artillery officer named Von
Reichenbach, of an Optical and Mechanical Institute at Munich. Here the
work of English instrumental artists was for the first time rivalled,
and that of English opticians--when Fraunhofer entered the new
establishment--far surpassed. The development given to the refracting
telescope by this extraordinary man was indispensable to the progress of
that fundamental part of astronomy which consists in the exact
determination of the places of the heavenly bodies. Reflectors are
brilliant engines of discovery, but they lend themselves with difficulty
to the prosaic work of measuring right ascensions and polar distances. A
signal improvement in the art of making and working flint-glass thus
most opportunely coincided with the rise of a German school of
scientific mechanicians, to furnish the instrumental means needed for
the reform which was at hand. Of the leader of that reform it is now
time to speak.
Friedrich Wilhelm Bessel was born at Minden, in Westphalia, July 22,
1784. A certain taste for figures, coupled with a still stronger
distaste for the Latin accidence, directed his inclination and his
father's choice towards a mercantile career. In his fifteenth year,
accordingly, he entered the house of Kuhlenkamp and Sons, in Bremen, as
an apprenticed clerk. He was now thrown completely upon his own
resources. From his father, a struggling Government official, heavily
weighted with a large family, he was well aware that he had nothing to
expect; his dormant faculties were roused by the necessity for
self-dependence, and he set himself to push manfully forward along the
path that lay before him. The post of supercargo on one of the trading
expeditions sent out from the Hanseatic towns to China and the East
Indies was the aim of his boyish ambition, for the attainment of which
he sought to qualify himself by the industrious acquisition of suitable
and useful knowledge. He learned English in two or three months; picked
up Spanish with the casual aid of a gunsmith's apprentice; studied the
geography of the distant lands which he hoped to visit; collected
information as to their climates, inhabitants, products, and the courses
of trade. He desired to add some acquaintance with the art (then much
neglected) of taking observations at sea; and thus, led on from
navigation to astronomy, and from astronomy to mathematics, he groped
his way into a new world.
It was characteristic of him that the practical problems of science
should have attracted him before his mind was as yet sufficiently
matured to feel the charm of its abstract beauties. His first attempt at
observation was made with a sextant, rudely constructed under his own
directions, and a common clock. Its object was the determination of the
longitude of Bremen, and its success, he tells us himself,[60] filled
him with a rapture of delight, which, by confirming his tastes, decided
his destiny. He now eagerly studied Bode's _Jahrbuch_ and Von Zach's
_Monatliche Correspondenz_, overcoming each difficulty as it arose with
the aid of Lalande's _Traité d'Astronomie_, and supplying, with amazing
rapidity, his early deficiency in mathematical training. In two years he
was able to attack a problem which would have tasked the patience, if
not the skill, of the most experienced astronomer. Amongst the Earl of
Egremont's papers Von Zach had discovered Harriot's observations on
Halley's comet at its appearance in 1607, and published them as a
supplement to Bode's Annual. With an elaborate care inspired by his
youthful ardour, though hardly merited by their loose nature, Bessel
deduced from them an orbit for that celebrated body, and presented the
work to Olbers, whose reputation in cometary researches gave a special
fitness to the proffered homage. The benevolent physician-astronomer of
Bremen welcomed with surprised delight such a performance emanating from
such a source. Fifteen years previously, the French Academy had crowned
a similar work; now its equal was produced by a youth of twenty, busily
engaged in commercial pursuits, self-taught, and obliged to snatch from
sleep the hours devoted to study. The paper was immediately sent to Von
Zach for publication, with a note from Olbers explaining the
circumstances of its author; and the name of Bessel became the common
property of learned Europe.
He had, however, as yet no intention of adopting astronomy as his
profession. For two years he continued to work in the counting-house by
day, and to pore over the _Mécanique Céleste_ and the Differential
Calculus by night. But the post of assistant in Schröter's observatory
at Lilienthal having become vacant by the removal of Harding to
Göttingen in 1805, Olbers procured for him the offer of it. It was not
without a struggle that he resolved to exchange the desk for the
telescope. His reputation with his employers was of the highest; he had
thoroughly mastered the details of the business, which his keen
practical intelligence followed with lively interest; his years of
apprenticeship were on the point of expiring, and an immediate, and not
unwelcome prospect of comparative affluence lay before him. The love of
science, however, prevailed; he chose poverty and the stars, and went to
Lilienthal with a salary of a hundred thalers yearly. Looking back over
his life's work, Olbers long afterwards declared that the greatest
service which he had rendered to astronomy was that of having discerned,
directed, and promoted the genius of Bessel.[61]
For four years he continued in Schröter's employment. At the end of that
time the Prussian Government chose him to superintend the erection of a
new observatory at Königsberg, which after many vexatious delays, caused
by the prostrate condition of the country, was finished towards the end
of 1813. Königsberg was the first really efficient German observatory.
It became, moreover, a centre of improvement, not for Germany alone, but
for the whole astronomical world. During two-and-thirty years it was the
scene of Bessel's labours, and Bessel's labours had for their aim the
reconstruction, on an amended and uniform plan, of the entire science of
observation.
A knowledge of the places of the stars is the foundation of
astronomy.[62] Their configuration lends to the skies their distinctive
features, and marks out the shifting tracks of more mobile objects with
relatively fixed, and generally unvarying points of light. A more
detailed and accurate acquaintance with the stellar multitude, regarded
from a purely uranographical point of view, has accordingly formed at
all times a primary object of celestial science, and was, during the
last century, cultivated with a zeal and success by which all previous
efforts were dwarfed into insignificance. In Lalande's _Histoire
Céleste_, published in 1801, the places of no less than 47,390 stars
were given, but in the rough, as it were, and consequently needing
laborious processes of calculation to render them available for exact
purposes. Piazzi set an example of improved methods of observation,
resulting in the publication, in 1803 and 1814, of two catalogues of
about 7,600 stars--the second being a revision and enlargement of the
first--which for their time were models of what such works should
be.[63] Stephen Groombridge at Blackheath was similarly and most
beneficially active. But something more was needed than the diligence of
individual observers. A systematic reform was called for; and it was
this which Bessel undertook and carried through.
Direct observation furnishes only what has been called the "raw
material" of the positions of the heavenly bodies.[64] A number of
highly complex corrections have to be applied before their _mean_ can be
disengaged from their _apparent_ places on the sphere. Of these, the
most considerable and familiar is atmospheric refraction, by which
objects seem to stand higher in the sky than they in reality do, the
effect being evanescent at the zenith, and attaining, by gradations
varying with conditions of pressure and temperature, a maximum at the
horizon. Moreover, the points to which measurements are referred are
themselves in motion, either continually in one direction, or
periodically to and fro. The _precession_ of the equinoxes is slowly
progressive, or rather retrogressive; the _nutation_ of the pole
oscillatory in a period of about eighteen years. Added to which, the
non-instantaneous transmission of light, combined with the movement of
the earth in its orbit, causes a small annual displacement known as
_aberration_.
Now it is easy to see that any uncertainty in the application of these
corrections saps the very foundations of exact astronomy. Extremely
minute quantities, it is true, are concerned; but the life and progress
of modern celestial science depends upon the sure recognition of
extremely minute quantities. In the early years of the nineteenth
century, however, no uniform system of "reduction" (so the complete
correction of observational results is termed) had been established.
Much was left to the individual caprice of observers, who selected for
the several "elements" of reduction such values as seemed best to
themselves. Hence arose much hurtful confusion, tending to hinder united
action and mar the usefulness of laborious researches. For this state of
things, Bessel, by the exercise of consummate diligence, sagacity, and
patience, provided an entirely satisfactory remedy.
His first step was an elaborate investigation of the precious series of
observations made by Bradley at Greenwich from 1750 until his death in
1762. The catalogue of 3,222 stars which he extracted from them gave the
earliest example of the systematic reduction on a uniform plan of such a
body of work. It is difficult, without entering into details out of
place in a volume like the present, to convey an idea of the arduous
nature of this task. It involved the formation of a theory of the errors
of each of Bradley's instruments, and a difficult and delicate inquiry
into the true value of each correction to be applied, before the entries
in the Greenwich journals could be developed into a finished and
authentic catalogue. Although completed in 1813, it was not until five
years later that the results appeared with the proud, but not
inappropriate title of _Fundamenta Astronomiæ_. The eminent value of the
work consisted in this, that by providing a mass of entirely reliable
information as to the state of the heavens at the epoch 1755, it threw
back the beginning of _exact_ astronomy almost half a century. By
comparison with Piazzi's catalogues the amount of precession was more
accurately determined, the proper motions of a considerable number of
stars became known with certainty, and definite prediction--the
certificate of initiation into the secrets of Nature--at last became
possible as regards the places of the stars. Bessel's final improvements
in the methods of reduction were published in 1830 in his _Tabulæ
Regiomontanæ_. They not only constituted an advance in accuracy, but
afforded a vast increase of facility in application, and were at once
and everywhere adopted. Thus astronomy became a truly universal science;
uncertainties and disparities were banished, and observations made at
all times and places rendered mutually comparable.[65]
More, however, yet remained to be done. In order to verify with greater
strictness the results drawn from the Bradley and Piazzi catalogues, a
third term of comparison was wanted, and this Bessel undertook to
supply. By a course of 75,011 observations, executed during the years
1821-33, with the utmost nicety of care, the number of accurately known
stars was brought up to above 50,000, and an ample store of trustworthy
facts laid up for the use of future astronomers. In this department
Argelander, whom he attracted from finance to astronomy, and trained in
his own methods, was his assistant and successor. The great "Bonn
Durchmusterung,"[66] in which 324,198 stars visible in the northern
hemisphere are enumerated, and the corresponding "Atlas" published in
1857-63, constituting a picture of our sidereal surroundings of
heretofore unapproached completeness, may be justly said to owe their
origin to Bessel's initiative, and to form a sequel to what he
commenced.
But his activity was not solely occupied with the promotion of a
comprehensive reform in astronomy; it embraced special problems as well.
The long-baffled search for a parallax of the fixed stars was resumed
with fresh zeal as each mechanical or optical improvement held out fresh
hopes of a successful issue. Illusory results abounded. Piazza in 1805
perceived, as he supposed, considerable annual displacements in Vega,
Aldebaran, Sirius, and Procyon; the truth being that his instruments
were worn out with constant use, and could no longer be depended
upon.[67] His countryman, Calandrelli, was similarly deluded. The
celebrated controversy between the Astronomer Royal and Dr. Brinkley,
Director of the Dublin College Observatory, turned on the same subject.
Brinkley, who was in possession of a first-rate meridian-circle,
believed himself to have discovered relatively large parallaxes for four
of the brightest stars; Pond, relying on the testimony of the Greenwich
instruments, asserted their nullity. The dispute, protracted for
fourteen years, from 1810 until 1824, was brought to no definite
conclusion; but the strong presumption on the negative side was
abundantly justified in the event.
There was good reason for incredulity in the matter of parallaxes.
Announcements of their detection had become so frequent as to be
discredited before they were disproved; and Struve, who investigated the
subject at Dorpat in 1818-21, had clearly shown that the quantities
concerned were too small to come within the reliable measuring powers of
any instrument then in use. Already, however, the means were being
prepared of giving to those powers a large increase.
On the 21st July, 1801, two old houses in an alley of Munich tumbled
down, burying in their ruins the occupants, of whom one alone was
extricated alive, though seriously injured. This was an orphan lad of
fourteen named Joseph Fraunhofer. The Elector Maximilian Joseph was
witness of the scene, became interested in the survivor, and consoled
his misfortune with a present of eighteen ducats. Seldom was money
better bestowed. Part of it went to buy books and a glass-polishing
machine, with the help of which young Fraunhofer studied mathematics and
optics, and secretly exercised himself in the shaping and finishing of
lenses; the remainder purchased his release from the tyranny of one
Weichselberger, a looking-glass maker by trade, to whom he had been
bound apprentice on the death of his parents. A period of struggle and
privation followed, during which, however, he rapidly extended his
acquirements; and was thus eminently fitted for the task awaiting him,
when, in 1806, he entered the optical department of the establishment
founded two years previously by Von Reichenbach and Utzschneider. He now
zealously devoted himself to the improvement of the achromatic
telescope; and, after a prolonged study of the theory of lenses, and
many toilsome experiments in the manufacture of flint-glass, he
succeeded in perfecting, December 12, 1817, an object-glass of exquisite
quality and finish, 9-1/2 inches in diameter, and of 14 feet focal
length.
This (as it was then considered) gigantic lens was secured by Struve for
the Russian Government, and the "great Dorpat refractor"--the first of
the large achromatics which have played such an important part in modern
astronomy--was, late in 1824, set up in the place which it still
occupies. By ingenious improvements in mounting and fitting, it was
adapted to the finest micrometrical work, and thus offered unprecedented
facilities both for the examination of double stars (in which Struve
chiefly employed it), and for such subtle measurements as might serve to
reveal or disprove the existence of a sensible stellar parallax.
Fraunhofer, moreover, constructed for the observatory at Königsberg the
first really available heliometer. The principle of this instrument
(termed with more propriety a "divided object-glass micrometer") is the
separation, by a strictly measurable amount, of two distinct images of
the same object. If a double star, for instance, be under examination,
the two half-lenses into which the object-glass is divided are shifted
until the upper star (say) in one image is brought into coincidence with
the lower star in the other, when their distance apart becomes known by
the amount of motion employed.[68]
This virtually new engine of research was delivered and mounted in 1829,
three years after the termination of the life of its deviser. The Dorpat
lens had brought to Fraunhofer a title of nobility and the sole
management of the Munich Optical Institute (completely separated since
1814 from the mechanical department). What he had achieved, however, was
but a small part of what he meant to achieve. He saw before him the
possibility of nearly quadrupling the light-gathering capacity of the
great achromatic acquired by Struve; he meditated improvements in
reflectors as important as those he had already effected in refractors;
and was besides eagerly occupied with investigations into the nature of
light, the momentous character of which we shall by-and-by have an
opportunity of estimating. But his health was impaired, it is said, from
the weakening effects of his early accident, combined with excessive and
unwholesome toil, and, still hoping for its restoration from a projected
journey to Italy, he died of consumption, June 7, 1826, aged thirty-nine
years. His tomb in Munich bears the concise eulogy, _Approximavit
sidera_.
Bessel had no sooner made himself acquainted with the exquisite defining
powers of the Königsberg heliometer, than he resolved to employ them in
an attack upon the now secular problem of star-distances. But it was not
until 1837 that he found leisure to pursue the inquiry. In choosing his
test-star he adopted a new principle. It had hitherto been assumed that
our nearest neighbours in space must be found among the brightest
ornaments of our skies. The knowledge of stellar proper motions afforded
by the critical comparison of recent with earlier star-places, suggested
a different criterion of distance. It is impossible to escape from the
conclusion that the apparently swiftest-moving stars are, _on the
whole_, also the nearest to us, however numerous the individual
exceptions to the rule. Now, as early as 1792,[69] Piazzi had noted as
an indication of relative vicinity to the earth, the unusually large
proper motion (5·2" annually) of a double star of the fifth magnitude in
the constellation of the Swan. Still more emphatically in 1812[70]
Bessel drew the attention of astronomers to the fact, and 61 Cygni
became known as the "flying star." The _seeming_ rate of its flight,
indeed, is of so leisurely a kind, that in a thousand years it will have
shifted its place by less than 3-1/2 lunar diameters, and that a quarter
of a million would be required to carry it round the entire circuit of
the visible heavens. Nevertheless, it has few rivals in rapidity of
movement, the apparent displacement of the vast majority of stars being,
by comparison, almost insensible.
This interesting, though inconspicuous object, then, was chosen by
Bessel to be put to the question with his heliometer, while Struve made
a similar and somewhat earlier trial with the bright gem of the Lyre,
whose Arabic title of the "Falling Eagle" survives as a time-worn
remnant in "Vega." Both astronomers agreed to use the "differential"
method, for which their instruments and the vicinity to their selected
stars of minute, physically detached companions offered special
facilities. In the last month of 1838 Bessel made known the result of
one year's observations, showing for 61 Cygni a parallax of about a
third of a second (0·3136").[71] He then had his heliometer taken down
and repaired, after which he resumed the inquiry, and finally terminated
a series of 402 measures in March 1840.[72] The resulting parallax of
0·3483" (corresponding to a distance about 600,000 times that of the
earth from the sun), seemed to be ascertained beyond the possibility of
cavil, and is memorable as the first _published_ instance of the
fathom-line, so industriously thrown into celestial space, having really
and indubitably _touched bottom_. It was confirmed in 1842-43 with
curious exactness by C. A. F. Peters at Pulkowa; but later researches
showed that it required increase to nearly half a second.[73]
Struve's measurements inspired less confidence. They extended over three
years (1835-38), but were comparatively few, and were frequently
interrupted. The parallax, accordingly, of about a quarter of a second
(0·2613") which he derived from them for Alpha Lyræ, and announced
in 1840,[74] has proved considerably too large.[75]
Meanwhile a result of the same kind, but of a more striking character
than either Bessel's or Struve's, had been obtained, one might almost
say casually, by a different method and in a distant region. Thomas
Henderson, originally an attorney's clerk in his native town of Dundee,
had become known for his astronomical attainments, and was appointed in
1831 to direct the recently completed observatory at the Cape of Good
Hope. He began observing in April, 1832, and, the serious shortcomings
of his instrument notwithstanding, executed during the thirteen months
of his tenure of office a surprising amount of first-rate work. With a
view to correcting the declination of the lustrous double star Alpha
Centauri (which ranks after Sirius and Canopus as the third brightest
orb in the heavens), he effected a number of successive determinations
of its position, and on being informed of its very considerable proper
motion (3·6" annually), he resolved to examine the observations already
made for possible traces of parallactic displacement. This was done on
his return to Scotland, where he filled the office of Astronomer Royal
from 1834 until his premature death in 1844. The result justified his
expectations. From the declination measurements made at the Cape and
duly reduced, a parallax of about one second of arc clearly emerged
(diminished by Gill's and Elkin's observations, 1882-1883, to O·75");
but, by perhaps an excess of caution, was withheld from publication
until fuller certainty was afforded by the concurrent testimony of
Lieutenant Meadows's determinations of the same star's right
ascension.[76] When at last, January 9, 1839, Henderson communicated his
discovery to the Astronomical Society, he could no longer claim the
priority which was his due. Bessel had anticipated him with the parallax
of 61 Cygni by just two months.
Thus from three different quarters, three successful and almost
simultaneous assaults were delivered upon a long-beleaguered citadel of
celestial secrets. The same work has since been steadily pursued, with
the general result of showing that, as regards their overwhelming
majority, the stars are far too remote to show even the slightest trace
of optical shifting from the revolution of the earth in its orbit. In
nearly a hundred cases, however, small parallaxes have been determined,
some certainly (that is, within moderate limits of error), others more
or less precariously. The list is an instructive one, in its omissions
no less than in its contents. It includes stars of many degrees of
brightness, from Sirius down to a nameless telescopic star in the Great
Bear;[77] yet the vicinity to the earth of this minute object is so much
greater than that of the brilliant Vega, that the latter transported to
its place would increase in lustre thirty-eight times. Moreover, many of
the brightest stars are found to have no sensible parallax, while the
majority of those ascertained to be nearest to the earth are of fifth,
sixth, even ninth magnitudes. The obvious conclusions follow that the
range of variety in the sidereal system is enormously greater than had
been supposed, and that estimates of distance based upon apparent
magnitude must be wholly futile. Thus, the splendid Canopus, Betelgeux,
and Rigel can be inferred, from their indefinite remoteness, to exceed
our sun thousands of times in size and lustre; while many inconspicuous
objects, which prove to be in our relative vicinity, must be notably his
inferiors. The limits of real stellar magnitude are then set very widely
apart. At the same time, the so-called "optical" and "geometrical"
methods of relatively estimating star-distances are both seen to have a
foundation of fact, although so disguised by complicated relations as to
be of very doubtful individual application. On the whole, the chances
are in favour of the superior vicinity of a bright star over a faint
one; and, on the whole, the stars in swiftest _apparent_ motion are
amongst those whose _actual_ remoteness is least. Indeed, there is no
escape from either conclusion, unless on the supposition of special
arrangements in themselves highly improbable, and, we may confidently
say, non-existent.
The distances even of the few stars found to have measurable parallaxes
are on a scale entirely beyond the powers of the human mind to conceive.
In the attempt both to realize them distinctly, and to express them
conveniently, a new unit of length, itself of bewildering magnitude, has
originated. This is what we may call the _light-journey_ of one year.
The subtle vibrations of the ether, propagated on all sides from the
surface of luminous bodies, travel at the rate of 186,300 miles a
second, or (in round numbers) six billions of miles a year. Four and a
third such measures are needed to span the abyss that separates us from
the nearest fixed star. In other words, light takes four years and four
months to reach the earth from Alpha Centauri; yet Alpha Centauri lies
some ten billions of miles nearer to us (so far as is yet known) than
any other member of the sidereal system!
The determination of parallax leads, in the case of stars revolving in
known orbits, to the determination of mass; for the distance from the
earth of the two bodies forming a binary system being ascertained, the
seconds of arc apparently separating them from each other can be
translated into millions of miles; and we only need to add a knowledge
of their period to enable us, by an easy sum in proportion, to find
their combined mass in terms of that of the sun. Thus, since--according
to Dr. Doberck's elements--the components of Alpha Centauri revolve
round their common centre of gravity at a mean distance nearly 25 times
the radius of the earth's orbit, in a period of 88 years, the attractive
force of the two together must be just twice the solar. We may gather
some idea of their relations by placing in imagination a second luminary
like our sun in circulation between the orbits of Neptune and Uranus.
But systems of still more majestic proportions are reduced by extreme
remoteness to apparent insignificance. A double star of the fourth
magnitude in Cassiopeia (Eta), to which a small parallax is ascribed on
the authority of O. Struve, appears to be above eight times as massive
as the central orb of our world; while a much less conspicuous pair--85
Pegasi--exerts, if the available data can be depended upon, no less than
thirteen times the solar gravitating power.
Further, the actual rate of proper motions, so far as regards that part
of them which is projected upon the sphere, can be ascertained for stars
at known distance. The annual journey, for instance, of 61 Cygni _across
the line of sight_ amounts to 1,000, and that of Alpha Centauri to
446 millions of miles. A small star, numbered 1,830 in Groombridge's
Circumpolar Catalogue, "devours the way" at the rate of at least 150
miles a second--a speed, in Newcomb's opinion, beyond the gravitating
power of the entire sidereal system to control; and Mu Cassiopeiæ
possesses above two-thirds of that surprising velocity; while for both
objects, radial movements of just sixty miles a second were disclosed
by Professor Campbell's spectroscopic measurements.
Herschel's conclusion as to the advance of the sun among the stars was
not admitted as valid by the most eminent of his successors. Bessel
maintained that there was absolutely no preponderating evidence in
favour of its supposed direction towards a point in the constellation
Hercules.[78] Biot, Burckhardt, even Herschel's own son, shared his
incredulity. But the appearance of Argelander's prize-essay in 1837[79]
changed the aspect of the question. Herschel's first memorable solution
in 1783 was based upon the motions of thirteen stars, imperfectly known;
his second, in 1805, upon those of no more than six. Argelander now
obtained an entirely concordant result from the large number of 390,
determined with the scrupulous accuracy characteristic of Bessel's work
and his own. The reality of the fact thus persistently disclosed could
no longer be doubted; it was confirmed five years later by the younger
Struve, and still more strikingly in 1847[80] by Galloway's
investigations, founded exclusively on the apparent displacements of
southern stars. In 1859 and 1863, Sir George Airy and Mr. Dunkin
(1821-1898),[81] employing all the resources of modern science, and
commanding the wealth of material furnished by 1,167 proper motions
carefully determined by Mr. Main, reached conclusions closely similar to
that indicated nearly eighty years previously by the first great
sidereal astronomer; which Mr. Plummer's reinvestigation of the subject
in 1883[82] served but slightly to modify. Yet astronomers were not
satisfied. Dr. Auwers of Berlin completed in 1866 a splendid piece of
work, for which he received in 1888 the Gold Medal of the Royal
Astronomical Society. It consisted in reducing afresh, with the aid of
the most refined modern data, Bradley's original stars, and comparing
their places thus obtained for the year 1755 with those assigned to them
from observations made at Greenwich after the lapse of ninety years. In
the interval, as was to be anticipated, most of them were found to have
travelled over some small span of the heavens, and there resulted a
stock of nearly three thousand highly authentic proper motions. These
ample materials were turned to account by M. Ludwig Struve[83] for a
discussion of the sun's motion, of which the upshot was to shift its
point of aim to the bordering region of the constellations Hercules and
Lyra. And the more easterly position of the solar apex was fully
confirmed by the experiments, with variously assorted lists of stars, of
Lewis Boss of Albany,[84] and Oscar Stumpe of Bonn.[85] Fresh
precautions of refinement were introduced into the treatment of the
subject by Ristenpart of Karlsruhe,[86] by Kapteyn of Groningen,[87] by
Newcomb[88] and Porter[89] in America, who ably availed themselves of
the copious materials accumulated before the close of the century. Their
results, although not more closely accordant than those of their
predecessors, combined to show that the journey of our system is
directed towards a point within a circle about ten degrees in radius,
having the brilliant Vega for its centre. To determine its rate was a
still more arduous problem. It involved the assumption, very much at
discretion, of an average parallax for the stars investigated; and Otto
Struve's estimate of 154 million miles as the span yearly traversed was
hence wholly unreliable. Fortunately, however, as will be seen further
on, a method of determining the sun's velocity independently of any
knowledge of star-distances, has now become available.
As might have been expected, speculation has not been idle regarding the
purpose and goal of the strange voyage of discovery through space upon
which our system is embarked; but altogether fruitlessly. The variety of
the conjectures hazarded in the matter is in itself a measure of their
futility. Long ago, before the construction of the heavens had as yet
been made the subject of methodical inquiry, Kant was disposed to regard
Sirius as the "central sun" of the Milky Way; while Lambert surmised
that the vast Orion nebula might serve as the regulating power of a
subordinate group including our sun. Herschel threw out the hint that
the great cluster in Hercules might prove to be the supreme seat of
attractive force;[90] Argelander placed his central body in the
constellation Perseus;[91] Fomalhaut, the brilliant of the Southern
Fish, was set in the post of honour by Boguslawski of Breslau. Mädler
(who succeeded Struve at Dorpat in 1839) concluded from a more formal
inquiry that the ruling power in the sidereal system resided, not in any
single prepondering mass, but in the centre of gravity of the
self-controlled revolving multitude.[92] In the former case (as we know
from the example of the planetary scheme), the stellar motions would be
most rapid near the centre; in the latter, they would become accelerated
with remoteness from it.[93] Mädler showed that no part of the heavens
could be indicated as a region of exceptionally swift movements, such as
would result from the presence of a gigantic (though possibly obscure)
ruling body; but that a community of extremely sluggish movements
undoubtedly existed in and near the group of the Pleiades, where,
accordingly, he placed the centre of gravity of the Milky Way.[94] The
bright star Alcyone thus became the "central sun," but in a purely
passive sense, its headship being determined by its situation at the
point of neutralisation of opposing tendencies, and of consequent rest.
By an avowedly conjectural method, the solar period of revolution round
this point was fixed at 18,200,000 years.
The scheme of sidereal government framed by the Dorpat astronomer was,
it may be observed, of the most approved constitutional type;
deprivation, rather than increase of influence accompanying the office
of chief dignitary. But while we are still ignorant, and shall perhaps
ever remain so, of the fundamental plan upon which the Galaxy is
organised, recent investigations tend more and more to exhibit it, not
as monarchical (so to speak), but as federative. The community of proper
motions detected by Mädler in the vicinity of the Pleiades may
accordingly possess a significance altogether different from what he
imagined.
Bessel's so-called "foundation of an Astronomy of the Invisible" now
claims attention.[95] His prediction regarding the planet Neptune does
not belong to the present division of our subject; a strictly analogous
discovery in the sidereal system was, however, also very clearly
foreshadowed by him. His earliest suspicions of non-uniformity in the
proper motion of Sirius dated from 1834; they extended to Procyon in
1840; and after a series of refined measurements with the new Repsold
circle, he announced in 1844 his conclusion that these irregularities
were due to the presence of obscure bodies round which the two bright
Dog-stars revolved as they pursued their way across the sphere.[96] He
even assigned to each an approximate period of half a century. "I adhere
to the conviction," he wrote later to Humboldt, "that Procyon and Sirius
form real binary systems, consisting of a visible and an invisible star.
There is no reason to suppose luminosity an essential quality of
cosmical bodies. The visibility of countless stars is no argument
against the invisibility of countless others."[97]
An inference so contradictory to received ideas obtained little credit,
until Peters found, in 1851,[98] that the apparent anomalies in the
movements of Sirius could be completely explained by an orbital
revolution in a period of fifty years. Bessel's prevision was destined
to be still more triumphantly vindicated. On the 31st of January, 1862,
while in the act of trying a new 18-inch refractor, Mr. Alvan G. Clark
(one of the celebrated firm of American opticians) actually discovered
the hypothetical Sirian companion in the precise position required by
theory. It has now been watched through nearly an entire revolution
(period 49·4 years), and proves to be very slightly luminous in
proportion to its mass. Its attractive power, in fact, is nearly half
that of its primary, while it emits only 1/10000th of its light. Sirius
itself, on the other hand, possesses a far higher radiative intensity
than our sun. It gravitates--admitting Sir David Gill's parallax of
0·38" to be exact--like two suns, but shines like twenty. Possibly it is
much distended by heat, and undoubtedly its atmosphere intercepts a very
much smaller proportion of its light than in stars of the solar class.
As regards Procyon, visual verification was awaited until November 13,
1896, when Professor Schaeberle, with the great Lick refractor, detected
the long-sought object in the guise of a thirteenth-magnitude star. Dr.
See's calculations[99] showed it to possess one-fifth the mass of its
primary, or rather more than half that of our sun.[100] Yet it gives
barely 1/20000th of the sun's light, so that it is still nearer to total
obscurity than the dusky satellite of Sirius. The period of forty years
assigned to the system by Auwers in 1862[101] appears to be singularly
exact.
But Bessel was not destined to witness the recognition of "the
invisible" as a legitimate and profitable field for astronomical
research. He died March 17, 1846, just six months before the discovery
of Neptune, of an obscure disease, eventually found to be occasioned by
an extensive fungus-growth in the stomach. The place which he left
vacant was not one easy to fill. His life's work might be truly
described as "epoch-making." Rarely indeed shall we find one who
reconciled with the same success the claims of theoretical and practical
astronomy, or surveyed the science which he had made his own with a
glance equally comprehensive, practical, and profound.
The career of Friedrich Georg Wilhelm Struve illustrates the maxim that
science _differentiates_ as it develops. He was, while much besides, a
specialist in double stars. His earliest recorded use of the telescope
was to verify Herschel's conclusion as to the revolving movement of
Castor, and he never varied from the predilection which this first
observation at once indicated and determined. He was born at Altona, of
a respectable yeoman family, April 15, 1793, and in 1811 took a degree
in philology at the new Russian University of Dorpat. He then turned to
science, was appointed in 1813 to a professorship of astronomy and
mathematics, and began regular work in the Dorpat Observatory just
erected by Parrot for Alexander I. It was not, however, until 1819 that
the acquisition of a 5-foot refractor by Troughton enabled him to take
the position-angles of double stars with regularity and tolerable
precision. The resulting catalogue of 795 stellar systems gave the
signal for a general resumption of the Herschelian labours in this
branch. His success, so far, and the extraordinary facilities for
observation afforded by the Fraunhofer achromatic encouraged him to
undertake, February 11, 1825, a review of the entire heavens down to 15°
south of the celestial equator, which occupied more than two years, and
yielded, from an examination of above 120,000 stars, a harvest of about
2,200 previously unnoticed composite objects. The ensuing ten years were
devoted to delicate and patient measurements, the results of which were
embodied in _Mensuræ Micrometricæ_, published at St. Petersburg in 1837.
This monumental work gives the places, angles of position, distances,
colours, and relative brightness of 3,112 double and multiple stars, all
determined with the utmost skill and care. The record is one which gains
in value with the process of time, and will for ages serve as a standard
of reference by which to detect change or confirm discovery.
It appears from Struve's researches that about one in forty of all stars
down to the ninth magnitude is composite, but that the proportion is
doubled in the brighter orders.[102] This he attributed to the
difficulty of detecting the faint companions of very remote orbs. It was
also noticed, both by him and Bessel, that double stars are in general
remarkable for large proper motions. Struve's catalogue included no star
of which the components were more than 32" apart, because beyond that
distance the chances of merely optical juxtaposition become
considerable; but the immense preponderance of extremely close over (as
it were) loosely yoked bodies is such as to demonstrate their physical
connection, even if no other proof were forthcoming. Many stars
previously believed to be single divided under the scrutiny of the
Dorpat refractor; while in some cases, one member of a supposed binary
system revealed itself as double, thus placing the surprised observer in
the unexpected presence of a triple group of suns. Five instances were
noted of two pairs lying so close together as to induce a conviction of
their mutual dependence;[103] besides which, 124 examples occurred of
triple, quadruple, and multiple combinations, the reality of which was
open to no reasonable doubt.[104]
It was first pointed out by Bessel that the fact of stars exhibiting a
common proper motion might serve as an unfailing test of their real
association into systems. This was, accordingly, one of the chief
criteria employed by Struve to distinguish true binaries from merely
optical couples. On this ground alone, 61 Cygni was admitted to be a
genuine double star; and it was shown that, although its components
appeared to follow almost strictly rectilinear paths, yet the
probability of their forming a connected pair is actually greater than
that of the sun rising to-morrow morning.[105] Moreover, this tie of an
identical movement was discovered to unite bodies[106] far beyond the
range of distance ordinarily separating the members of binary systems,
and to prevail so extensively as to lead to the conclusion that single
do not outnumber conjoined stars more than twice or thrice.[107]
In 1835 Struve was summoned by the Emperor Nicholas to superintend the
erection of a new observatory at Pulkowa, near St. Petersburg, destined
for the special cultivation of sidereal astronomy. Boundless resources
were placed at his disposal, and the institution created by him was
acknowledged to surpass all others of its kind in splendour, efficiency,
and completeness. Its chief instrumental glory was a refractor of
fifteen inches aperture by Merz and Mahler (Fraunhofer's successors),
which left the famous Dorpat telescope far behind, and remained long
without a rival. On the completion of this model establishment, August
19, 1839, Struve was installed as its director, and continued to fulfil
the important duties of the post with his accustomed vigour until 1858,
when illness compelled his virtual resignation in favour of his son Otto
Struve, born at Dorpat in 1819. He died November 23, 1864.
An inquiry into the laws of stellar distribution, undertaken during the
early years of his residence at Pulkowa, led Struve to confirm in the
main the inferences arrived at by Herschel as to the construction of the
heavens. According to his view, the appearance known as the Milky Way is
produced by a collection of irregularly condensed star-clusters, within
which the sun is somewhat eccentrically placed. The nebulous ring which
thus integrates the light of countless worlds was supposed by him to be
made up of stars scattered over a bent or "broken plane," or to lie in
two planes slightly inclined to each other, our system occupying a
position near their intersection.[108] He further attempted to show that
the limits of this vast assemblage must remain for ever shrouded from
human discernment, owing to the gradual extinction of light in its
passage through space,[109] and sought to confer upon this celebrated
hypothesis a definiteness and certainty far beyond the aspirations of
its earlier advocates, Chéseaux and Olbers; but arbitrary assumptions
vitiated his reasonings on this, as well as on some other points.[110]
In his special line as a celestial explorer of the most comprehensive
type, Sir William Herschel had but one legitimate successor, and that
successor was his son. John Frederick William Herschel was born at
Slough, March 17, 1792, graduated with the highest honours from St.
John's College, Cambridge, in 1813, and entered upon legal studies with
a view to being called to the Bar. But his share in an early compact
with Peacock and Babbage, "to do their best to leave the world wiser
than they found it," was not thus to be fulfilled. The acquaintance of
Dr. Wollaston decided his scientific vocation. Already, in 1816, we find
him reviewing some of his father's double stars; and he completed in
1820 the 18-inch speculum which was to be the chief instrument of his
investigations. Soon afterwards, he undertook, in conjunction with Mr.
(later Sir James) South, a series of observations, issuing in the
presentation to the Royal Society of a paper[111] containing
micrometrical measurements of 380 binary stars, by which the elder
Herschel's inferences of orbital motion were, in many cases, strikingly
confirmed. A star in the Northern Crown, for instance (Eta Coronæ),
had completed more than one entire circuit since its first discovery;
another, Tau Ophiuchi, had _closed up_ into apparent singleness; while
the motion of a third, Xi Ursæ Majoris, in an obviously eccentric orbit,
was so rapid as to admit of being traced and measured from month to month.
It was from the first confidently believed that the force retaining
double stars in curvilinear paths was identical with that governing the
planetary revolutions. But that identity was not ascertained until
Savary of Paris showed, in 1827,[112] that the movements of the
above-named binary in the Great Bear could be represented with all
attainable accuracy by an ellipse calculated on orthodox gravitational
principles with a period of 58-1/4 years. Encke followed at Berlin with
a still more elegant method; and Sir John Herschel, pointing out the
uselessness of analytical refinements where the data were necessarily so
imperfect, described in 1832 a graphical process by which "the aid of
the eye and hand" was brought in "to guide the judgment in a case where
judgment only, and not calculation, could be of any avail."[113]
Improved methods of the same kind were published by Dr. See in
1893,[114] and by Mr. Burnham in 1894;[115] and our acquaintance with
stellar orbits is steadily gaining precision, certainty, and extent.
In 1825 Herschel undertook, and executed with great assiduity during the
ensuing eight years, a general survey of the northern heavens, directed
chiefly towards the verification of his father's nebular discoveries.
The outcome was a catalogue of 2,306 nebulæ and clusters, of which 525
were observed for the first time, besides 3,347 double stars discovered
almost incidentally.[116] "Strongly invited," as he tells us himself,
"by the peculiar interest of the subject, and the wonderful nature of
the objects which presented themselves," he resolved to attempt the
completion of the survey in the southern hemisphere. With this noble
object in view, he embarked his family and instruments on board the
_Mount Stewart Elphinstone_, and, after a prosperous voyage, landed at
Cape Town on the 16th of January, 1834. Choosing as the scene of his
observations a rural spot under the shelter of Table Mountain, he began
regular "sweeping" on the 5th of March. The site of his great reflector
is now marked with an obelisk, and the name of Feldhausen has become
memorable in the history of science; for the four years' work done there
may truly be said to open the chapter of our knowledge as regards the
southern skies.
The full results of Herschel's journey to the Cape were not made public
until 1847, when a splendid volume[117] embodying them was brought out
at the expense of the Duke of Northumberland. They form a sequel to his
father's labours such as the investigations of one man have rarely
received from those of another. What the elder observer did for the
northern heavens, the younger did for the southern, and with generally
concordant results. Reviving the paternal method of "star-gauging," he
showed, from a count of 2,299 fields, that the Milky Way surrounds the
solar system as a complete annulus of minute stars; not, however, quite
symmetrically, since the sun was thought to lie somewhat nearer to those
portions visible in the southern hemisphere, which display a brighter
lustre and a more complicated structure than the northern branches. The
singular cosmical agglomerations known as the "Magellanic Clouds" were
now, for the first time, submitted to a detailed, though admittedly
incomplete, examination, the almost inconceivable richness and variety
of their contents being such that a lifetime might with great profit be
devoted to their study. In the Greater Nubecula, within a compass of
forty-two square degrees, Herschel reckoned 278 distinct nebulæ and
clusters, besides fifty or sixty outliers, and a large number of stars
intermixed with diffused nebulosity--in all, 919 catalogued objects,
and, for the Lesser Cloud, 244. Yet this was only the most conspicuous
part of what his twenty-foot revealed. Such an extraordinary
concentration of bodies so various led him to the inevitable conclusion
that "the Nubeculæ are to be regarded as systems _sui generis_, and
which have no analogues in our hemisphere."[118] He noted also the
blankness of surrounding space, especially in the case of Nubecula
Minor, "the access to which on all sides," he remarked, "is through a
desert;" as if the cosmical material in the neighbourhood had been swept
up and garnered in these mighty groups.[119]
Of southern double stars, he discovered and gave careful measurements of
2,102, and described 1,708 nebulæ, of which at least 300 were new. The
list was illustrated with a number of drawings, some of them extremely
beautiful and elaborate.
Sir John Herschel's views as to the nature of nebulæ were considerably
modified by Lord Rosse's success in "resolving" with his great
reflectors a crowd of these objects into stars. His former somewhat
hesitating belief in the existence of phosphorescent matter,
"disseminated through extensive regions of space in the manner of a
cloud or fog,"[120] was changed into a conviction that no valid
distinction could be established between the faintest wisp of cosmical
vapour just discernible in a powerful telescope, and the most brilliant
and obvious cluster. He admitted, however, an immense range of possible
variety in the size and mode of aggregation of the stellar constituents
of various nebulæ. Some might appear nebulous from the closeness of
their parts; some from their smallness. Others, he suggested, might be
formed of "discrete luminous bodies floating in a non-luminous
medium;"[121] while the annular kind probably consisted of "hollow
shells of stars."[122] That a physical, and not merely an optical,
connection unites nebulæ with the _embroidery_ (so to speak) of small
stars with which they are in many instances profusely decorated, was
evident to him, as it must be to all who look as closely and see as
clearly as he did. His description of No. 2,093 in his northern
catalogue as "a network or tracery of nebula following the lines of a
similar network of stars,"[123] would alone suffice to dispel the idea
of accidental scattering; and many other examples of a like import might
be quoted. The remarkably frequent occurrence of one or more minute
stars in the close vicinity of "planetary" nebulæ led him to infer their
dependent condition; and he advised the maintenance of a strict watch
for evidences of circulatory movements, not only over these supposed
stellar satellites, but also over the numerous "double nebulæ," in
which, as he pointed out, "all the varieties of double stars as to
distance, position, and relative brightness, have their counterparts."
He, moreover, investigated the subject of nebular distribution by the
simple and effectual method of graphic delineation or "charting," and
succeeded in showing that while a much greater uniformity of scattering
prevails in the southern than in the northern heavens, a condensation is
nevertheless perceptible about the constellations Pisces and Cetus,
roughly corresponding to the "nebular region" in Virgo by its vicinity
(within 20° or 30°) to the opposite pole of the Milky Way. He concluded
"that the nebulous system is distinct from the sidereal, though
involving, and perhaps to a certain extent intermixed with, the
latter."[124]
Towards the close of his residence at Feldhausen, Herschel was fortunate
enough to witness one of those singular changes in the aspect of the
firmament which occasionally challenge the attention even of the
incurious, and excite the deepest wonder of the philosophical observer.
Immersed apparently in the Argo nebula is a star denominated Eta
Carinæ. When Halley visited St. Helena in 1677, it seemed of the fourth
magnitude; but Lacaille in the middle of the following century, and
others after him, classed it as of the second. In 1827 the traveller
Burchell, being then at St. Paul, near Rio Janeiro, remarked that it had
unexpectedly assumed the first rank--a circumstance the more surprising
to him because he had frequently, when in Africa during the years 1811
to 1815, noted it as of only fourth magnitude. This observation,
however, did not become generally known until later. Herschel, on his
arrival at Feldhausen, registered the star as a bright second, and had
no suspicion of its unusual character until December 16, 1837, when he
suddenly perceived its light to be almost tripled. It then far outshone
Rigel in Orion, and on the 2nd of January following it very nearly
matched Alpha Centauri. From that date it declined; but a second
and even brighter maximum occurred in April, 1843, when Maclear, then
director of the Cape Observatory, saw it blaze out with a splendour
approaching that of Sirius. Its waxings and wanings were marked by
curious "trepidations" of brightness extremely perplexing to theory. In
1863 it had sunk below the fifth magnitude, and in 1869 was barely
visible to the naked eye; yet it was not until eighteen years later that
it touched a minimum of 7·6 magnitude. Soon afterwards a recovery of
brightness set in, but was not carried very far; and the star now shines
steadily as of the seventh magnitude, its reddish light contrasting
effectively with the silvery rays of the surrounding nebula. An attempt
to include its fluctuations within a cycle of seventy years[125] has
signally failed; the extent and character of the vicissitudes to which
it is subject stamping it rather as a species of connecting link between
periodic and temporary stars.[126]
Among the numerous topics which engaged Herschel's attention at the Cape
was that of relative stellar brightness. Having contrived an
"astrometer" in which an "artificial star," formed by the total
reflection of moonlight from the base of a prism, served as a standard
of comparison, he was able to estimate the lustre of the _natural_ stars
examined by the distances at which the artificial object appeared equal
respectively to each. He thus constructed a table of 191 of the
principal stars,[127] both in the northern and southern hemispheres,
setting forth the numerical values of their apparent brightness
relatively to that of Alpha Centauri, which he selected as a unit
of measurement. Further, the light of the full moon being found by him
to exceed that of his standard star 27,408 times, and Dr. Wollaston
having shown that the light of the full moon is to that of the sun as
1:801,072[128] (Zöllner made the ratio 1:618,000), it became possible to
compare stellar with solar radiance. Hence was derived, in the case of
the few stars at ascertained distances, a knowledge of real lustre.
Alpha Centauri, for example, emits less than twice, Capella one hundred
times as much light as our sun; while Arcturus, at its enormous
distance, must display the splendour of 1,300 such luminaries.
Herschel returned to England in the spring of 1838, bringing with him a
wealth of observation and discovery such as had perhaps never before
been amassed in so short a time. Deserved honours awaited him. He was
created a baronet on the occasion of the Queen's coronation (he had been
knighted in 1831); universities and learned societies vied with each
other in showering distinctions upon him; and the success of an
enterprise in which scientific zeal was tinctured with an attractive
flavour of adventurous romance, was justly regarded as a matter of
national pride. His career as an observing astronomer was now virtually
closed, and he devoted his leisure to the collection and arrangement of
the abundant trophies of his father's and his own activity. The
resulting great catalogue of 5,079 nebulæ (including all then certainly
known), published in the _Philosophical Transactions_ for 1864, is, and
will probably long remain, the fundamental source of information on the
subject;[129] but he unfortunately did not live to finish the companion
work on double stars, for which he had accumulated a vast store of
materials.[130] He died at Collingwood in Kent, May 11, 1871, in the
eightieth year of his age, and was buried in Westminster Abbey, close
beside the grave of Sir Isaac Newton.
The consideration of Sir John Herschel's Cape observations brings us to
the close of the period we are just now engaged in studying. They were
given to the world, as already stated, three years before the middle of
the century, and accurately represent the condition of sidereal science
at that date. Looking back over the fifty years traversed, we can see at
a glance how great was the stride made in the interval. Not alone was
acquaintance with individual members of the cosmos vastly extended, but
their mutual relations, the laws governing their movements, their
distances from the earth, masses, and intrinsic lustre, had begun to be
successfully investigated. _Begun to be_; for only regarding a scarcely
perceptible minority had even approximate conclusions been arrived at.
Nevertheless the whole progress of the future lay in that beginning; it
was the thin end of the wedge of exact knowledge. The principle of
measurement had been substituted for that of probability; a basis had
been found large and strong enough to enable calculation to ascend from
it to the sidereal heavens; and refinements had been introduced,
fruitful in performance, but still more in promise. Thus, rather the
kind than the amount of information collected was significant for the
time to come--rather the methods employed than the results actually
secured rendered the first half of the nineteenth century of epochal
importance in the history of our knowledge of the stars.
FOOTNOTES:
[Footnote 58: Bessel, _Populäre Vorlesungen_, pp. 6, 408.]
[Footnote 59: Fitted to the old transit instrument, July 11, 1772.]
[Footnote 60: _Briefwechsel mit Olbers_, p. xvi.]
[Footnote 61: R. Wolf, _Gesch. der Astron._, p. 518.]
[Footnote 62: Bessel, _Pop. Vorl._, p. 22.]
[Footnote 63: A new reduction of the observations upon which they were
founded was undertaken in 1896 by Herman S. Davis, of the U.S. Coast
Survey.]
[Footnote 64: Bessel, _Pop. Vorl._, p. 440.]
[Footnote 65: Durège, _Bessel's Leben und Wirken_, p. 28.]
[Footnote 66: _Bonner Beobachtungen_, Bd. iii.-v., 1859-62.]
[Footnote 67: Bessel, _Pop. Vorl._, p. 238.]
[Footnote 68: The heads of the screws applied to move the halves of the
object-glass in the Königsberg heliometer are of so considerable a size
that a thousandth part of a revolution, equivalent to 1/20 of a second
of arc, can be measured with the utmost accuracy. Main, _R. A. S. Mem._,
vol. xii., p. 53.]
[Footnote 69: _Specola Astronomica di Palermo_, lib. vi., p. 10,
_note_.]
[Footnote 70: _Monatliche Correspondenz_, vol. xxvi., p. 162.]
[Footnote 71: _Astronomische Nachrichten_, Nos. 365-366. It should be
explained that what is called the "annual parallax" of a star is only
half its apparent displacement. In other words, it is the angle
subtended at the distance of that particular star by the _radius_ of the
earth's orbit.]
[Footnote 72: _Astr. Nach._, Nos. 401-402.]
[Footnote 73: Sir R. Ball's measurements at Dunsink gave to 61 Cygni a
parallax of 0·47"; Professor Pritchard obtained, by photographic
determinations, one of 0·43".]
[Footnote 74: _Additamentum in Mensuras Micrometricas_, p. 28.]
[Footnote 75: Elkin's corrected result (in 1897) for the parallax of
Vega is 0·082".]
[Footnote 76: _Mem. Roy. Astr. Soc._, vol. xi., p. 61.]
[Footnote 77: That numbered 21,185 in Lalande's _Hist. Cél._, found by
Argelander to have a proper motion of 4·734", and by Winnecke a parallax
of O·511". _Month. Not._, vol. xviii., p. 289.]
[Footnote 78: _Fund. Astr._, p. 309.]
[Footnote 79: _Mém. Prés. à l'Ac. de St. Pétersb._, t. iii.]
[Footnote 80: _Phil. Trans._, vol. cxxxvii., p. 79.]
[Footnote 81: _Mem. Roy. Astr. Soc._, vols. xxviii. and xxxii.]
[Footnote 82: _Ibid._, vol. xlvii., p. 327.]
[Footnote 83: _Mémoires de St. Pétersbourg_, t. xxxv., No. 3, 1887;
revised in _Astr. Nach._, Nos. 3,729-30, 1901.]
[Footnote 84: _Astronomical Journal_, Nos. 213, 501.]
[Footnote 85: _Astr. Nach._, Nos. 2,999, 3,000.]
[Footnote 86: _Veröffentlichungen der Grossh. Sternwarte zu Karlsruhe_,
Bd. iv., 1892.]
[Footnote 87: _Proceedings Amsterdam Acad. of Sciences_, Jan. 27, 1900.]
[Footnote 88: _Astr. Jour._, No. 457.]
[Footnote 89: _Ibid._, Nos. 276, 497.]
[Footnote 90: _Phil. Trans._, vol. xcvi., p. 230.]
[Footnote 91: _Mém. Prés. à l'Ac. de St. Pétersbourg_, t. iii., p. 603
(read Feb. 5, 1837).]
[Footnote 92: _Die Centralsonne, Astr. Nach._, Nos. 566-567, 1846.]
[Footnote 93: Sir J. Herschel, note to _Treatise on Astronomy_, and
_Phil. Trans._, vol. cxxiii., part ii., p. 502.]
[Footnote 94: The position is (as Sir J. Herschel pointed out, _Outlines
of Astronomy_, p. 631, 10th ed.) placed beyond the range of reasonable
probability by its remoteness (fully 26°) from the galactic plane.]
[Footnote 95: Mädler in _Westermann's Jahrbuch_, 1867, p. 615.]
[Footnote 96: Letter from Bessel to Sir J. Herschel, _Month. Not._, vol.
vi., p. 139.]
[Footnote 97: Wolf, _Gesch. d. Astr._, p. 743, _note_.]
[Footnote 98: _Astr. Nach._, Nos. 745-748.]
[Footnote 99: _Astr. Jour._, No. 440.]
[Footnote 100: Adopting Elkin's revised parallax for Procyon of 0·325".]
[Footnote 101: _Astr. Nach._, Nos. 1371-1373.]
[Footnote 102: _Ueber die Doppelsterne_, Bericht, 1827, p. 22.]
[Footnote 103: _Ueber die Doppelsterne_, Bericht, 1827, p. 25.]
[Footnote 104: _Mensuræ Micr._, p. xcix.]
[Footnote 105: _Stellarum Fixarum imprimis Duplicium et Multiplicum
Positiones Mediæ_, pp. cxc., cciii.]
[Footnote 106: For instance, the southern stars, 36A Ophiuchi (itself
double) and 30 Scorpii, which are 12' 10" apart. _Ibid._, p. cciii.]
[Footnote 107: _Stellarum Fixarum_, etc., p. ccliii.]
[Footnote 108: _Études d'Astronomie Stellaire_, 1847, p. 82.]
[Footnote 109: _Ibid._, p. 86.]
[Footnote 110: See Encke's criticism in _Astr. Nach._, No. 622.]
[Footnote 111: _Phil. Trans._, vol. cxiv., part iii., 1824.]
[Footnote 112: _Conn. d. Temps_, 1830.]
[Footnote 113: _R. A. S. Mem._, vol. v., p. 178, 1833.]
[Footnote 114: _Astr. and Astrophysics_, vol. xii., p. 581.]
[Footnote 115: _Popular Astr._, vol. i., p. 243.]
[Footnote 116: _Phil. Trans._, vol. cxxiii., and _Results_, etc.,
Introd.]
[Footnote 117: _Results of Astronomical Observations made during the
years 1834-8 at the Cape of Good Hope._]
[Footnote 118: _Results_, etc., p. 147.]
[Footnote 119: See Proctor's _Universe of Stars_, p. 92.]
[Footnote 120: _A Treatise on Astronomy_, 1833, p. 406.]
[Footnote 121: _Results_, etc., p. 139.]
[Footnote 122: _Ibid._, pp. 24, 142.]
[Footnote 123: _Phil. Trans._, vol. cxxiii., p. 503.]
[Footnote 124: _Results_, etc., p. 136.]
[Footnote 125: Loomis, _Month. Not._, vol. xxix., p. 298.]
[Footnote 126: See the Author's _System of the Stars_, pp. 116-120.]
[Footnote 127: _Outlines of Astr._, App. I.]
[Footnote 128: _Phil. Trans._, vol. cxix., p. 27.]
[Footnote 129: Dr. Dreyer's New General Catalogue, published in 1888 as
vol. xlix. of the Royal Astronomical Society's _Memoirs_, is an
enlargement of Herschel's work. It includes 7,840 entries, and was
supplemented, in 1895, by an "Index Catalogue" of 1,529 nebulæ
discovered 1888 to 1894. _Mem. R. A. S._, vol. li.]
[Footnote 130: A list of 10,320 composite stars was drawn out by him in
order of right ascension, and has been published in vol. xl. of _Mem. R.
A. S._; but the data requisite for their formation into a catalogue were
not forthcoming. See Main's and Pritchard's _Preface_ to above, and
Dunkin's _Obituary Notices_, p. 73.]
CHAPTER III
_PROGRESS OF KNOWLEDGE REGARDING THE SUN_
The discovery of sun-spots in 1610 by Fabricius and Galileo first opened
a way for inquiry into the solar constitution; but it was long before
that way was followed with system or profit. The seeming irregularity of
the phenomena discouraged continuous attention; casual observations were
made the basis of arbitrary conjectures, and real knowledge received
little or no increase. In 1620 we find Jean Tarde, Canon of Sarlat,
arguing that because the sun is "the eye of the world," and the eye of
the world _cannot suffer from ophthalmia_, therefore the appearances in
question must be due, not to actual specks or stains on the bright solar
disc, but to the transits of a number of small planets across it! To
this new group of heavenly bodies he gave the name of "Borbonia Sidera,"
and they were claimed in 1633 for the House of Hapsburg, under the title
of "Austriaca Sidera" by Father Malapertius, a Belgian Jesuit.[131] A
similar view was temporarily maintained against Galileo by the justly
celebrated Father Scheiner of Ingolstadt, and later by William
Gascoigne, the inventor of the micrometer; but most of those who were
capable of thinking at all on such subjects (and they were but few)
adhered either to the _cloud theory_ or to the _slag theory_ of
sun-spots. The first was championed by Galileo, the second by Simon
Marius, "astronomer and physician" to the brother Margraves of
Brandenburg. The latter opinion received a further notable development
from the fact that in 1618, a year remarkable for the appearance of
three bright comets, the sun was almost free from spots; whence it was
inferred that the cindery refuse from the great solar conflagration,
which usually appeared as dark blotches on its surface, was occasionally
thrown off in the form of comets, leaving the sun, like a snuffed taper,
to blaze with renewed brilliancy.[132]
In the following century, Derham gathered from observations carried on
during the years 1703-11, "That the spots on the sun are caused by the
eruption of some new volcano therein, which at first pouring out a
prodigious quantity of smoke and other opacous matter, causeth the
spots; and as that fuliginous matter decayeth and spendeth itself, and
the volcano at last becomes more torrid and flaming, so the spots decay,
and grow to umbræ, and at last to faculæ."[133]
The view, confidently upheld by Lalande,[134] that spots were rocky
elevations uncovered by the casual ebbing of a luminous ocean, the
surrounding penumbræ representing shoals or sandbanks, had even less to
recommend it than Derham's volcanic theory. Both were, however,
significant of a growing tendency to bring solar phenomena within the
compass of terrestrial analogies.
For 164 years, then, after Galileo first levelled his telescope at the
setting sun, next to nothing was learned as to its nature; and the facts
immediately ascertained, of its rotation on an axis nearly erect to the
plane of the ecliptic, in a period of between twenty-five and twenty-six
days, and of the virtual limitation of the spots to a so-called "royal"
zone extending some thirty degrees north and south of the solar equator,
gained little either in precision or development from five generations
of astronomers.
But in November, 1769, a spot of extraordinary size engaged the
attention of Alexander Wilson, professor of astronomy in the University
of Glasgow. He watched it day by day, and to good purpose. As the great
globe slowly revolved, carrying the spot towards its western edge, he
was struck with the gradual contraction and final disappearance of the
penumbra _on the side next the centre of the disc_; and when on the 6th
of December the same spot re-emerged on the eastern limb, he perceived,
as he had anticipated, that the shady zone was now deficient _on the
opposite side_, and resumed its original completeness as it returned to
a central position. In other spots subsequently examined by him, similar
perspective effects were visible, and he proved in 1774,[135] by strict
geometrical reasoning, that they could only arise in vast photospheric
excavations. It was not, indeed, the first time that such a view had
been suggested. Father Scheiner's later observations plainly
foreshadowed it;[136] a conjecture to the same effect was emitted by
Leonard Rost of Nuremburg early in the eighteenth century;[137] both by
Lahire in 1703 and by J. Cassini in 1719 spots had been seen as notches
on the solar limb; while in 1770 Pastor Schülen of Essingen, from the
careful study of phenomena similar to those noted by Wilson, concluded
their depressed nature.[138] Modern observations, nevertheless, prove
those phenomena to be by no means universally present.
Wilson's general theory of the sun was avowedly tentative. It took the
modest form of an interrogatory. "Is it not reasonable to think," he
asks, "that the great and stupendous body of the sun is made up of two
kinds of matter, very different in their qualities; that by far the
greater part is solid and dark, and that this immense and dark globe is
encompassed with a thin covering of that resplendent substance from
which the sun would seem to derive the whole of his vivifying heat and
energy?"[139] He further suggests that the excavations or spots may be
occasioned "by the working of some sort of elastic vapour which is
generated within the dark globe," and that the luminous matter, being in
some degree fluid, and being acted upon by gravity, tends to flow down
and cover the nucleus. From these hints, supplemented by his own
diligent observations and sagacious reasonings, Herschel elaborated a
scheme of solar constitution which held its ground until the physics of
the sun were revolutionised by the spectroscope.
A cool, dark, solid globe, its surface diversified with mountains and
valleys, clothed in luxuriant vegetation, and "richly stored with
inhabitants," protected by a heavy cloud-canopy from the intolerable
glare of the upper luminous region, where the dazzling coruscations of a
solar aurora some thousands of miles in depth evolved the stores of
light and heat which vivify our world--such was the central luminary
which Herschel constructed with his wonted ingenuity, and described with
his wonted eloquence.
"This way of considering the sun and its atmosphere," he says,[140]
"removes the great dissimilarity we have hitherto been used to find
between its condition and that of the rest of the great bodies of the
solar system. The sun, viewed in this light, appears to be nothing else
than a very eminent, large, and lucid planet, evidently the first, or,
in strictness of speaking, the only primary one of our system; all
others being truly secondary to it. Its similarity to the other globes
of the solar system with regard to its solidity, its atmosphere, and its
diversified surface, the rotation upon its axis, and the fall of heavy
bodies, leads us on to suppose that it is most probably also inhabited,
like the rest of the planets, by beings whose organs are adapted to the
peculiar circumstances of that vast globe."
We smile at conclusions which our present knowledge condemns as
extravagant and impossible, but such incidental flights of fancy in no
way derogate from the high value of Herschel's contributions to solar
science. The cloud-like character which he attributed to the radiant
shell of the sun (first named by Schröter the "photosphere") is borne
out by all recent investigations; he observed its mottled or corrugated
aspect, resembling, as he described it, the roughness on the rind of an
orange; showed that "faculæ" are elevations or heaped-up ridges of the
disturbed photospheric matter; and threw out the idea that spots may
ensue from an excess of the ordinary luminous emissions. A certain
"empyreal" gas was, he supposed (very much as Wilson had done),
generated in the body of the sun, and rising everywhere by reason of its
lightness, made for itself, when in moderate quantities, small openings
or "pores,"[141] abundantly visible as dark points on the solar disc.
But should an uncommon quantity be formed, "it will," he maintained,
"burst through the planetary[142] regions of clouds, and thus will
produce great openings; then, spreading itself above them, it will
occasion large shallows (penumbræ), and mixing afterwards gradually with
other superior gases, it will promote the increase, and assist in the
maintenance, of the general luminous phenomena."[143]
This partial anticipation of the modern view that the solar radiations
are maintained by some process of circulation within the solar mass, was
reached by Herschel through prolonged study of the phenomena in
question. The novel and important idea contained in it, however, it was
at that time premature to attempt to develop. But though many of the
subtler suggestions of Herschel's genius passed unnoticed by his
contemporaries, the main result of his solar researches was an
unmistakable one. It was nothing less than the definitive introduction
into astronomy of the paradoxical conception of the central fire and
hearth of our system as a cold, dark, terrestrial mass, wrapt in a
mantle of innocuous radiance--an earth, so to speak, within--a sun
without.
Let us pause for a moment to consider the value of this remarkable
innovation. It certainly was not a step in the direction of truth. On
the contrary, the crude notions of Anaxagoras and Xeno approached more
nearly to what we now know of the sun, than the complicated structure
devised for the happiness of a nobler race of beings than our own by the
benevolence of eighteenth-century astronomers. And yet it undoubtedly
constituted a very important advance in science. It was the first
earnest attempt to bring solar phenomena within the compass of a
rational system; to put together into a consistent whole the facts
ascertained; to fabricate, in short, a solar machine that would in some
fashion work. It is true that the materials were inadequate and the
design faulty. The resulting construction has not proved strong enough
to stand the wear and tear of time and discovery, but has had to be
taken to pieces and remodelled on a totally different plan. But the work
was not therefore done in vain. None of Bacon's aphorisms show a clearer
insight into the relations between the human mind and the external world
than that which declares "Truth to emerge sooner from error than from
confusion."[144] A definite theory (even if a false one) gives
holding-ground to thought. Facts acquire a meaning with reference to it.
It affords a motive for accumulating them and a means of co-ordinating
them; it provides a framework for their arrangement, and a receptacle
for their preservation, until they become too strong and numerous to be
any longer included within arbitrary limits, and shatter the vessel
originally framed to contain them.
Such was the purpose subserved by Herschel's theory of the sun. It
helped to _clarify_ ideas on the subject. The turbid sense of groping
and viewless ignorance gave place to the lucidity of a possible scheme.
The persuasion of knowledge is a keen incentive to its increase. Few men
care to investigate what they are obliged to admit themselves entirely
ignorant of; but once started on the road of knowledge, real or
supposed, they are eager to pursue it. By the promulgation of a
confident and consistent view regarding the nature of the sun,
accordingly, research was encouraged, because it was rendered hopeful,
and inquirers were shown a path leading indefinitely onwards where an
impassable thicket had before seemed to bar the way.
We have called the "terrestrial" theory of the sun's nature an
innovation, and so, as far as its general acceptance is concerned, it
may justly be termed; but, like all successful innovations, it was a
long time brewing. It is extremely curious to find that Herschel had a
predecessor in its advocacy who never looked through a telescope (nor,
indeed, imagined the possibility of such an instrument), who knew
nothing of sun-spots, was still (mistaken assertions to the contrary
notwithstanding) in the bondage of the geocentric system, and regarded
nature from the lofty standpoint of an idealist philosophy. This was the
learned and enlightened Cardinal Cusa, a fisherman's son from the banks
of the Moselle, whose distinguished career in the Church and in
literature extended over a considerable part of the fifteenth century
(1401-64). In his singular treatise _De Doctâ Ignorantiâ_, one of the
most notable literary monuments of the early Renaissance, the following
passage occurs:--"To a spectator on the surface of the sun, the
splendour which appears to us would be invisible, since it contains, as
it were, an earth for its central mass, with a circumferential envelope
of light and heat, and between the two an atmosphere of water and clouds
and translucent air." The luminary of Herschel's fancy could scarcely be
more clearly portrayed; some added words, however, betray the origin of
the Cardinal's idea. "The earth also," he says, "would appear as a
shining star to any one outside the fiery element." It was, in fact, an
extension to the sun of the ancient elemental doctrine; but an extension
remarkable at that period, as premonitory of the tendency, so powerfully
developed by subsequent discoveries, to assimilate the orbs of heaven to
the model of our insignificant planet, and to extend the brotherhood of
our system and our species to the farthest limit of the visible or
imaginable universe.
In later times we find Flamsteed communicating to Newton, March 7, 1681,
his opinion "that the substance of the sun is terrestrial matter, his
light but the liquid menstruum encompassing him."[145] Bode in 1776
arrived independently at the conclusion that "the sun is neither burning
nor glowing, but in its essence a dark planetary body, composed like our
earth of land and water, varied by mountains and valleys, and enveloped
in a vaporous atmosphere";[146] and the learned in general applauded and
acquiesced. The view, however, was in 1787 still so far from popular,
that the holding of it was alleged as a proof of insanity in Dr. Elliot
when accused of a murderous assault on Miss Boydell. His friend Dr.
Simmons stated on his behalf that he had received from him in the
preceding January a letter giving evidence of a deranged mind, wherein
he asserted "that the sun is not a body of fire, as hath been hitherto
supposed, but that its light proceeds from a dense and universal aurora,
which may afford ample light to the inhabitants of the surface beneath,
and yet be at such a distance aloft as not to annoy them. No objection,
he saith, ariseth to that great luminary's being inhabited; vegetation
may obtain there as well as with us. There may be water and dry land,
hills and dales, rain and fair weather; and as the light, so the season
must be eternal, consequently it may easily be conceived to be by far
the most blissful habitation of the whole system!" The Recorder,
nevertheless, objected that if an extravagant hypothesis were to be
adduced as proof of insanity, the same might hold good with regard to
some other speculators, and desired Dr. Simmons to tell the court what
he thought of the theories of Burnet and Buffon.[147]
Eight years later, this same "extravagant hypothesis," backed by the
powerful recommendation of Sir William Herschel, obtained admittance to
the venerable halls of science, there to abide undisturbed for nearly
seven decades. Individual objectors, it is true, made themselves heard,
but their arguments had little effect on the general body of opinion.
Ruder blows were required to shatter an hypothesis flattering to human
pride of invention in its completeness, in the plausible detail of
observations by which it seemed to be supported, and in its
condescension to the natural pleasure in discovering resemblance under
all but total dissimilarity.
Sir John Herschel included among the results of his multifarious labours
at the Cape of Good Hope a careful study of the sun-spots conspicuously
visible towards the end of the year 1836 and in the early part of 1837.
They were remarkable, he tells us, for their forms and arrangement, as
well as for their number and size; one group, measured on the 29th of
March in the latter year, covering (apart from what may be called its
outlying dependencies) the vast area of five square minutes or 3,780
million square miles.[148] We have at present to consider, however, not
so much these observations in themselves, as the chain of theoretical
suggestions by which they were connected. The distribution of spots, it
was pointed out, on two zones parallel to the equator, showed plainly
their intimate connection with the solar rotation, and indicated as
their cause fluid circulations analogous to those producing the
terrestrial trade and anti-trade winds.
"The spots, in this view of the subject," he went on to say,[149] "would
come to be assimilated to those regions on the earth's surface where,
for the moment, hurricanes and tornadoes prevail; the upper stratum
being temporarily carried downwards, displacing by its impetus the two
strata of luminous matter beneath, the upper of course to a greater
extent than the lower, and thus wholly or partially denuding the opaque
surface of the sun below. Such processes cannot be unaccompanied by
vorticose motions, which, left to themselves, die away by degrees and
dissipate, with the peculiarity that their lower portions come to rest
more speedily than their upper, by reason of the greater resistance
below, as well as the remoteness from the point of action, which lies in
a higher region, so that their centres (as seen in our waterspouts,
which are nothing but small tornadoes) appear to retreat upwards. Now
this agrees perfectly with what is observed during the obliteration of
the solar spots, which appear as if filled in by the collapse of their
sides, the penumbra closing in upon the spot and disappearing after it."
But when it comes to be asked whether a cause can be found by which a
diversity of solar temperature might be produced corresponding with that
which sets the currents of the terrestrial atmosphere in motion, we are
forced to reply that we know of no such cause. For Sir John Herschel's
hypothesis of an increased retention of heat at the sun's equator, due
to the slightly spheroidal or bulging form of its outer atmospheric
envelope, assuredly gives no sufficient account of such circulatory
movements as he supposed to exist. Nevertheless, the view that the sun's
rotation is intimately connected with the formation of spots is so
obviously correct, that we can only wonder it was not thought of sooner,
while we are even now unable to explain with any certainty _how_ it is
so connected.
Mere scrutiny of the solar surface, however, is not the only means of
solar observation. We have a satellite, and that satellite from time to
time acts most opportunely as a screen, cutting off a part or the whole
of those dazzling rays in which the master-orb of our system veils
himself from over-curious regards. The importance of eclipses to the
study of the solar surroundings is of comparatively recent recognition;
nevertheless, much of what we know concerning them has been snatched, as
it were, by surprise under favour of the moon. In former times, the sole
astronomical use of such incidents was the correction of the received
theories of the solar and lunar movements; the precise time of their
occurrence was the main fact to be noted, and subsidiary phenomena
received but casual attention. Now, their significance as a geometrical
test of tabular accuracy is altogether overshadowed by the interest
attaching to the physical observations for which they afford propitious
occasions. This change may be said to date, in its pronounced form, from
the great eclipse of 1842. Although a necessary consequence of the
general direction taken by scientific progress, it remains associated in
a special manner with the name of Francis Baily.
The "philosopher of Newbury" was by profession a London stockbroker, and
a highly successful one. Nevertheless, his services to science were
numerous and invaluable, though not of the brilliant kind which attract
popular notice. Born at Newbury in Berkshire, April 28, 1774, and placed
in the City at the age of fourteen, he derived from the acquaintance of
Dr. Priestley a love of science which never afterwards left him. It was,
however, no passion such as flames up in the brain of the destined
discoverer, but a regulated inclination, kept well within the bounds of
an actively pursued commercial career. After travelling for a year or
two in what were then the wilds of North America, he went on the Stock
Exchange in 1799, and earned during twenty-four years of assiduous
application to affairs a high reputation for integrity and ability, to
which corresponded an ample fortune. In the meantime the Astronomical
Society (largely through his co-operation) had been founded; he had for
three years acted as its secretary, and he now felt entitled to devote
himself exclusively to a subject which had long occupied his leisure
hours. He accordingly in 1825 retired from business, purchased a house
in Tavistock Place, and fitted up there a small observatory. He was,
however, by preference a computator rather than an observer. What Sir
John Herschel calls the "archæology of practical astronomy" found in him
an especially zealous student. He re-edited the star-catalogues of
Ptolemy, Ulugh Beigh, Tycho Brahe, Hevelius, Halley, Flamsteed,
Lacaille, and Mayer; calculated the eclipse of Thales and the eclipse of
Agathocles, and vindicated the memory of the first Astronomer Royal. But
he was no less active in meeting present needs than in revising past
performances. The subject of the reduction of observations, then, as we
have already explained,[150] in a state of deplorable confusion,
attracted his most earnest attention, and he was close on the track of
Bessel when made acquainted with the method of simplification devised at
Königsberg. Anticipated as an inventor, he could still be of eminent use
as a promoter of these valuable improvements; and, carrying them out on
a large scale in the star-catalogue of the Astronomical Society
(published in 1827), "he put" (in the words of Herschel) "the
astronomical world in possession of a power which may be said, without
exaggeration, to have changed the face of sidereal astronomy."[151]
His reputation was still further enhanced by his renewal, with vastly
improved apparatus, of the method, first used by Henry Cavendish in
1797-98, for determining the density of the earth. From a series of no
less than 2,153 delicate and difficult experiments, conducted at
Tavistock Place during the years 1838-42, he concluded our planet to
weigh 5·66 as much as a globe of water of the same bulk; and this result
slightly corrected is still accepted as a very close approximation of
the truth.
What we have thus glanced at is but a fragment of the truly surprising
mass of work accomplished by Baily in the course of a variously occupied
life. A rare combination of qualities fitted him for his task. Unvarying
health, undisturbed equanimity, methodical habits, the power of directed
and sustained thought, combined to form in him an intellectual toiler of
the surest, though not perhaps of the highest quality. He was in harness
almost to the end. He was destined scarcely to know the miseries of
enforced idleness or of consciously failing powers. In 1842 he completed
the laborious reduction of Lalande's great catalogue, undertaken at the
request of the British Association, and was still engaged in seeing it
through the press when he was attacked with what proved his last, as it
was probably his first serious illness. He, however, recovered
sufficiently to attend the Oxford Commemoration of July 2, 1844, where
an honorary degree of D.C.L. was conferred upon him in company with Airy
and Struve; but sank rapidly after the effort, and died on the 30th of
August following, at the age of seventy, lamented and esteemed by all
who knew him.
It is now time to consider his share in the promotion of solar research.
Eclipses of the sun, both ancient and modern, were a speciality with
him, and he was fortunate in those which came under his observation.
Such phenomena are of three kinds--partial, annular, and total. In a
partial eclipse, the moon, instead of passing directly between us and
the sun, slips by, as it were, a little on one side, thus cutting off
from our sight only a portion of his surface. An annular eclipse, on the
other hand, takes place when the moon is indeed centrally interposed,
but falls short of the apparent size required for the entire concealment
of the solar disc, which consequently remains visible as a bright ring
or annulus, even when the obscuration is at its height. In a total
eclipse, on the contrary, the sun completely disappears behind the dark
body of the moon. The difference of the two latter varieties is due to
the fact that the apparent diameter of the sun and moon are so nearly
equal as to gain alternate preponderance one over the other through the
slight periodical changes in their respective distances from the earth.
Now, on the 15th of May, 1836, an annular eclipse was visible in the
northern parts of Great Britain, and was observed by Baily at Inch
Bonney, near Jedburgh. It was here that he saw the phenomenon which
obtained the name of "Baily's Beads," from the notoriety conferred upon
it by his vivid description.
"When the cusps of the sun," he writes, "were about 40° asunder, a row
of lucid points, like a string of bright beads, irregular in size and
distance from each other, _suddenly_ formed round that part of the
circumference of the moon that was about to enter on the sun's disc. Its
formation, indeed, was so rapid that it presented the appearance of
having been caused by the ignition of a fine train of gunpowder.
Finally, as the moon pursued her course, the dark intervening spaces
(which, at their origin, had the appearance of lunar mountains in high
relief, and which still continued attached to the sun's border) were
stretched out into long, black, thick, parallel lines, joining the limbs
of the sun and moon; when all at once they _suddenly_ gave way, and left
the circumference of the sun and moon in those points, as in the rest,
comparatively smooth and circular, and the moon perceptibly advanced on
the face of the sun."[152]
These curious appearances were not an absolute novelty. Weber in 1791,
and Von Zach in 1820, had seen the "beads"; Van Swinden had described
the "belts" or "threads."[153] These last were, moreover (as Baily
clearly perceived), completely analogous to the "black ligament" which
formed so troublesome a feature in the transits of Venus in 1764 and
1769, and which, to the regret and confusion, though no longer to the
surprise of observers, was renewed in that of 1874. The phenomenon is
largely an effect of what is called _irradiation_, by which a bright
object seems to encroach upon a dark one; but under good atmospheric and
instrumental conditions it becomes inconspicuous. The "Beads" must
always appear when the projected lunar edge is serrated with mountains.
In Baily's observation, they were exaggerated and distorted by an
irradiative _clinging together_ of the limbs of sun and moon.
The immediate result, however, was powerfully to stimulate attention to
solar eclipses in their _physical_ aspect. Never before had an
occurrence of the kind been expected so eagerly or prepared for so
actively as that which was total over Central and Southern Europe on the
8th of July, 1842. Astronomers hastened from all quarters to the
favoured region. The Astronomer Royal (Airy) repaired to Turin; Baily to
Pavia; Otto Struve threw aside his work amidst the stars at Pulkowa, and
went south as far as Lipeszk; Schumacher travelled from Altona to
Vienna; Arago from Paris to Perpignan. Nor did their trouble go
unrewarded. The expectations of the most sanguine were outdone by the
wonders disclosed.
Baily (to whose narrative we again have recourse) had set up his
Dollond's achromatic in an upper room of the University of Pavia, and
was eagerly engaged in noting a partial repetition of the singular
appearances seen by him in 1836, when he was "astounded by a tremendous
burst of applause from the streets below, and at the same moment was
electrified at the sight of one of the most brilliant and splendid
phenomena that can well be imagined. For at that instant the dark body
of the moon was suddenly surrounded with a corona, or kind of bright
glory similar in shape and relative magnitude to that which painters
draw round the heads of saints, and which by the French is designated an
_auréole_. Pavia contains many thousand inhabitants, the major part of
whom were, at this early hour, walking about the streets and squares or
looking out of windows, in order to witness this long-talked-of
phenomenon; and when the total obscuration took place, which was
_instantaneous_, there was a universal shout from every observer, which
'made the welkin ring,' and, for the moment, withdrew my attention from
the object with which I was immediately occupied. I had indeed
anticipated the appearance of a luminous circle round the moon during
the time of total obscurity; but I did not expect, from any of the
accounts of preceding eclipses that I had read, to witness so
magnificent an exhibition as that which took place.... The breadth of
the corona, measured from the circumference of the moon, appeared to me
to be nearly equal to half the moon's diameter. It had the appearance of
brilliant rays. The light was most dense close to the border of the
moon, and became gradually and uniformly more attenuate as its distance
therefrom increased, assuming the form of diverging rays in a
rectilinear line, which at the extremity were more divided, and of an
unequal length; so that in no part of the corona could I discover the
regular and well-defined shape of a ring at its _outer_ margin. It
appeared to me to have the sun for its centre, but I had no means of
taking any accurate measures for determining this point. Its colour was
quite white, not pearl-colour, nor yellow, nor red, and the rays had a
vivid and flickering appearance, somewhat like that which a gaslight
illumination might be supposed to assume if formed into a similar
shape.... Splendid and astonishing, however, as this remarkable
phenomenon really was, and although it could not fail to call forth the
admiration and applause of every beholder, yet I must confess that there
was at the same time something in its singular and wonderful appearance
that was appalling; and I can readily imagine that uncivilised nations
may occasionally have become alarmed and terrified at such an object,
more especially at times when the true cause of the occurrence may have
been but faintly understood, and the phenomenon itself wholly
unexpected.
"But the most remarkable circumstance attending the phenomenon was the
appearance of _three large protuberances_ apparently emanating from the
circumference of the moon, but evidently forming a portion of the
corona. They had the appearance of mountains of a prodigious elevation;
their colour was red, tinged with lilac or purple; perhaps the colour of
the peach-blossom would more nearly represent it. They somewhat
resembled the snowy tops of the Alpine mountains when coloured by the
rising or setting sun. They resembled the Alpine mountains also in
another respect, inasmuch as their light was perfectly steady, and had
none of that flickering or sparkling motion so visible in other parts of
the corona. All the three projections were of the same roseate cast of
colour, and very different from the brilliant vivid white light that
formed the corona; but they differed from each other in magnitude....
The whole of these three protuberances were visible even to the last
moment of total obscuration; at least, I never lost sight of them when
looking in that direction; and when the first ray of light was admitted
from the sun, they vanished, with the corona, altogether, and daylight
was instantaneously restored."[154]
Notwithstanding unfavourable weather, the "red flames" were perceived
with little less clearness and no less amazement from the Superga than
at Pavia, and were even discerned by Mr. Airy with the naked eye. "Their
form" (the Astronomer Royal wrote) "was nearly that of saw-teeth in the
position proper for a circular saw turned round in the same direction in
which the hands of a watch turn.... Their colour was a full lake-red,
and their brilliancy greater than that of any other part of the
ring."[155]
The height of these extraordinary objects was estimated by Arago at two
minutes of arc, representing, at the sun's distance, an actual elevation
of 54,000 miles. When carefully watched, the rose-flush of their
illumination was perceived to fade through violet to white as the light
returned, the same changes in a reversed order having accompanied their
first appearance. Their forms, however, during about three minutes of
visibility, showed no change, although of so apparently unstable a
character as to suggest to Arago "mountains on the point of crumbling
into ruins" through topheaviness.[156]
The corona, both as to figure and extent, presented very different
appearances at different stations. This was no doubt due to varieties in
atmospheric conditions. At the Superga, for instance, all details of
structure seem to have been effaced by the murky air, only a
comparatively feeble ring of light being seen to encircle the moon.
Elsewhere, a brilliant radiated formation was conspicuous, spreading at
four opposite points into four vast luminous expansions, compared to
feather-plumes or _aigrettes_.[157] Arago at Perpignan noticed
considerable irregularities in the divergent rays. Some appeared curved
and twisted, a few lay _across_ the others, in a direction almost
tangential to the moon's limb, the general effect being described as
that of a "hank of thread in disorder."[158] At Lipeszk, where the sun
stood much higher above the horizon than in Italy or France, the corona
showed with surprising splendour. Its apparent extent was judged by
Struve to be no less than twenty-five minutes (more than six times
Airy's estimate), while the great plumes spread their radiance to three
or four degrees from the dark lunar edge. So dazzling was the light that
many well-instructed persons denied the totality of the eclipse. Nor was
the error without precedent, although the appearances attending
respectively a total and an annular eclipse are in reality wholly
dissimilar. In the latter case, the surviving ring of sunlight becomes
so much enlarged by irradiation, that the interposed dark lunar body is
reduced to comparative insignificance, or even invisibility. Maclaurin
tells us[159] that during an eclipse of this character which he observed
at Edinburgh in 1737, "gentlemen by no means shortsighted declared
themselves unable to discern the moon upon the sun without the aid of a
smoked glass;" and Baily (who, however, _was_ shortsighted) could
distinguish, in 1836, with the naked eye, no trace of "the globe of
purple velvet" which the telescope revealed as projected upon the face
of the sun.[160] Moreover, the diminution of light is described by him
as "little more than might be caused by a temporary cloud passing over
the sun"; the birds continued in full song, and "one cock in particular
was crowing with all his might while the annulus was forming."
Very different were the effects of the eclipse of 1842, as to which some
interesting particulars were collected by Arago.[161] Beasts of burthen,
he tells us, paused in their labour, and could by no amount of
punishment be induced to move until the sun reappeared. Birds and beasts
abandoned their food; linnets were found dead in their cages; even ants
suspended their toil. Diligence-horses, on the other hand, seemed as
insensible to the phenomenon as locomotives. The convolvulus and some
other plants closed their leaves, but those of the mimosa remained open.
The little light that remained was of a livid hue. One observer
described the general coloration as resembling the lees of wine, but
human faces showed pale olive or greenish. We may, then, rest assured
that none of the remarkable obscurations recorded in history were due to
eclipses of the annular kind.
The existence of the corona is no modern discovery. Indeed, it is too
conspicuous an apparition to escape notice from the least attentive or
least practised observer of a total eclipse. Nevertheless, explicit
references to it are rare in early times. Plutarch, however, speaks of a
"certain splendour" compassing round the hidden edge of the sun, as a
regular feature of total eclipses;[162] and the corona is expressly
mentioned in a description of an eclipse visible at Corfu in 968
A.D.[163] The first to take the phenomenon into scientific consideration
was Kepler. He showed, from the orbital positions at the time of the sun
and moon, that an eclipse observed by Clavius at Rome in 1567 could not
have been annular,[164] as the dazzling coronal radiance visible during
the obscuration had caused it to be believed. Although he himself never
witnessed a total eclipse of the sun, he carefully collected and
compared the remarks of those more fortunate, and concluded that the
ring of "flame-like splendour" seen on such occasions was caused by the
reflection of the solar rays from matter condensed in the neighbourhood
either of the sun or moon.[165] To the solar explanation he gave his own
decided preference; but, with one of those curious flashes of
half-prophetic insight characteristic of his genius, declared that "it
should be laid by ready for use, not brought into immediate
requisition."[166] So literally was his advice acted upon, that the
theory, which we now know to be (broadly speaking) the correct one, only
emerged from the repository of anticipated truths after 236 years of
almost complete retirement, and even then timorously and with
hesitation.
The first eclipse of which the attendant phenomena were observed with
tolerable exactness was that which was central in the South of France,
May 12, 1706. Cassini then put forward the view that the "crown of pale
light" seen round the lunar disc was caused by the illumination of the
zodiacal light;[167] but it failed to receive the attention which, as a
step in the right direction, it undoubtedly merited. Nine years later we
meet with Halley's comments on a similar event, the first which had
occurred in London since March 20, 1140. By nine in the morning of May
3, 1715, the obscuration, he tells us, "was about ten digits,[168] when
the face and colour of the sky began to change from perfect serene azure
blue to a more dusky livid colour, having an eye of purple intermixt....
A few seconds before the sun was all hid, there discovered itself round
the moon a luminous ring, about a digit or perhaps a tenth part of the
moon's diameter in breadth. It was of a pale whiteness, or rather pearl
colour, seeming to be a little tinged with the colours of the iris, and
to be concentric with the moon, whence I concluded it the moon's
atmosphere. But the great height thereof, far exceeding our earth's
atmosphere, and the observation of some, who found the breadth of the
ring to increase on the west side of the moon as emersion approached,
together with the contrary sentiments of those whose judgment I shall
always revere" (Newton is most probably referred to), "makes me less
confident, especially in a matter whereto I confess I gave not all the
attention requisite." He concludes by declining to decide whether the
"enlightened atmosphere," which the appearance "in all respects
resembled," "belonged to sun or moon."[169]
A French Academician, who happened to be in London at the time, was less
guarded in expressing an opinion. The Chevalier de Louville declared
emphatically for the lunar atmospheric theory of the corona,[170] and
his authority carried great weight. It was, however, much discredited by
an observation made by Maraldi in 1724, to the effect that the luminous
ring, instead of travelling _with_ the moon, was traversed _by_ it.[171]
This was in reality decisive, though, as usual, belief lagged far behind
demonstration. In 1715 a novel explanation had been offered by Delisle
and Lahire,[172] supported by experiments regarded at the time as
perfectly satisfactory. The aureola round the eclipsed sun, they argued,
is simply a result of the _diffraction_, or apparent bending of the
sunbeams that graze the surface of the lunar globe--an effect of the
same kind as the coloured fringes of shadows. And this view prevailed
amongst men of science until (and even after) Brewster showed, with
clear and simple decisiveness, that such an effect could by no
possibility be appreciable at our distance from the moon.[173] Don José
Joaquim de Ferrer, however, who observed a total eclipse of the sun at
Kinderhook, in the State of New York, on June 16, 1806, ignoring this
refined optical _rationale_, considered two alternative explanations of
the phenomenon as alone possible. The bright ring round the moon must be
due to the illumination either of a lunar or of a solar atmosphere. If
the former, he calculated that it should have a height fifty times that
of the earth's gaseous envelope. "Such an atmosphere," he rightly
concluded, "cannot belong to the moon, but must without any doubt belong
to the sun."[174] But he stood alone in this unhesitating assertion.
The importance of the problem was first brought fully home to
astronomers by the eclipse of 1842. The brilliant and complex appearance
which on that occasion challenged the attention of so many observers,
demanded and received, no longer the casual attention hitherto bestowed
upon it, but the most earnest study of those interested in the progress
of science. Nevertheless, it was only by degrees, and through a process
of "exclusions" (to use a Baconian phrase) that the corona was put in
its right place as a solar appendage. As every other available
explanation proved inadmissible and dropped out of sight, the broad
presentation of fact remained, which, though of sufficiently obvious
interpretation, was long and persistently misconstrued. Nor was it until
1869 that absolutely decisive evidence on the subject was forthcoming,
as we shall see further on.
Sir John Herschel, writing to his venerable aunt, relates that when the
brilliant red flames burst into view behind the dark moon on the morning
of the 8th of July, 1842, the populace of Milan, with the usual
inconsequence of a crowd, raised the shout, "_Es leben die
Astronomen!_"[175] In reality, none were less prepared for their
apparition than the class to whom the applause due to the magnificent
spectacle was thus adjudged. And in some measure through their own
fault, for many partial hints and some distinct statements from earlier
observers had given unheeded notice that some such phenomenon might be
expected to attend a solar eclipse.
What we now call the "chromosphere" is an envelope of glowing gases, by
which the sun is completely covered, and from which the "prominences"
are emanations, eruptive or flame-like. Now, continual indications of
the presence of this fire-ocean had been detected during eclipses in the
eighteenth and nineteenth centuries. Captain Stannyan, describing in a
letter to Flamsteed an occurrence of the kind witnessed by him at Berne
on May 1 (o.s.), 1706, says that the sun's "getting out of the eclipse
was preceded by a blood-red streak of light from its left limb."[176] A
precisely similar appearance was noted by both Halley and De Louville in
1715; during annular eclipses by Lord Aberdour in 1737,[177] and by
Short in 1748,[178] the tint of the ruby border being, however, subdued
to "brown" or "dusky red" by the surviving sunlight; while observations
identical in character were made at Amsterdam in 1820,[179] at Edinburgh
by Henderson in 1836, and at New York in 1838.[180]
"Flames" or "prominences," if more conspicuous, are less constant in
their presence than the glowing stratum from which they spring. The
first to describe them was a Swedish professor named Vassenius, who
observed a total eclipse at Gothenburg, May 2 (o.s.), 1733.[181] His
astonishment equalled his admiration when he perceived, just outside the
edge of the lunar disc, and suspended, as it seemed, in the coronal
atmosphere, three or four reddish spots or clouds, one of which was so
large as to be detected with the naked eye. As to their nature, he did
not even offer a speculation, further than by tacitly referring them to
the moon. The observation was repeated in 1778 by a Spanish Admiral, but
with no better success in directing efficacious attention to the
phenomenon. Don Antonio Ulloa was on board his ship the _Espagne_ in
passage from the Azores to Cape St. Vincent on the 24th of June in that
year, when a total eclipse of the sun occurred, of which he has left a
valuable description. His notices of the corona are full of interest;
but what just now concerns us is the appearance of "a red luminous
point" "near the edge of the moon," which gradually increased in size as
the moon moved away from it, and was visible during about a minute and a
quarter.[182] He was satisfied that it belonged to the sun because of
its fiery colour and growth in magnitude, and supposed that it was
occasioned by some crevice or inequality in the moon's limb, through
which the solar light penetrated.
Allusions less precise, both prior and subsequent, which it is now easy
to refer to similar objects (such as the "slender columns of smoke" seen
by Ferrer)[183] might be detailed; but the evidence already adduced
suffices to show that the prominences viewed with such amazement in 1842
were no unprecedented or even unusual phenomenon.
It was more important, however, to decide what was their nature than
whether their appearance might have been anticipated. They were
generally, and not very incorrectly, set down as solar clouds. Arago
believed them to shine by reflected light,[184] but the Abbé Peytal
rightly considered them to be self-luminous. Writing in a Montpellier
paper of July 16, 1842, he declared that we had now become assured of
the existence of a third or outer solar envelope, composed of a glowing
substance of a bright rose tint, forming mountains of prodigious
elevation, analogous in character to the clouds piled above our
horizons.[185] This first distinct recognition of a very important
feature of our great luminary was probably founded on an observation
made by Bérard at Toulon during the then recent eclipse, "of a very fine
red band, irregularly dentelated, or, as it were, crevassed here and
there,"[186] encircling a large arc of the moon's circumference. It can
hardly, however, be said to have attracted general notice until July 28,
1851. On that day a total eclipse took place, which was observed with
considerable success in various parts of Sweden and Norway by a number
of English astronomers. Mr. Hind saw, on the south limb of the moon, "a
long range of rose-coloured flames,"[187] described by Dawes as "a low
ridge of red prominences, resembling in outline the tops of a very
irregular range of hills."[188] Airy termed the portion of this "rugged
lines of projections" visible to him the _sierra_, and was struck with
its brilliant light and "nearly scarlet" colour.[189] Its true character
of a continuous solar envelope was inferred from these data by Grant,
Swan, and Littrow, and was by Father Secchi, after the great eclipse of
1860,[190] formally accepted as established.
Several prominences of remarkable forms, especially one variously
compared to a Turkish scimitar, a sickle, and a boomerang, were seen in
1851. In connection with them two highly significant circumstances were
pointed out. First, that of the approximate coincidence between their
positions and those of sun-spots previously observed.[191] Next, that
"the moon passed over them, leaving them behind, and revealing
successive portions as she advanced."[192] This latter perfectly
well-attested fact was justly considered by the Astronomer Royal and
others as affording absolute certainty of the solar dependence of these
singular objects. Nevertheless sceptics were still found. M. Faye, of
the French Academy, inclined to a lunar origin for them;[193] Feilitsch
of Greifswald published in 1852 a treatise for the express purpose of
proving all the luminous phenomena attendant on solar eclipses--corona,
prominences and "sierra"--to be purely optical appearances.[194]
Happily, however, the unanswerable arguments of the photographic camera
were soon to be made available against such hardy incredulity.
Thus, the virtual discovery of the solar appendages, both coronal and
chromospheric, may be said to have been begun in 1842, and completed in
1851. The current Herschelian theory of the solar constitution remained,
however, for the time, intact. Difficulties, indeed, were thickening
around it; but their discussion was perhaps felt to be premature, and
they were permitted to accumulate without debate, until fortified by
fresh testimony into unexpected and overwhelming preponderance.
FOOTNOTES:
[Footnote 131: Kosmos, Bd. iii., p. 409; Lalande, _Bibliographie
Astronomique_, pp. 179, 202.]
[Footnote 132: R. Wolf, _Die Sonne und ihre Flecken_, p. 9.
Marius himself, however, seems to have held the Aristotelian
terrestrial-exhalation theory of cometary origin. See his curious little
tract, _Astronomische und Astrologische Beschreibung der Cometen_,
Nürnberg, 1619.]
[Footnote 133: _Phil. Trans._, vol. xxvii., p. 274. _Umbræ_ (now called
_penumbræ_) are spaces of half-shadow which usually encircle spots.
_Faculæ_ ("little torches," so named by Scheiner) are bright streaks or
patches closely associated with spots.]
[Footnote 134: _Mém. Ac. Sc._, 1776 (pub. 1779), p. 507. D. Cassini,
however, first put forward about 1671 the hypothesis alluded to in the
text. See Delambre, _Hist. de l'Astr. Mod._, t. ii., p. 694; and
_Kosmos_, Bd. iii., p. 410.]
[Footnote 135: _Phil. Trans._, vol. lxiv., part i., pp. 7-11.]
[Footnote 136: _Rosa Ursina_, lib. iv., p. 507.]
[Footnote 137: R. Wolf, _Die Sonne und ihre Flecken_, p. 12.]
[Footnote 138: Schellen, _Die Spectralanalyse_, Bd. ii., p. 56 (3rd
ed.).]
[Footnote 139: _Phil. Trans._, vol. lxiv., p. 20.]
[Footnote 140: _Ibid._, vol. lxxxv., 1795, p. 63.]
[Footnote 141: _Phil. Trans._, vol. xci., 1801, p. 303.]
[Footnote 142: The supposed opaque or protective stratum beneath the
photosphere was named by him "planetary," from the analogy of
terrestrial clouds.]
[Footnote 143: _Ibid._, p. 305.]
[Footnote 144: _Novum Organum_, lib. ii. aph. 20.]
[Footnote 145: Brewster's _Life of Newton_, vol. ii., p. 103.]
[Footnote 146: _Beschäftigungen d. Berl. Ges. Naturforschender Freunde_,
Bd. ii., p. 233.]
[Footnote 147: _Gentleman's Magazine_, 1787, vol. ii., p. 636.]
[Footnote 148: _Results_, etc., p. 432.]
[Footnote 149: _Ibid._, p. 434.]
[Footnote 150: See _ante_, p. 31.]
[Footnote 151: _Memoir of Francis Baily, Mem. R. A. S._, vol. xv., p.
524.]
[Footnote 152: _Mem. R. A. S._, vol. x., pp. 5-6.]
[Footnote 153: _Ibid._, pp. 14-17.]
[Footnote 154: _Mem. R. A. S._, vol. xv., pp. 4-6.]
[Footnote 155: _Ibid._, p. 16.]
[Footnote 156: _Annuaire_, 1846, p. 409.]
[Footnote 157: _Ibid._, p. 317.]
[Footnote 158: _Ibid._, p. 322.]
[Footnote 159: _Phil. Trans._, vol. xl., p. 192.]
[Footnote 160: _Mem. R. A. S._, vol. x., p. 17.]
[Footnote 161: _Ann. du Bureau des Long._, 1846, p. 309.]
[Footnote 162: _De Facie in Orbe Lunæ_, xix., 10. Cf. Grant, _Astr.
Nach._, No. 1838. As to the phenomenon mentioned by Philostratus in his
_Life of Apollonius_ (viii. 23), see W. T. Lynn, _Observatory_, vol.
ix., p. 128.]
[Footnote 163: Schmidt, _Astr. Nach._, No. 1832.]
[Footnote 164: _Astronomiæ Pars Optica, Op. omnia_, t. ii., p. 317.]
[Footnote 165: _De Stellâ Novâ, Op._, t. ii., pp. 696, 697.]
[Footnote 166: _Astr. Pars Op._, p. 320.]
[Footnote 167: _Mém. de l'Ac. des Sciences_, 1706, p. 119.]
[Footnote 168: A digit = 1/12 of the solar diameter.]
[Footnote 169: _Phil. Trans._, vol. xxix., pp. 247-249.]
[Footnote 170: _Mém. de l'Ac. des Sciences_, 1715; _Histoire_, p. 49;
_Mémoires_, pp. 93-98.]
[Footnote 171: _Ibid._, 1724, p. 178.]
[Footnote 172: _Mém. de l'Ac. des Sciences_, 1715, pp. 161, 166-169.]
[Footnote 173: _Ed. Ency._, art. _Astronomy_, p. 635.]
[Footnote 174: _Trans. Am. Phil. Soc._, vol. vi., p. 274.]
[Footnote 175: _Memoir of Caroline Herschel_, p. 327.]
[Footnote 176: _Phil. Trans._, vol. xxv., p. 2240.]
[Footnote 177: _Ibid._, vol. xl., p. 182.]
[Footnote 178: _Ibid._, vol. xlv., p. 586.]
[Footnote 179: _Mem. R. A. S._, vol. i., pp. 145, 148.]
[Footnote 180: _American Journal of Science_, vol. xlii., p. 396.]
[Footnote 181: _Phil. Trans._, vol. xxxviii., p. 134. Father Secchi,
however, adverted to a distinct mention of a prominence observed in 1239
A.D. A description of a total eclipse of that date includes the remark,
"Et quoddam foramen erat ignitum in circulo solis ex parte inferiore"
(Muratori, _Rer. It. Scriptores_, t. xiv., col. 1097). The "circulus
solis" of course signifies the corona.]
[Footnote 182: _Phil. Trans._, vol. lxix., p. 114.]
[Footnote 183: _Trans. Am. Phil. Soc._, vol. vi., 1809, p. 267.]
[Footnote 184: _Annuaire_, 1846, p. 460.]
[Footnote 185: _Ibid._, p. 439, _note_.]
[Footnote 186: _Ibid._, p. 416.]
[Footnote 187: _Mem. R. A. S._, vol. xxi., p. 82.]
[Footnote 188: _Ibid._, p. 90.]
[Footnote 189: _Ibid._, pp. 7, 8.]
[Footnote 190: _Le Soleil_, t. i., p. 386.]
[Footnote 191: By Williams and Stanistreet, _Mem. R. A. S._, vol. xxi.,
pp. 54, 56. Santini had made a similar observation at Padua in 1842.
Grant, _Hist. Astr._, p. 401.]
[Footnote 192: Lassell in _Month. Not._, vol. xii., p. 53.]
[Footnote 193: _Comptes Rendus_, t. xxxiv., p. 155.]
[Footnote 194: _Optische Untersuchungen_, and _Zeitschrift für populäre
Mittheilungen_, Bd. i., 1860, p. 201.]
CHAPTER IV
_PLANETARY DISCOVERIES_
In the course of his early gropings towards a law of the planetary
distances, Kepler tried the experiment of setting a planet, invisible by
reason of its smallness, to revolve in the vast region of seemingly
desert space separating Mars from Jupiter.[195] The disproportionate
magnitude of the same interval was explained by Kant as due to the
overweening size of Jupiter. The zone in which each planet moved was,
according to the philosopher of Königsberg, to be regarded as the empty
storehouse from which its materials had been derived. A definite
relation should thus exist between the planetary masses and the
planetary intervals.[196] Lambert, on the other hand, sportively
suggested that the body or bodies (for it is noticeable that he speaks
of them in the plural) which once bridged this portentous gap in the
solar system, might, in some remote age, have been swept away by a great
comet, and forced to attend its wanderings through space.[197]
These speculations were destined before long to assume a more definite
form. Johann Daniel Titius, a professor at Wittenberg (where he died in
1796), pointed out in 1772, in a note to a translation of Bonnet's
_Contemplation de la Nature_,[198] the existence of a remarkable
symmetry in the disposition of the bodies constituting the solar system.
By a certain series of numbers, increasing in regular progression,[199]
he showed that the distances of the six known planets from the sun might
be represented with a close approach to accuracy. But with one striking
interruption. The term of the series succeeding that which corresponded
to the orbit of Mars was without a celestial representative. The orderly
flow of the sequence was thus singularly broken. The space where a
planet should--in fulfilment of the "Law"--have revolved, was, it
appeared, untenanted. Johann Elert Bode, then just about to begin his
long career as leader of astronomical thought and work at Berlin, marked
at once the anomaly, and filled the vacant interval with a hypothetical
planet. The discovery of Uranus, at a distance falling but slightly
short of perfect conformity with the law of Titius, lent weight to a
seemingly hazardous prediction, and Von Zach was actually at the pains,
in 1785, to calculate what he termed "analogical" elements[200] for this
unseen and (by any effect or influence) _unfelt_ body. The search for
it, through confessedly scarcely less chimerical than that of alchemists
for the philosopher's stone, he kept steadily in view for fifteen years,
and at length (September 21, 1800) succeeded in organising, in
combination with five other German astronomers assembled at Lilienthal,
a force of what he jocularly termed celestial police, for the express
purpose of tracking and intercepting the fugitive subject of the sun.
The zodiac was accordingly divided for purposes of scrutiny into
twenty-four zones; their apportionment to separate observers was in part
effected, and the association was rapidly getting into working order,
when news arrived that the missing planet had been found, through no
systematic plan of search, but by the diligent, though otherwise
directed labours of a distant watcher of the skies.
Giuseppe Piazzi was born at Ponte in the Valtelline, July 16, 1746. He
studied at various places and times under Tiraboschi, Beccaria,
Jacquier, and Le Sueur; and having entered the Theatine order of monks
at the age of eighteen, he taught philosophy, science, and theology in
several of the Italian cities, as well as in Malta, until 1780, when the
chair of mathematics in the University of Palermo was offered to and
accepted by him. Prince Caramanico, then viceroy of Sicily, had
scientific leanings, and was easily won over to the project of building
an observatory, a commodious foundation for which was afforded by one of
the towers of the viceregal palace. This architecturally incongruous
addition to an ancient Saracenic edifice--once the abode of Kelbite and
Zirite Emirs--was completed in February, 1791. Piazzi, meanwhile, had
devoted nearly three years to the assiduous study of his new profession,
acquiring a practical knowledge of Lalande's methods at the École
Militaire, and of Maskelyne's at the Royal Observatory; and returned to
Palermo in 1789, bringing with him, in the great five-foot circle which
he had prevailed upon Ramsden to construct, the most perfect measuring
instrument hitherto employed by an astronomer.
He had been above nine years at work on his star-catalogue, and was
still profoundly unconscious that a place amongst the Lilienthal
band[201] of astronomical detectives was being held in reserve for him,
when, on the first evening of the nineteenth century, January 1, 1801,
he noticed the position of an eighth-magnitude star in a part of the
constellation Taurus to which an error of Wollaston's had directed his
special attention. Reobserving, according to his custom, the same set of
fifty stars on four consecutive nights, it seemed to him, on the 2nd,
that the one in question had slightly shifted its position to the west;
on the 3rd he assured himself of the fact, and believed that he had
chanced upon a new kind of comet without tail or coma. The wandering
body, whatever its nature, exchanged retrograde for direct motion on
January 14,[202] and was carefully watched by Piazzi until February 11,
when a dangerous illness interrupted his observations. He had, however,
not omitted to give notice of his discovery; but so precarious were
communications in those unpeaceful times, that his letter to Oriani of
January 23 did not reach Milan until April 5, while a missive of one day
later addressed to Bode came to hand at Berlin, March 20. The delay just
afforded time for the publication, by a young philosopher of Jena named
Hegel, of a "Dissertation" showing, by the clearest light of reason,
that the number of the planets could not exceed seven, and exposing the
folly of certain devotees of induction who sought a new celestial body
merely to fill a gap in a numerical series.[203]
Unabashed by speculative scorn, Bode had scarcely read Piazzi's letter
when he concluded that it referred to the precise body in question. The
news spread rapidly, and created a profound sensation, not unmixed with
alarm lest this latest addition to the solar family should have been
found only to be again lost. For by that time Piazzi's moving star was
too near the sun to be any longer visible, and in order to rediscover it
after conjunction a tolerably accurate knowledge of its path was
indispensable. But a planetary orbit had never before been calculated
from such scanty data as Piazzi's observation afforded;[204] and the
attempts made by nearly every astronomer of note in Germany to compass
the problem were manifestly inadequate, failing even to account for the
positions in which the body had been actually seen, and _à fortiori_
serving only to mislead as to the places where, from September, 1801, it
ought once more to have become discernible. It was in this extremity
that the celebrated mathematician Gauss came to the rescue. He was then
in his twenty-fifth year, and was earning his bread by tuition at
Brunswick, with many possibilities, but no settled career before him.
The news from Palermo may be said to have converted him from an
arithmetician into an astronomer. He was already in possession of a new
and more general method of computing elliptical orbits; and the system
of "least squares," which he had devised though not published, enabled
him to extract the most probable result from a given set of
observations. Armed with these novel powers, he set to work; and the
communication in November of his elements and ephemeris for the lost
object revived the drooping hopes of the little band of eager searchers.
Their patience, however, was to be still further tried. Clouds, mist,
and sleet seemed to have conspired to cover the retreat of the fugitive;
but on the last night of the year the sky cleared unexpectedly with the
setting in of a hard frost, and there, in the north-western part of
Virgo, nearly in the position assigned by Gauss to the runaway planet, a
strange star was discerned by Von Zach[205] at Gotha, and on a
subsequent evening--the anniversary of the original discovery--by Olbers
at Bremen. The name of Ceres (as the tutelary goddess of Sicily) was, by
Piazzi's request, bestowed upon this first known of the numerous, and
probably all but innumerable family of the minor planets.
The recognition of the second followed as the immediate consequence of
the detection of the first. Olbers had made himself so familiar with the
positions of the small stars along the track of the long-missing body,
that he was at once struck (March 28, 1802) with the presence of an
intruder near the spot where he had recently identified Ceres. He at
first believed the new-comer to be a variable star usually
inconspicuous, but just then at its maximum of brightness; but within
two hours he had convinced himself that it was no _fixed_ star, but a
rapidly moving object. The aid of Gauss was again invoked, and his
prompt calculations showed that this fresh celestial acquaintance (named
"Pallas" by Olbers), revolved round the sun at nearly the same mean
distance as Ceres, and was beyond question of a strictly analogous
character.
This result was perplexing in the extreme. The symmetry and simplicity
of the planetary scheme appeared fatally compromised by the admission of
many, where room could, according to old-fashioned rules, only be found
for one. A daring hypothesis of Olbers's invention provided an exit from
the difficulty. He supposed that both Ceres and Pallas were fragments of
a primitive trans-Martian planet, blown to pieces in the remote past,
either by the action of internal forces or by the impact of a comet; and
predicted that many more such fragments would be found to circulate in
the same region. He, moreover, pointed out that these numerous orbits,
however much they might differ in other respects, must all have a common
line of intersection,[206] and that the bodies moving in them must
consequently pass, at each revolution, through two opposite points of
the heavens, one situated in the Whale, the other in the constellation
of the Virgin, where already Pallas had been found and Ceres recaptured.
The intimation that fresh discoveries might be expected in those
particular regions was singularly justified by the detection of two
bodies now known respectively as Juno and Vesta. The first was found
near the predicted spot in Cetus by Harding, Schröter's assistant at
Lilienthal, September 2, 1804; the second by Olbers himself in Virgo,
after three years of persistent scrutiny, March 29, 1807.
The theory of an exploded planet now seemed to have everything in its
favour. It required that the mean or average distances of the
newly-discovered bodies should be nearly the same, but admitted a wide
range of variety in the shapes and positions of their orbits, provided
always that they preserved common points of intersection. These
conditions were fulfilled with a striking approach to exactness. Three
of the four "asteroids" (a designation introduced by Sir. W.
Herschel[207]) conformed with very approximate precision to "Bode's law"
of distances; they all traversed, in their circuits round the sun,
nearly the same parts of Cetus and Virgo; while the eccentricities and
inclinations of their paths departed widely from the planetary
type--that of Pallas, to take an extreme instance, making with the
ecliptic an angle of nearly 35°. The minuteness of these bodies appeared
further to strengthen the imputation of a fragmentary character.
Herschel estimated the diameter of Ceres at 162, that of Pallas at 147
miles.[208] But these values are now known to be considerably too small.
A suspected variability of brightness in some of the asteroids, somewhat
hazardously explained as due to the irregularities of figure to be
expected in cosmical _potsherds_ (so to speak), was added to the
confirmatory evidence.[209] The strong point of the theory, however, lay
not in what it explained, but in what it had predicted. It had been
twice confirmed by actual exploration of the skies, and had produced, in
the recognition of Vesta, the first recorded instance of the
_premeditated_ discovery of a heavenly body.
The view not only commended itself to the facile imagination of the
unlearned, but received the sanction of the highest scientific
authority. The great Lagrange bestowed upon it his analytical
_imprimatur_, showing that the explosive forces required to produce the
supposed catastrophe came well within the bounds of possibility; since a
velocity of less than twenty times that of a cannon-ball leaving the
gun's mouth would have sufficed, according to his calculation, to launch
the asteroidal fragments on their respective paths. Indeed, he was
disposed to regard the hypothesis of disruption as more generally
available than its author had designed it to be, and proposed to
supplement with it, as explanatory of the eccentric orbits of comets,
the nebular theory of Laplace, thereby obtaining, as he said, "a
complete view of the origin of the planetary system more conformable to
Nature and mechanical laws than any yet proposed."[210]
Nevertheless the hypothesis of Olbers has not held its ground. It seemed
as if all the evidence available for its support had been produced at
once and spontaneously, while the unfavourable items were elicited
slowly, and, as it were, by cross-examination. A more extended
acquaintance with the group of bodies whose peculiarities it was framed
to explain has shown them, after all, as recalcitrant to any such
explanation. Coincidences at the first view significant and striking
have been swamped by contrary examples; and a hasty general conclusion
has, by a not uncommon destiny, at last perished under the accumulation
of particulars. Moreover, as has been remarked by Professor
Newcomb,[211] mutual perturbations would rapidly efface all traces of a
common disruptive origin, and the catastrophe, to be perceptible in its
effects, should have been comparatively recent.
A new generation of astronomers had arisen before any additions were
made to the little family of the minor planets. Piazzi died in 1826,
Harding in 1834, Olbers in 1840; all those who had prepared or
participated in the first discoveries passed away without witnessing
their resumption. In 1830, however, a certain Hencke, ex-postmaster in
the Prussian town of Driessen, set himself to watch for new planets, and
after fifteen long years his patience was rewarded. The asteroid found
by him, December 8, 1845, received the name of Astræa, and his further
prosecution of the search resulted, July 1, 1847, in the discovery of
Hebe. A few weeks later (August 13), John Russell Hind (1823-1893),
after many months' exploration from Mr. Bishop's observatory in the
Regent's Park, picked up Iris, and October 18, Flora.[212] The next on
the list was Metis, found by Mr. Graham, April 25, 1848, at Markree, in
Ireland.[213] At the close of the period to which our attention is at
present limited, the number of these small bodies known to astronomy was
thirteen; and the course of discovery has since proceeded far more
rapidly and with less interruption.
Both in itself and in its consequences the recognition of the minor
planets was of the highest importance to science. The traditional ideas
regarding the constitution of the solar system were enlarged by the
admission of a new class of bodies, strongly contrasted, yet strictly
co-ordinate with the old-established planetary order; the profusion of
resource, so conspicuous in the living kingdoms of Nature, was seen to
prevail no less in the celestial spaces; and some faint preliminary
notion was afforded of the indefinite complexity of relations underlying
the apparent simplicity of the majestic scheme to which our world
belongs. Both theoretical and practical astronomy derived profit from
the admission of these apparently insignificant strangers to the rights
of citizenship of the solar system. The disturbance of their motions by
their giant neighbours afforded a more accurate knowledge of the Jovian
mass, which Laplace had taken about 1/50 too small; the anomalous
character of their orbits presented geometers with highly stimulating
problems in the theory of perturbation; while the exigencies of the
first discovery had produced the _Theoria Motus_, and won Gauss over to
the ranks of calculating astronomy. Moreover, the sure prospect of
further detections powerfully incited to the exploration of the skies;
observers became more numerous and more zealous in view of the prizes
held out to them; star-maps were diligently constructed, and the
sidereal multitude strewn along the great zodiacal belt acquired a fresh
interest when it was perceived that its least conspicuous member might
be a planetary shred or projectile in the dignified disguise of a
distant sun. Harding's "Celestial Atlas," designed for the special
purpose of facilitating asteroidal research, was the first systematic
attempt to represent to the eye the _telescopic_ aspect of the heavens.
It was while engaged on its construction that the Lilienthal observer
successfully intercepted Juno on her passage through the Whale in 1804;
whereupon promoted to Göttingen, he there completed, in 1822, the
arduous task so opportunely entered upon a score of years previously.
Still more important were the great star-maps of the Berlin Academy,
undertaken at Bessel's suggestion, with the same object of
distinguishing errant from fixed stars, and executed, under Encke's
supervision, during the years 1830-59. They have played a noteworthy
part in the history of planetary discovery, nor of the minor kind alone.
We have now to recount an event unique in scientific history. The
discovery of Neptune has been characterised as the result of a "movement
of the age,"[214] and with some justice. It had become necessary to the
integrity of planetary theory. Until it was accomplished, the phantom of
an unexplained anomaly in the orderly movements of the solar system must
have continued to haunt astronomical consciousness. Moreover, it was
prepared by many, suggested as possible by not a few, and actually
achieved, simultaneously, independently, and completely, by two
investigators.
The position of the planet Uranus was recorded as that of a fixed star
no less than twenty times between 1690 and the epoch of its final
detection by Herschel. But these early observations, far from affording
the expected facilities for the calculation of its orbit, proved a
source of grievous perplexity. The utmost ingenuity of geometers failed
to combine them satisfactorily with the later Uranian places, and it
became evident, either that they were widely erroneous, or that the
revolving body was wandering from its ancient track. The simplest course
was to reject them altogether, and this was done in the new Tables
published in 1821 by Alexis Bouvard, the indefatigable computating
partner of Laplace. But the trouble was not thus to be got rid of. After
a few years fresh irregularities began to appear, and continued to
increase until absolutely "intolerable." It may be stated as
illustrative of the perfection to which astronomy had been brought, that
divergencies regarded as menacing the very foundation of its theories
never entered the range of unaided vision. In other words, if the
theoretical and the real Uranus had been placed side by side in the sky,
they would have seemed, to the sharpest eye, to form a single body.[215]
The idea that these enigmatical disturbances were due to the attraction
of an unknown exterior body was a tolerably obvious one; and we
accordingly find it suggested in many different quarters. Bouvard
himself was perhaps the first to conceive it. He kept the possibility
continually in view, and bequeathed to his nephew's diligence the
inquiry into its reality when he felt that his own span was drawing to a
close; but before any progress had been made with it, he had already
(June 7, 1843) "ceased to breathe and to calculate." The Rev. T. J.
Hussey actually entertained in 1834 the notion, but found his powers
inadequate to the task, of assigning an approximate place to the
disturbing body; and Bessel, in 1840, laid his plans for an assault in
form upon the Uranian difficulty, the triumphant exit from which fatal
illness frustrated his hopes of effecting or even witnessing.
The problem was practically untouched when, in 1841, an undergraduate of
St. John's College, Cambridge, formed the resolution of grappling with
it. The projected task was an arduous one. There were no guiding
precedents for its conduct. Analytical obstacles had to be encountered
so formidable as to appear invincible even to such a mathematician as
Airy. John Couch Adams, however, had no sooner taken his degree, which
he did as senior wrangler in January, 1843, than he set resolutely to
work, and on October 21, 1845, was able to communicate to the Astronomer
Royal numerical estimates of the elements and mass of the unknown
planet, together with an indication of its actual place in the heavens.
These results, it has been well said,[216] gave "the final and
inexorable proof" of the validity of Newton's Law. The date October 21,
1845, "may therefore be regarded as marking a distinct epoch in the
history of gravitational astronomy."
Sir George Biddell Airy had begun in 1835 his long and energetic
administration of the Royal Observatory, and was already in possession
of data vitally important to the momentous inquiry then on foot. At his
suggestion, and under his superintendence, the reduction of all the
planetary observations made at Greenwich from 1750 onwards had been
undertaken in 1833. The results, published in 1846, constituted a
permanent and universal stock of materials for the correction of
planetary theory. But in the meantime, investigators, both native and
foreign, were freely supplied with the "places and errors," which,
clearly exhibiting the discrepancies between observation and
calculation--between what _was_ and what was _expected_--formed the very
groundwork of future improvements.
Mr. Adams had no reason to complain of official discourtesy. His labours
received due and indispensable aid; but their purpose was regarded as
chimerical. "I have always," Sir George Airy wrote,[217] "considered the
correctness of a distant mathematical result to be a subject rather of
moral than of mathematical evidence." And that actually before him
seemed, from its very novelty, to incur a suspicion of unlikelihood. No
problem in planetary disturbance had heretofore been attacked, so to
speak, from the rear. The inverse method was untried, and might well be
deemed impracticable. For the difficulty of determining the
perturbations produced by a given planet is small compared with the
difficulty of finding a planet by its resulting perturbations. Laplace
might have quailed before it; yet it was now grappled with as a first
essay in celestial dynamics. Moreover, Adams unaccountably neglected to
answer until too late a question regarded by Airy in the light of an
_experimentum crucis_ as to the soundness of the new theory. Nor did he
himself take any steps to obtain a publicity which he was more anxious
to merit than to secure. The investigation consequently remained buried
in obscurity. It is now known that had a search been instituted in the
autumn of 1845 for the remote body whose existence had been so
marvellously foretold, it would have been found within _three and a half
lunar diameters_ (1° 49') of the spot assigned to it by Adams.
A competitor, however, equally daring and more fortunate--_audax fortunâ
adjutus_, as Gauss said of him--was even then entering the field. Urbain
Jean Joseph Leverrier, the son of a small Government _employé_ in
Normandy, was born at Saint-Lô, March 11, 1811. He studied with
brilliant success at the École Polytechnique, accepted the post of
astronomical teacher there in 1837, and, "docile to circumstance,"
immediately concentrated the whole of his vast, though as yet
undeveloped powers upon the formidable problems, of celestial mechanics.
He lost no time in proving to the mathematical world that the race of
giants was not extinct. Two papers on the stability of the solar system,
presented to the Academy of Sciences, September 16 and October 14, 1839,
showed him to be the worthy successor of Lagrange and Laplace, and
encouraged hopes destined to be abundantly realised. His attention was
directed by Arago to the Uranian difficulty in 1845, when he cheerfully
put aside certain intricate cometary researches upon which he happened
to be engaged, in order to obey with dutiful promptitude the summons of
the astronomical chief of France. In his first memoir on the subject
(communicated to the Academy, November 10, 1845), he proved the
inadequacy of all known causes of disturbance to account for the
vagaries of Uranus; in a second (June 1, 1848), he demonstrated that
only an exterior body, occupying at a certain date a determinate
position in the zodiac, could produce the observed effects; in a third
(August 31, 1846), he assigned the orbit of the disturbing body, and
announced its visibility as an object with a sensible disc about as
bright as a star of the eighth magnitude.
The question was now visibly approaching an issue. On September 10, Sir
John Herschel declared to the British Association respecting the
hypothetical new planet: "We see it as Columbus saw America from the
coast of Spain. Its movements have been felt, trembling along the
far-reaching line of our analysis with a certainty hardly inferior to
that of ocular demonstration." Less than a fortnight later, September
23, Professor Galle, of the Berlin Observatory, received a letter from
Leverrier requesting his aid in the telescopic part of the inquiry
already analytically completed. He directed his refractor to the heavens
that same night, and perceived, within less than a degree of the spot
indicated, an object with a measurable disc nearly three seconds in
diameter. Its absence from Bremiker's recently-completed map of that
region of the sky showed it to be no star, and its movement in the
predicted direction confirmed without delay the strong persuasion of its
planetary nature.[218]
In this remarkable manner the existence of the remote member of our
system known as "Neptune" was ascertained. But the discovery, which
faithfully reflected the duplicate character of the investigation which
led to it, had been already secured at Cambridge before it was announced
from Berlin. Sir George Airy's incredulity vanished in the face of the
striking coincidence between the position assigned by Leverrier to the
unknown planet in June, and that laid down by Adams in the previous
October; and on the 9th of July he wrote to Professor Challis, director
of the Cambridge Observatory, recommending a search with the great
Northumberland equatoreal. Had a good star-map been at hand, the process
would have been a simple one; but of Bremiker's "Hora XXI." no news had
yet reached England, and there was no other sufficiently comprehensive
to be available for an inquiry which, in the absence of such aid,
promised to be both long and laborious. As the event proved, it might
have been neither. "After four days of observing," Challis wrote,
October 12, 1846, to Airy, "the planet was in my grasp if only I had
examined or mapped the observations."[219] Had he done so, the first
honours in the discovery, both theoretical and optical, would have
fallen to the University of Cambridge. But Professor Challis had other
astronomical avocations to attend to, and, moreover, his faith in the
precision of the indications furnished to him was, by his own
confession, a very feeble one. For both reasons he postponed to a later
stage of the proceedings the discussion and comparison of the data
nightly furnished to him by his telescope, and thus allowed to lie, as
it were, latent in his observations the momentous result which his
diligence had insured, but which his delay suffered to be
anticipated.[220]
Nevertheless, it should not be forgotten that the Berlin astronomer had
two circumstances in his favour apart from which his swift success could
hardly have been achieved. The first was the possession of a good
star-map; the second was the clear and confident nature of Leverrier's
instructions. "Look where I tell you," he seemed authoritatively to say,
"and you will see an object such as I describe."[221] And in fact, not
only Galle on the 23rd of September, but also Challis on the 29th,
immediately after reading the French geometer's lucid and impressive
treatise, picked out from among the stellar points strewing the zodiac,
a small planetary disc, which eventually proved to be that of the
precise body he had been in search of during two months.
The controversy that ensued had its ignominious side; but it was entered
into by neither of the parties principally concerned. Adams bore the
disappointment, which the dilatory proceedings at Greenwich and
Cambridge had inflicted upon him, with quiet heroism. His silence on the
subject of what another man would have called his wrongs remained
unbroken to the end of his life;[222] and he took every opportunity of
testifying his admiration for the genius of Leverrier.
Personal questions, however, vanish in the magnitude of the event they
relate to. By it the last lingering doubts as to the absolute exactness
of the Newtonian Law were dissipated. Recondite analytical methods
received a confirmation brilliant and intelligible even to the minds of
the vulgar, and emerged from the patient solitude of the study to enjoy
an hour of clamorous triumph. For ever invisible to the unaided eye of
man, a sister-globe to our earth was shown to circulate, in perpetual
frozen exile, at thirty times its distance from the sun. Nay, the
possibility was made apparent that the limits of our system were not
even thus reached, but that yet profounder abysses of space might
shelter obedient, though little favoured, members of the solar family,
by future astronomers to be recognised through the sympathetic
thrillings of Neptune, even as Neptune himself was recognised through
the tell-tale deviations of Uranus.
It is curious to find that the fruit of Adams's and Leverrier's
laborious investigations had been accidentally all but snatched half a
century before it was ripe to be gathered. On the 8th, and again on the
10th of May, 1795, Lalande noted the position of Neptune as that of a
fixed star, but perceiving that the two observations did not agree, he
suppressed the first as erroneous, and pursued the inquiry no further.
An immortality which he would have been the last to despise hung in the
balance; the feather-weight of his carelessness, however, kicked the
beam, and the discovery was reserved to be more hardly won by later
comers.
Bode's Law did good service in the quest for a trans-Uranian planet by
affording ground for a probable assumption as to its distance. A
starting-point for approximation was provided by it; but it was soon
found to be considerably at fault. Even Uranus is about 36 millions of
miles nearer to the sun than the order of progression requires; and
Neptune's vast distance of 2,800 million should be increased by no less
than 800 million miles, and its period of 165 lengthened out to 225
years,[223] in order to bring it into conformity with the curious and
unexplained rule which planetary discoveries have alternately tended to
confirm and to invalidate.
Within seventeen days of its identification with the Berlin achromatic,
Neptune was found to be attended by a satellite. This discovery was the
first notable performance of the celebrated two-foot reflector[224]
erected by Mr. Lassell at his suggestively named residence of Starfield,
near Liverpool. William Lassell was a brewer by profession, but by
inclination an astronomer. Born at Bolton in Lancashire, June 18, 1799,
he closed a life of eminent usefulness to science, October 5, 1818, thus
spanning with his well-spent years four-fifths of the momentous period
which we have undertaken to traverse. At the age of twenty-one, being
without the means to purchase, he undertook to construct telescopes, and
naturally turned his attention to the reflecting sort, as favouring
amateur efforts by the comparative simplicity of its structure. His
native ingenuity was remarkable, and was developed by the hourly
exigencies of his successive enterprises. Their uniform success
encouraged him to enlarge his aims, and in 1844 he visited Birr Castle
for the purpose of inspecting the machine used in polishing the giant
speculum of Parsonstown. In the construction of his new instrument,
however, he eventually discarded the model there obtained, and worked on
a method of his own, assisted by the supreme mechanical skill of James
Nasmyth. The result was a Newtonian of exquisite definition, with an
aperture of two, and a focal length of twenty feet, provided by a novel
artifice with the equatoreal mounting, previously regarded as available
only for refractors.
This beautiful instrument afforded to its maker, October 10, 1846, a
cursory view of a Neptunian attendant. But the planet was then
approaching the sun, and it was not until the following July that the
observation could be verified, which it was completely, first by Lassell
himself, and somewhat later by Otto Stuve and Bond of Cambridge (U.S.).
When it is considered that this remote object shines by reflecting
sunlight reduced by distance to 1/900th of the intensity with which it
illuminates our moon, the fact of its visibility, even in the most
perfect telescopes, is a somewhat surprising one. It can only, indeed,
be accounted for by attributing to it dimensions very considerable for a
body of the secondary order. It shares with the moons of Uranus the
peculiarity of retrograde motion; that is to say, its revolutions,
running counter to the grand current of movement in the solar system,
are performed from east to west, in a plane inclined at an angle of 35°
to that of the ecliptic. Their swiftness serves to measure the mass of
the globe round which they are performed. For while our moon takes
twenty-seven days and nearly eight hours to complete its circuit of the
earth, the satellite of Neptune, at a distance not greatly inferior,
sweeps round its primary in five days and twenty-one hours, showing
(according to a very simple principle of computation) that it is urged
by a force seventeen times greater than the terrestrial pull upon the
lunar orb. Combining this result with those of Professor Barnard's[225]
and Dr. See's[226] recent measurements of the small telescopic disc of
this farthest known planet, it is found that while in _mass_ Neptune
equals seventeen, in _bulk_ it is equivalent to forty-nine earths. This
is as much as to say that it is composed of relatively very light
materials, or more probably of materials distended by internal heat, as
yet unwasted by radiation into space, to about five times the volume
they would occupy in the interior of our globe. The fact, at any rate,
is fairly well ascertained, that the average density of Neptune is about
twice that of water.
We must now turn from this late-recognised member of our system to
bestow some brief attention upon the still fruitful field of discovery
offered by one of the immemorial five. The family of Saturn, unlike that
of its brilliant neighbour, has been gradually introduced to the notice
of astronomers. Titan, the sixth Saturnian moon in order of distance,
led the way, being detected by Huygens, March 25, 1655; Cassini made the
acquaintance of four more between 1671 and 1684; while Mimas and
Enceladus, the two innermost, were caught by Herschel in 1789, as they
threaded their lucid way along the edge of the almost vanished ring. In
the distances of these seven revolving bodies from their primary, an
order of progression analogous to that pointed out by Titius in the
planetary intervals was found to prevail; but with one conspicuous
interruption, similar to that which had first suggested the search for
new members of the solar system. Between Titan and Japetus--the sixth
and seventh reckoning outwards--there was obviously room for another
satellite. It was discovered on both sides of the Atlantic
simultaneously, on the 19th of September, 1848. Mr. W. C. Bond,
employing the splendid 15-inch refractor of the Harvard Observatory,
noticed, September 16, a minute star situated in the plane of Saturn's
rings. The same object was discerned by Mr. Lassell on the 18th. On the
following evening, both observers perceived that the problematical speck
of light kept up with, instead of being left behind by the planet as it
moved, and hence inferred its true character.[227] Hyperion, the seventh
by distance and eighth by recognition of Saturn's attendant train, is of
so insignificant a size when compared with some of its fellow-moons
(Titan is but little inferior to the planet Mars), as to have suggested
to Sir John Herschel[228] the idea that it might be only one of several
bodies revolving very close together--in fact, an _asteroidal
satellite_; but the conjecture has, so far, not been verified.
The coincidence of its duplicate discovery was singularly paralleled two
years later. Galileo's amazement when his "optic glass" revealed to him
the "triple" form of Saturn--_planeta tergeminus_--has proved to be,
like the laughter of the gods, "inextinguishable." It must revive in
every one who contemplates anew the unique arrangements of that world
apart known to us as the Saturnian system. The resolution of the
so-called _ansæ_, or "handles," into one encircling ring by Huygens in
1655, the discovery by Cassini in 1675 of the division of that ring into
two concentric ones, together with Laplace's investigation of the
conditions of stability of such a formation, constituted, with some
minor observations, the sum of the knowledge obtained, up to the middle
of the last century, on the subject of this remarkable formation. The
first place in the discovery now about to be related belongs to an
American astronomer.
William Cranch Bond, born in 1789 at Portland, in the State of Maine,
was a watchmaker, whom the solar eclipse of 1806 attracted to study the
wonders of the heavens. When, in 1815, the erection of an observatory in
connection with Harvard College, Cambridge, was first contemplated, he
undertook a mission to England for the purpose of studying the working
of similar institutions there, and on his return erected a private
observatory at Dorchester, where he worked diligently for many years.
Then at last, in 1843, the long-postponed design of the Harvard
authorities was resumed, and on the completion of the new establishment,
Bond, who had been from 1838 officially connected with the College and
had carried on his scientific labours within its precincts, was offered
and accepted the post of its director. Placed in 1847 in possession of
one of the finest instruments in the world--a masterpiece of Merz and
Mahler--he headed the now long list of distinguished Transatlantic
observers. Like the elder Struve, he left an heir to his office and to
his eminence, but George Bond unfortunately died in 1865, at the early
age of thirty-nine, having survived his father but six years.
On the night of November 15, 1850--the air, remarkably enough, being so
hazy that only the brightest stars could be perceived with the naked
eye--William Bond discerned a dusky ring, extending about halfway
between the inner brighter one and the globe of Saturn. A fortnight
later, but before the observation had been announced in England, the
same appearance was seen by the Rev. W. R. Dawes with the comparatively
small refractor of his observatory at Wateringbury, and on December 3
was described by Mr. Lassell (then on a visit to him) as "something like
a crape veil covering a part of the sky within the inner ring."[229]
Next morning the _Times_ containing the report of Bond's discovery
reached Wateringbury. The most surprising circumstance in the matter was
that the novel appendage had remained so long unrecognised. As the rings
opened out to their full extent, it became obvious with very moderate
optical assistance; yet some of the most acute observers who have ever
lived, using instruments of vast power, had heretofore failed to detect
its presence. It soon appeared, however, that Galle of Berlin[230] had
noticed, June 10, 1838, a veil-like extension of the lucid ring across
half the dark space separating it from the planet; but the observation,
although communicated at the time to the Berlin Academy of Sciences, had
remained barren. Traces of the dark ring, moreover, were found in
drawings executed by Campani in 1664[231] and by Hooke in 1666;[232]
while Picard (June 15, 1673),[233] Hadley (spring of 1720),[234] and
Herschel,[235] had all undoubtedly seen it under the aspect of a dark
bar or belt crossing the Saturnian globe. It was, then, of no recent
origin; but there seemed reason to think that it had lately gained
considerably in brightness. The full meaning of this suspected change it
was reserved for later investigations to develop.
What we may, in a certain sense, call the closing result of the race for
discovery, in which several observers seemed at that time to be engaged,
was the establishment, on a satisfactory footing, of our acquaintance
with the dependent system of Uranus. Sir William Herschel, whose
researches formed, in so many distinct lines of astronomical inquiry,
the starting-points of future knowledge, detected, January 11,
1787,[236] two Uranian moons, since called Oberon and Titania, and
ascertained the curious circumstance of their motion in a plane almost
at right angles to the ecliptic, in a direction contrary to that of all
previously known denizens (other than cometary) of the solar kingdom. He
believed that he caught occasional glimpses of four more, but never
succeeded in assuring himself of their substantial existence. Even the
two first remained unseen save by himself until 1828, when his son
re-observed them with a 20-foot reflector, similar to that with which
they had been originally discovered. Thenceforward they were kept fairly
within view, but their four questionable companions, in spite of some
false alarms of detection, remained in the dubious condition in which
Herschel had left them. At last, on October 24, 1851,[237] after some
years of fruitless watching, Lassell espied "Ariel" and "Umbriel," two
Uranian attendants, interior to Oberon and Titania, and of about half
their brightness; so that their disclosure is still reckoned amongst the
very highest proofs of instrumental power and perfection. In all
probability they were then for the first time seen; for although
Professor Holden[238] made out a plausible case in favour of the fitful
visibility to Herschel of each of them in turn, Lassell's argument[239]
that the glare of the planet in Herschel's great specula must have
rendered almost impossible the perception of objects so minute and so
close to its disc, appears tolerably decisive to the contrary. Uranus is
thus attended by four moons, and, so far as present knowledge extends,
by no more. Among the most important of the "negative results"[240]
secured by Lassell's observations at Malta during the years 1852-53 and
1861-65, were the convincing evidence afforded by them that, without
great increase of optical power, no further Neptunian or Uranian
satellites can be perceived, and the consequent relegation of Herschel's
baffling quartette, notwithstanding the unquestioned place long assigned
to them in astronomical text-books, to the Nirvana of non-existence.
FOOTNOTES:
[Footnote 195: _Op._, t. i., p. 107. He interposed, but tentatively
only, another similar body between Mercury and Venus.]
[Footnote 196: _Allgemeine Naturgeschichte_ (ed. 1798), pp. 118, 119.]
[Footnote 197: _Cosmologische Briefe_, No. 1 (quoted by Von Zach,
_Monat. Corr._, vol. iii., p. 592).]
[Footnote 198: Second ed., p. 7. See Bode, _Von dem neuen
Hauptplaneten_, p. 43, _note_.]
[Footnote 199: The representative numbers are obtained by adding 1 to
the following series (irregular, it will be observed, in its first
member, which should be 1/2 instead of 0); 0, 3, 6, 12, 24, 48, etc. The
formula is a purely empirical one, and is, moreover, completely at fault
as regards the distance of Neptune.]
[Footnote 200: _Monat. Corr._, vol. iii., p. 596.]
[Footnote 201: Wolf, _Geschichte der Astronomie_, p. 648.]
[Footnote 202: Such reversals of direction in the apparent movements of
the planets are a consequence of the earth's revolution in its orbit.]
[Footnote 203: _Dissertatio Philosophica de Orbitis Planetarum_, 1801.
See Wolf, _Gesch. d. Astr._, p. 685.]
[Footnote 204: Observations on Uranus, as a supposed fixed star, went
back to 1690.]
[Footnote 205: He had caught a glimpse of it on December 7, but was
prevented by bad weather from verifying his suspicion. _Monat. Corr._,
vol. v., p. 171.]
[Footnote 206: Planetary fragments, hurled _in any direction_, and _with
any velocity_ short of that which would for ever release them from the
solar sway, would continue to describe elliptic orbits round the sun,
all passing through the scene of the explosion, and thus possessing a
common line of intersection.]
[Footnote 207: _Phil. Trans._, vol. xcii., part ii., p. 228.]
[Footnote 208: _Ibid._, p. 218. In a letter to Von Zach of June 24,
1802, he speaks of Pallas as "almost incredibly small," and makes it
only seventy English miles in diameter. _Monat. Corr._, vol. vi., pp.
89, 90.]
[Footnote 209: Olbers, _Monat. Corr._, vol. vi., p. 88.]
[Footnote 210: _Conn. d. Tems_ for 1814, p. 218.]
[Footnote 211: _Popular Astronomy_, p. 327.]
[Footnote 212: _Month. Not._, vol. vii., p. 299; vol. viii., p. 1.]
[Footnote 213: _Ibid._, p. 146.]
[Footnote 214: Airy, _Mem. R. A. S._, vol. xvi., p. 386.]
[Footnote 215: See Newcomb's _Pop. Astr._, p. 359. The error of Uranus
amounted, in 1844, to 2'; but even the tailor of Breslau, whose
extraordinary powers of vision Humboldt commemorates (_Kosmos_, Bd. ii.,
p. 112), could only see Jupiter's first satellite at its greatest
elongation, 2' 15". He might, however, possibly have distinguished two
objects of _equal_ lustre at a lesser interval.]
[Footnote 216: J. W. L. Glaisher, _Observatory_, vol. xv., p. 177.]
[Footnote 217: _Mem. R. A. S._, vol. xvi., p. 399.]
[Footnote 218: For an account of D'Arrest's share in the detection see
_Copernicus_, vol. ii., pp. 63, 96.]
[Footnote 219: _Mem. R. A. S._, vol. xvi., p. 412.]
[Footnote 220: He had recorded the places of 3,150 stars (three of which
were different positions of the planet), and was preparing to map them,
when, October 1, news of the discovery arrived from Berlin. Prof.
Challis's _Report_, quoted in Obituary Notice, _Month. Not._, Feb.,
1883, p. 170.]
[Footnote 221: See Airy in _Mem. R. A. S._, vol. xvi., p. 411.]
[Footnote 222: He died January 21, 1892, in his 71st year.]
[Footnote 223: Ledger, _The Sun, its Planets and their Satellites_, p.
414.]
[Footnote 224: Presented by the Misses Lassell, after their father's
death, to the Greenwich Observatory.]
[Footnote 225: _Astr. Jour._, No. 508.]
[Footnote 226: _Report of U.S. Naval Observatory for 1900_, p. 15.]
[Footnote 227: Grant, _Hist. of Astr._, p. 271.]
[Footnote 228: _Month. Not._, vol. ix., p. 91.]
[Footnote 229: _Month. Not._, vol. xi., p. 21.]
[Footnote 230: _Astr. Nach._, No. 756 (May 2, 1851).]
[Footnote 231: _Phil. Trans._, vol. i., p. 246. See H. T. Vivian, _Engl.
Mech._, April 20, 1894.]
[Footnote 232: Secchi, _Month. Not._, vol. xiii., p. 248.]
[Footnote 233: Hind, _ibid._, vol. xv., p. 32.]
[Footnote 234: Lynn, _Observatory_, Oct. 1, 1883; Hadley, _Phil.
Trans._, vol. xxxii., p. 385.]
[Footnote 235: Proctor, _Saturn and its System_, p. 64.]
[Footnote 236: _Phil. Trans._, vol. lxxvii., p. 125.]
[Footnote 237: _Month. Not._, vol. xi., p. 248.]
[Footnote 238: _Ibid._, vol. xxxv., pp. 16-22.]
[Footnote 239: _Ibid._, p. 26.]
[Footnote 240: _Ibid._, vol. xli., p. 190.]
CHAPTER V
_COMETS_
Newton showed that the bodies known as "comets," or _hirsute_ stars,
obey the law of gravitation; but it was by no means certain that the
individual of the species observed by him in 1680 formed a permanent
member of the solar system. The velocity, in fact, of its rush round the
sun was quite possibly sufficient to carry it off for ever into the
depths of space, there to wander, a celestial casual, from star to star.
With another comet, however, which appeared two years later, the case
was different. Edmund Halley, who afterwards succeeded Flamsteed as
Astronomer Royal, calculated the elements of its orbit on Newton's
principles, and found them to resemble so closely those similarly
arrived at for comets observed by Peter Apian in 1531, and by Kepler in
1607, as almost to compel the inference that all three were apparitions
of a single body. This implied its revolution in a period of about
seventy-six years, and Halley accordingly fixed its return for 1758-9.
So fully alive was he to the importance of the announcement that he
appealed to a "candid posterity," in the event of its verification, to
acknowledge that the discovery was due to an Englishman. The prediction
was one of the test-questions put by Science to Nature, on the replies
to which largely depend both the development of knowledge and the
conviction of its reality. In the present instance, the answer afforded
may be said to have laid the foundation of this branch of astronomy.
Halley's comet punctually reappeared on Christmas Day, 1758, and
effected its perihelion passage on the 12th of March following, thus
proving beyond dispute that some at least of these erratic bodies are
domesticated within our system, and strictly conform, if not to its
unwritten customs (so to speak), at any rate to its fundamental laws.
Their movements, in short, were demonstrated by the most unanswerable of
all arguments--that of verified calculation--to be _calculable_, and
their investigation was erected into a legitimate department of
astronomical science.
This notable advance was the chief _result_ obtained in the field of
inquiry just now under consideration during the eighteenth century. But
before it closed, its cultivation had received a powerful stimulus
through the invention of an improved _method_. The name of Olbers has
already been brought prominently before our readers in connection with
asteroidal discoveries; these, however, were but chance excursions from
the path of cometary research which he steadily pursued through life. An
early predilection for the heavens was fixed in this particular
direction by one of the happy inspirations of genius. As he was
watching, one night in the year 1779, by the sick-bed of a
fellow-student in medicine at Göttingen, an important simplification in
the mode of computing the paths of comets occurred to him. Although not
made public until 1797, "Olbers's method" was then universally adopted,
and is still regarded as the most expeditious and convenient in cases
where absolute rigour is not required. By its introduction, not only
many a toilsome and thankless hour was spared, but workers were
multiplied, and encouraged in the prosecution of labours more useful
than attractive.
The career of Heinrich Olbers is a brilliant example of what may be done
by an amateur in astronomy. He at no time did regular work in an
observatory; he was never the possessor of a transit or any other fixed
instrument; moreover, all the best years of his life were absorbed in
the assiduous exercise of a toilsome profession. Born in 1758 at the
village of Arbergen, where his father was pastor, he settled in 1781 as
a physician in the neighbouring town of Bremen, and continued in active
practice there for over forty years. It was thus only the hours which
his robust constitution enabled him to spare from sleep that were
available for his intellectual pleasures. Yet his recreation was, as Von
Zach remarked,[241] no less prolific of useful results than the severest
work of other men. The upper part of his house in the Sandgasse was
fitted up with such instruments and appliances as restrictions of space
permitted, and there, night after night during half a century and
upwards, he discovered, calculated, or observed the cometary visitants
of northern skies. Almost as effective in promoting the interests of
science as the valuable work actually done by him, was the influence of
his genial personality. He engaged confidence by his ready and
discerning sympathy; he inspired affection by his benevolent
disinterestedness; he quickened thought and awakened zeal by the
suggestions of a lively and inventive spirit, animated with the warmest
enthusiasm for the advancement of knowledge. Nearly every astronomer in
Germany enjoyed the benefits of a frequently active correspondence with
him, and his communications to the scientific periodicals of the time
were numerous and striking. The motive power of his mind was thus widely
felt and continually in action. Nor did it wholly cease to be exerted
even when the advance of age and the progress of infirmity rendered him
incapable of active occupation. He was, in fact, _alive_ even to the
last day of his long life of eighty-one years; and his death, which
occurred March 2, 1840, left vacant a position which a rare combination
of moral and intellectual qualities had conspired to render unique.
Amongst the many younger men who were attracted and stimulated by
intercourse with him was Johann Franz Encke. But while Olbers became a
mathematician because he was an astronomer, Encke became an astronomer
because he was a mathematician. A born geometer, he was naturally sent
to Göttingen and placed under the tuition of Gauss. But geometers are
men; and the contagion of patriotic fervour which swept over Germany
after the battle of Leipsic did not spare Gauss's promising pupil. He
took up arms in the Hanseatic Legion, and marched and fought until the
oppressor of his country was safely ensconced behind the ocean-walls of
St. Helena. In the course of his campaigning he met Lindenau, the
militant director of the Seeberg Observatory, and by his influence was
appointed his assistant, and eventually, in 1822, became his successor.
Thence he was promoted in 1825 to Berlin, where he superintended the
building of the new observatory, so actively promoted by Humboldt, and
remained at its head until within some eighteen months of his death in
August, 1865.
On the 26th of November, 1818, Pons of Marseilles discovered a comet,
whose inconspicuous appearance gave little promise of its becoming one
of the most interesting objects in our system. Encke at once took the
calculation of its elements in hand, and brought out the unexpected
result that it revolved round the sun in a period of about 3-1/3
years.[242] He, moreover, detected its identity with comets seen by
Méchain in 1786, by Caroline Herschel in 1795, by Pons, Huth, and
Bouvard in 1805, and after six laborious weeks of research into the
disturbances experienced by it from the planets during the entire
interval since its first ascertained appearance, he fixed May 24, 1822,
as the date of its next return to perihelion. Although on that occasion,
owing to the position of the earth, invisible in the northern
hemisphere, Sir Thomas Brisbane's observatory at Paramatta was
fortunately ready equipped for its recapture, which Rümker effected
quite close to the spot indicated by Encke's ephemeris.
The importance of this event can be better understood when it is
remembered that it was only the second instance of the recognised return
of a comet (that of Halley's, sixty-three years previously, having, as
already stated, been the first); and that it, moreover, established the
existence of a new class of celestial objects, somewhat loosely
distinguished as "comets of short period." These bodies (of which about
thirty have been found to circulate within the orbit of Saturn) are
remarkable as showing certain planetary affinities in the manners of
their motions not at all perceptible in the wider travelling members of
their order. They revolve, without exception, in the same direction as
the planets--from west to east; they exhibit a marked tendency to
conform to the zodiacal track which limits planetary excursions north
and south; and their paths round the sun, although much more eccentric
than the approximately circular planetary orbits, are far less so than
the extravagantly long ellipses in which comets comparatively untrained
(as it were) in the habits of the solar system ordinarily perform their
revolutions.
No _great_ comet is of the "planetary" kind. These are, indeed, only by
exception visible to the naked eye; they possess extremely feeble
tail-producing powers, and give small signs of central condensation.
Thin wisps of cosmical cloud, they flit across the telescopic field of
view without sensibly obscuring the smallest star. Their appearance, in
short, suggests--what some notable facts in their history will presently
be shown to confirm--that they are bodies already effete, and verging
towards dissolution. If it be asked what possible connection can be
shown to exist between the shortness of period by which they are
essentially characterised, and what we may call their _superannuated_
condition, we are not altogether at a loss for an answer. Kepler's
remark,[243] that comets are consumed by their own emissions, has
undoubtedly a measure of truth in it. The substance ejected into the
tail must, in overwhelmingly large proportion, be for ever lost to the
central mass from which it issues. True, it is of a nature inconceivably
tenuous; but unrepaired waste, however small in amount, cannot be
persisted in with impunity. The incitement to such self-spoliation
proceeds from the sun; it accordingly progresses more rapidly the more
numerous are the returns to the solar vicinity. Comets of short period
may thus reasonably be expected to _wear out_ quickly.
They are, moreover, subject to many adventures and vicissitudes. Their
aphelia--or the farthest points of their orbits from the sun--are
usually, if not invariably, situated so near to the path either of
Jupiter or of Saturn, as to permit these giant planets to act as
secondary rulers of their destinies. By their influence they were, in
all likelihood, originally fixed in their present tracks; and by their
influence, exerted in an opposite sense, they may, in some cases, be
eventually ejected from them. Careers so varied, as can easily be
imagined, are apt to prove instructive, and astronomers have not been
backward in extracting from them the lessons they are fitted to convey.
Encke's comet, above all, has served as an index to much curious
information, and it may be hoped that its function in that respect is by
no means at an end. The great extent of the solar system traversed by
its eccentric path makes it peculiarly useful for the determination of
the planetary masses. At perihelion it penetrates within the orbit of
Mercury; it considerably transcends at aphelion the farthest excursion
of Pallas. Its vicinity to the former planet in August, 1835, offered
the first convenient opportunity of placing that body in the
astronomical balance. Its weight or mass had previously been assumed,
not ascertained; and the comparatively slight deviation from its regular
course impressed upon the comet by its attractive power showed that it
had been assumed nearly twice too great.[244] That fundamental datum of
planetary astronomy--the mass of Jupiter--was corrected by similar
means; and it was reassuring to find the correction in satisfactory
accord with that already introduced from observations of the asteroidal
movements.
The fact that comets contract in approaching the sun had been noticed by
Hevelius; Pingré admitted it with hesitating perplexity;[245] the
example of Encke's comet rendered it conspicuous and undeniable. On the
28th of October, 1828, the diameter of the nebulous matter composing
this body was estimated at 312,000 miles. It was then about one and a
half times further from the sun than the earth is at the time of the
equinox. On the 24th of December following, its distance being reduced
by nearly two-thirds, it was found to be only 14,000 miles across.[246]
That is to say, it had shrunk during those two months of approach to
1/11000th part of its original volume! Yet it had still seventeen days'
journey to make before reaching perihelion. The same curious
circumstance was even more markedly apparent at its return in 1838. Its
bulk, or the actual space occupied by it, appeared to be reduced, as it
drew near the hearth of our system, in the enormous proportion of
800,000 to 1. A corresponding expansion accompanied on each occasion its
retirement from the sphere of observation. Similar changes of volume,
though rarely to the same astounding extent, have been perceived in
other comets. They still remain unexplained; but it can scarcely be
doubted that they are due to the action of the same energetic internal
forces which reveal themselves in so many splendid and surprising
cometary phenomena.
Another question of singular interest was raised by Encke's acute
inquiries into the movements and disturbances of the first known "comet
of short period." He found from the first that its revolutions were
subject to some influence besides that of gravity. After every possible
allowance had been made for the pulls, now backward, now forward,
exerted upon it by the several planets, there was still a surplus of
acceleration left unaccounted for. Each return to perihelion took place
about two and a half hours sooner than received theories warranted.
Here, then, was a "residual phenomenon" of the utmost promise for the
disclosure of novel truths. Encke (in accordance with the opinion of
Olbers) explained it as due to the presence in space of some such
"subtle matter" as was long ago invoked by Euler[247] to be the agent of
eventual destruction for the fair scheme of planetary creation. The
apparent anomaly of accounting for an accelerative effect by a retarding
cause disappears when it is considered that any check to the motion of
bodies revolving round a centre of attraction causes them to draw closer
to it, thus shortening their periods and quickening their circulation.
If space were filled with a resisting medium capable of impeding, even
in the most infinitesimal degree, the swift course of the planets, their
orbits should necessarily be, not ellipses, but very close elliptical
spirals along which they would slowly, but inevitably, descend into the
burning lap of the sun. The circumstance that no such tendency can be
traced in their revolutions by no means sets the question at rest. For
it might well be that an effect totally imperceptible until after the
lapse of countless ages, as regards the solid orbs of our system, might
be obvious in the movements of bodies like comets of small mass and
great bulk; just as a feather or a gauze veil at once yields its motion
to the resistance of the air, while a cannon-ball cuts its way through
with comparatively slight loss of velocity.
It will thus be seen that issues of the most momentous character hang on
the _time-keeping_ of comets; for plainly all must in some degree suffer
the same kind of hindrance as Encke's, if the cause of that hindrance be
the one suggested. None of its congeners, however, show any trace of
similar symptoms. True, the late Professor Oppolzer announced,[248] in
1880, that a comet, first seen by Pons in 1819, and rediscovered by
Winnecke in 1858, having a period of 2,052 days (5·6 years), was
accelerated at each revolution precisely in the manner required by
Encke's theory. But M. von Haerdtl's subsequent investigation, the
materials for which included numerous observations of the body in
question at its return to the sun in 1886, decisively negatived the
presence of any such effect.[249] Moreover, the researches of Von Asten
and Backlund[250] into the movements of Encke's comet revealed a
perplexing circumstance. They confirmed Encke's results for the period
covered by them, but exhibited the acceleration as having _suddenly
diminished_ by nearly one-half in 1868. The reality and permanence of
this change were fully established by observations of the ensuing return
in March, 1885. Some physical alteration of the retarded body seems
indicated; but visual evidence countenances no such assumption. In
aspect the comet is no less thin and diffuse than in 1795 or in 1848.
The character of the supposed resistance in inter-planetary space has,
it may be remarked, been often misapprehended. What Encke stipulated for
was not a medium equally diffused throughout the visible universe, such
as the ethereal vehicle of the vibrations of light, but a rare fluid,
rapidly increasing in density towards the sun.[251] This cannot be a
solar atmosphere, since it is mathematically certain, as Laplace has
shown,[252] that no envelope partaking of the sun's axial rotation can
extend farther from his surface than nine-tenths of the mean distance of
Mercury; while physical evidence assures us that the _actual_ depth of
the solar atmosphere bears a very minute proportion to the _possible_
depth theoretically assigned to it. That matter, however, not
atmospheric in its nature--that is, neither forming one body with the
sun nor altogether aëriform--exists in its neighbourhood, can admit of
no reasonable doubt. The great lens-shaped mass of the zodiacal light,
stretching out at times far beyond the earth's orbit, may indeed be
regarded as an extension of the corona, the streamers of which
themselves mark the wide diffusion, all round the solar globe, of
granular or gaseous materials. Yet comets have been known to penetrate
the sphere occupied by them without perceptible loss of velocity. The
hypothesis, then, of a resisting medium receives at present no
countenance from the movements of comets, whether of short or of long
periods.
Although Encke's comet has made thirty-five complete rounds of its orbit
since its first detection in 1786, it shows no certain signs of decay.
Variations in its brightness are, it is true, conspicuous, but they do
not proceed continuously.[253]
The history of the next known planet-like comet has proved of even more
curious interest than that of the first. It was discovered by an
Austrian officer named Wilhelm von Biela at Josephstadt in Bohemia,
February 27, 1826, and ten days later by the French astronomer Gambart
at Marseilles. Both observers computed its orbit, showed its remarkable
similarity to that traversed by comets visible in 1772 and 1805, and
connected them together as previous appearances of the body just
detected by assigning to its revolutions a period of between six and
seven years. The two brief letters conveying these strikingly similar
inferences were printed side by side in the same number of the
_Astronomische Nachrichten_ (No. 94); but Biela's priority in the
discovery of the comet was justly recognised by the bestowal upon it of
his name.
The object in question was at no time, subsequently to 1805, visible to
the naked eye. Its aspect in Sir John Herschel's great reflector on the
23rd of September, 1832, was described by him as that of a "conspicuous
nebula," nearly 3 minutes in diameter. No trace of a tail was
discernible. While he was engaged in watching it, a small knot of minute
stars was directly traversed by it, "and when on the cluster," he tells
us,[254] it "presented the appearance of a nebula resolvable and partly
resolved into stars, the stars of the cluster being visible through the
comet." Yet the depth of cometary matter through which such faint
stellar rays penetrated undimmed, was, near the central parts of the
globe, not less than 50,000 miles.
It is curious to find that this seemingly harmless, and we may perhaps
add effete body, gave occasion to the first (and not the last) cometary
"scare" of an enlightened century. Its orbit, at the descending node,
may be said to have intersected that of the earth; since, according as
it _bulged in or out_ under the disturbing influence of the planets, the
passage of the comet was affected _inside_ or _outside_ the terrestrial
track. Now, certain calculations published by Olbers in 1828[255] showed
that, on October 29, 1832, a considerable portion of its nebulous
surroundings would actually sweep over the spot which, a month later,
would be occupied by our planet. It needed no more to set the popular
imagination in a ferment. Astronomers, after all, could not, by an
alarmed public, be held to be infallible. Their computations, it was
averred, which a trifling oversight would suffice to vitiate, exhibited
clearly enough the danger, but afforded no guarantee of safety from a
collision, with all the terrific consequences frigidly enumerated by
Laplace. Nor did the panic subside until Arago formally demonstrated
that the earth and the comet could by no possibility approach within
less than fifty millions of miles.[256]
The return of the same body in 1845-46 was marked by an extraordinary
circumstance. When first seen, November 28, it wore its usual aspect of
a faint round patch of cosmical fog; but on December 19, Mr. Hind
noticed that it had become distorted somewhat into the form of a pear;
and ten days later, it had divided into two separate objects. This
singular duplication was first perceived at New Haven in America,
December 29,[257] by Messrs. Herrick and Bradley, and by Lieutenant
Maury at Washington, January 13, 1846. The earliest British observer of
the phenomenon (noticed by Wichmann the same evening at Königsberg) was
Professor Challis. "I see _two_ comets!" he exclaimed, putting his eye
to the great equatoreal of the Cambridge Observatory on the night of
January 15; then, distrustful of what his senses had told him, he called
in his judgment to correct their improbable report by resolving one of
the dubious objects into a hazy star.[258] On the 23rd, however, both
were again seen by him in unmistakable cometary shape, and until far on
in March (Otto Struve caught a final glimpse of the pair on the 16th of
April),[259] continued to be watched with equal curiosity and amazement
by astronomers in every part of the northern hemisphere. What Seneca
reproved Ephorus for supposing to have taken place in 373 b.c.--what
Pingré blamed Kepler for conjecturing in 1618--had then actually
occurred under the attentive eyes of science in the middle of the
nineteenth century!
At a distance from each other of about two-thirds the distance of the
moon from the earth, the twin comets meantime moved on tranquilly, so
far, at least, as their course through the heaven was concerned. Their
extreme _lightness_, or the small amount of matter contained in each,
could not have received a more signal illustration than by the fact that
their revolutions round the sun were performed independently; that is to
say, they travelled side by side without experiencing any appreciable
mutual disturbance, thus plainly showing that at an interval of only
157,250 miles their attractive power was virtually inoperative. Signs of
internal agitation, however, were not wanting. Each fragment threw out a
short tail in a direction perpendicular to the line joining their
centres, and each developed a bright nucleus, although the original
comet had exhibited neither of these signs of cometary vitality. A
singular interchange of brilliancy was, besides, observed to take place
between the coupled objects, each of which alternately outshone and was
outshone by the other, while an arc of light, apparently proceeding from
the more lustrous, at times bridged the intervening space. Obviously,
the gravitational tie, rendered powerless by exiguity of matter, was
here replaced by some other form of mutual action, the nature of which
can as yet be dealt with only by conjecture.
Once more, in August, 1852, the double comet returned to the
neighbourhood of the sun, but under circumstances not the most
advantageous for observation. Indeed, the companion was not detected
until September 16, when Father Secchi at Rome perceived it to have
increased its distance from the originating body to a million and a
quarter of miles, or about eight times the average interval at the
former appearance. Both vanished shortly afterwards, and have never
since been seen, notwithstanding the eager watch kept for objects of
such singular interest, and the accurate knowledge of their track
supplied by Santini's investigations. A dangerously near approach to
Jupiter in 1841 is believed to have occasioned their disruption, and the
disaggregating process thus started was likely to continue. We can
scarcely doubt that the fate has overtaken them which Newton assigned as
the end of all cometary existence. _Diffundi tandem et spargi per
coelos universos._[260]
Biela's is not the only vanished comet. Brorsen's, discovered at Kiel in
1846, and observed at four subsequent returns, failed unaccountably to
become visible in 1890.[261] Yet numerous sentinels were on the alert to
surprise its approach along a well-ascertained track, traversed in five
and a half years. The object presented from the first a somewhat
time-worn aspect. It was devoid of tail, or any other kind of appendage;
and the rapid loss of the light acquired during perihelion passage was
accompanied by inordinate expansion of an already tenuous globular mass.
Another lost or mislaid comet is one found by De Vico at Rome, August
22, 1844. It was expected to return early in 1850, but did not, and has
never since been seen; unless its re-appearance as E. Swift's comet of
1894 should be ratified by closer inquiry.[262]
A telescopic comet with a period of 7-1/2 years, discovered November 22,
1843, by M. Faye of the Paris Observatory, formed the subject of a
characteristically patient and profound inquiry on the part of
Leverrier, designed to test its suggested identity with Lexell's comet
of 1770. The result was decisive against the hypothesis of Valz, the
divergences between the orbits of the two bodies being found to increase
instead of to diminish, as the history of the new-comer was traced
backward into the previous century.[263] Faye's comet pursues the most
nearly circular path of any similar known object; even at its nearest
approach to the sun it remains farther off than Mars when he is most
distant from it; and it was proved by the admirable researches of
Professor Axel Möller,[264] director of the Swedish observatory of Lund,
to exhibit no trace of the action of a resisting medium.
Periodical comets are evidently bodies which have each lived through a
chapter of accidents, and a significant hint as to the nature of their
adventures can be gathered from the fact that their aphelia are pretty
closely grouped about the tracks of the major planets. Halley's, and
five other comets are thus related to Neptune; three connect themselves
with Uranus, two with Saturn, above a score with Jupiter. Some form of
dependence is plainly indicated, and the researches of Tisserand,[265]
Callandreau,[266] and Newton[267] of Yale College, leave scarcely a
doubt that the "capture-theory" represents the essential truth in the
matter. The original parabolic paths of these comets were then changed
into ellipses by the backward pull of a planet, whose sphere of
attraction they chanced to enter when approaching the sun from outer
space. Moreover, since a body thus affected should necessarily return at
each revolution to the scene of encounter, the same process of
retardation may, in some cases, have been repeated many times, until the
more restricted cometary orbits were reduced to their present
dimensions. The prevalence, too, among periodical comets, of direct
motion, is shown to be inevitable by M. Callandreau's demonstration that
those travelling in a retrograde direction would, by planetary action,
be thrown outside the probable range of terrestrial observation. The
scarcity of hyperbolic comets can be similarly explained. They would be
created whenever the attractive influence of the disturbing planet was
exerted in a forward or accelerative sense, but could come only by a
rare exception to our notice. The inner planets, including the earth,
have also unquestionably played their parts in modifying cometary
orbits; and Mr. Plummer suggests, with some show of reason, that the
capture of Encke's comet may be a feat due to Mercury.[268]
No _great_ comet appeared between the "star" which presided at the birth
of Napoleon and the "vintage" comet of 1811. The latter was first
described by Flaugergues at Viviers, March 26, 1811; Wisniewski, at
Neu-Tscherkask in Southern Russia, caught a final glimpse of it, August
17, 1812. Two disappearances in the solar rays as the earth moved round
in its orbit, and two reappearances after conjunction, were included in
this unprecedentedly long period of visibility of 510 days. This
relative permanence (so far as the inhabitants of Europe were concerned)
was due to the high northern latitude attained near perihelion, combined
with a certain leisureliness of movement along a path everywhere
external to that of the earth. The magnificent luminous train of this
body, on October 15, the day of its nearest terrestrial approach,
covered an arc of the heavens 23-1/2 degrees in length, corresponding to
a real extension of one hundred millions of miles. Its form was
described by Sir William Herschel as that of "an inverted hollow cone,"
and its colour as yellowish, strongly contrasted with the bluish-green
tint of the "head," round which it was flung like a transparent veil.
The planetary disc of the head, 127,000 miles across, appeared to be
composed of strongly-condensed nebulous matter; but somewhat
eccentrically situated within it was a star-like nucleus of a reddish
tinge, which Herschel presumed to be solid, and ascertained, with his
usual care, to have a diameter of 428 miles. From the total absence of
phases, as well as from the vivacity of its radiance, he confidently
inferred that its light was not borrowed, but inherent.[269]
This remarkable apparition formed the subject of a memoir by
Olbers,[270] the striking yet steadily reasoned out suggestions
contained in which there was at that time no means of following up with
profit. Only of late has the "electrical theory," of which Zöllner[271]
regarded Olbers as the founder, assumed a definite and measurable form,
capable of being tested by the touchstone of fact, as knowledge makes
its slow inroads on the fundamental mystery of the physical universe.
The paraboloidal shape of the bright envelope separated by a dark
interval from the head of the great comet of 1811, and constituting, as
it were, the _root_ of its tail, seemed to the astronomer of Bremen to
reveal the presence of a double repulsion; the expelled vapours
accumulating where the two forces, solar and cometary, balanced each
other, and being then swept backwards in a huge train. He accordingly
distinguished three classes of these bodies:--First, comets which
develop _no_ matter subject to solar repulsion. These have no tails, and
are probably mere nebulosities, without solid nuclei. Secondly, comets
which are acted upon by solar repulsion _only_, and consequently throw
out no emanations _towards_ the sun. Of this kind was a bright comet
visible in 1807.[272] Thirdly, comets like that of 1811, giving evidence
of action of both kinds. These are distinguished by a dark _hoop_
encompassing the head and dividing it from the luminous envelope, as
well as by an obscure caudal axis, resulting from the hollow, cone-like
structure of the tail.
Again, the ingenious view subsequently propounded by M. Bredikhin as to
the connection between the _form_ of these appendages and the _kind_ of
matter composing them, was very clearly anticipated by Olbers. The
amount of tail-curvature, he pointed out, depends in each case upon the
proportion borne by the velocity of the ascending particles to that of
the comet in its orbit; the swifter the outrush, the straighter the
resulting tail. But the velocity of the ascending particles varies with
the energy of their repulsion by the sun, and this again, it may be
presumed, with their quality. Thus multiple tails are developed when the
same comet throws off, as it approaches perihelion, specifically
distinct substances. The long, straight ray which proceeded from the
comet of 1807, for example, was doubtless made up of particles subject
to a much more vigorous solar repulsion than those formed into the
shorter curved emanation issuing from it nearly in the same direction.
In the comet of 1811 he calculated that the particles expelled from the
head travelled to the remote extremity of the tail in eleven minutes,
indicating by this enormous rapidity of movement (comparable to that of
the transmission of light) the action of a force much more powerful than
the opposing one of gravity. The not uncommon phenomena of multiple
envelopes, on the other hand, he explained as due to the varying amounts
of repulsion exercised by the nucleus itself on the different kinds of
matter developed from it.
The movements and perturbations of the comet of 1811 were no less
profoundly studied by Argelander than its physical constitution by
Olbers. The orbit which he assigned to it is of such vast dimensions as
to require no less that 3,065 years for the completion of its circuit;
and to carry the body describing it at each revolution to fourteen times
the distance from the sun of the frigid Neptune. Thus, when it last
visited our neighbourhood, Achilles may have gazed on its imposing train
as he lay on the sands all night bewailing the loss of Patroclus; and
when it returns, it will perhaps be to shine upon the ruins of empires
and civilizations still deep buried among the secrets of the coming
time.[273]
On the 26th of June, 1819, while the head of a comet passed across the
face of the sun, the earth was in all probability involved in its tail.
But of this remarkable double event nothing was known until more than a
month later, when the fact of its past occurrence emerged from the
calculations of Olbers.[274] Nor had the comet itself been generally
visible previous to the first days of July. Several observers, however,
on the publication of these results, brought forward accounts of
singular spots perceived by them upon the sun at the time of the
transit, and an original drawing of one of them, by Pastorff of
Buchholtz, has been preserved. This undoubtedly authentic
delineation[275] represents a round nebulous object with a _bright_ spot
in the centre, of decidedly cometary aspect, and not in the least like
an ordinary solar "macula." Mr. Hind,[276] nevertheless, showed its
position on the sun to be irreconcilable with that which the comet must
have occupied; and Mr. Ranyard's discovery of a similar smaller drawing
by the same author, dated May 26, 1828,[277] reduces to evanescence the
probability of its connection with that body. Indeed, recent experience
renders very doubtful the possibility of such an observation.
The return of Halley's comet in 1835 was looked forward to as an
opportunity for testing the truth of floating cometary theories, and did
not altogether disappoint expectation. As early as 1817, its movements
and disturbances since 1759 were proposed by the Turin Academy of
Sciences as the subject of a prize ultimately awarded to Baron
Damoiseau. Pontécoulant was adjudged a similar distinction by the Paris
Academy in 1829; while Rosenberger's calculations were rewarded with the
gold medal of the Royal Astronomical Society.[278]
They were verified by the detection at Rome, August 6, 1835, of a nearly
circular misty object not far from the predicted place of the comet. It
was not, however, until the middle of September that it began to throw
out a tail, which by the 15th of October had attained a length of about
24 degrees (on the 19th, at Madras, it extended to fully 30),[279] the
head showing to the naked eye as a reddish star rather brighter than
Aldebaran or Antares.[280] Some curious phenomena accompanied the
process of tail-formation. An outrush of luminous matter, resembling in
shape a partially opened fan, issued from the nucleus _towards_ the sun,
and at a certain point, like smoke driven before a high wind, was
vehemently swept backwards in a prolonged train. The appearance of the
comet at this time was compared by Bessel,[281] who watched it with
minute attention, to that of a blazing rocket. He made the singular
observation that this fan of light, which seemed the source of supply
for the tail, oscillated like a pendulum to and fro across a line
joining the sun and nucleus, in a period of 4-3/5 days; and he was
unable to escape from the conclusion[282] that a repulsive force, about
twice as powerful as the attractive force of gravity, was concerned in
the production of these remarkable effects. Nor did he hesitate to recur
to the analogy of magnetic polarity, or to declare, still more
emphatically than Olbers, "the emission of the tail to be a purely
electrical phenomenon."[283]
The transformations undergone by this body were almost as strange and
complete as those which affected the brigands in Dante's _Inferno_. When
first seen, it wore the aspect of a nebula; later it put on the
distinctive garb of a comet; it next appeared as a star; finally, it
dilated, first in a spherical, then in a paraboloidal form, until May 5,
1836, when it vanished from Herschel's observation at Feldhausen as if
by melting into adjacent space from the excessive diffusion of its
light. A very uncommon circumstance in its development was that it lost
all trace of tail _previous_ to its arrival at perihelion on the 16th of
November. Nor did it begin to recover its elongated shape for more than
two months afterwards. On the 23rd of January, Boguslawski perceived it
as a star of the sixth magnitude, _without measurable disc_.[284] Only
two nights later, Maclear, director of the Cape Observatory, found the
head to be 131 seconds across.[285] And so rapidly did the augmentation
of size progress, that Sir John Herschel estimated the actual bulk of
this singular object to have increased forty-fold in the ensuing week.
"I can hardly doubt," he remarks, "that the comet was fairly evaporated
in perihelio by the heat, and resolved into transparent vapour, and is
now in process of rapid condensation and re-precipitation on the
nucleus."[286] A plausible, but no longer admissible, interpretation of
this still unexplained phenomenon. The next return of this body, which
will be considerably accelerated by Jupiter's influence, is expected to
take place in 1910.[287]
By means of an instrument devised to test the quality of light, Arago
obtained decisive evidence that some at least of the radiance proceeding
from Halley's comet was derived by reflection from the sun.[288]
Indications of the same kind had been afforded[289] by the comet which
suddenly appeared above the north-western horizon of Paris, July 3,
1819, after having enveloped (as already stated) our terrestrial abode
in its filmy appendages; but the "polariscope" had not then reached the
perfection subsequently given to it, and its testimony was accordingly
far less reliable than in 1835. Such experiments, however, are in
reality more beautiful and ingenious than instructive, since ignited as
well as obscure bodies possess the power of throwing back light incident
upon them, and will consequently transmit to us from the neighbourhood
of the sun rays partly direct, partly reflected, of which a certain
proportion will exhibit the peculiarity known as polarisation.
The most brilliant comets of the century were suddenly rivalled if not
surpassed by the extraordinary object which blazed out beside the sun,
February 28, 1843. It was simultaneously perceived in Mexico and the
United States, in Southern Europe, and at sea off the Cape of Good Hope,
where the passengers on board the _Owen Glendower_ were amazed by the
sight of a "short, dagger-like object," closely following the sun
towards the western horizon.[290] At Florence, Amici found its distance
from the sun's centre at noon to be only 1° 23'; and spectators at Parma
were able, when sheltered from the direct glare of mid-day, to trace the
tail to a length of four or five degrees. The full dimensions of this
astonishing appurtenance began to be disclosed a few days later. On the
3rd of March it measured 25°, and on the 11th, at Calcutta, Mr. Clerihew
observed a second streamer, nearly twice as long as the first, and
making an angle with it of 18°, to have been emitted in a single day.
This rapidity of projection, Sir John Herschel remarked, "conveys an
astounding impression of the intensity of the forces at work." "It is
clear," he continued, "that _if we have to deal here with matter, such
as we conceive it_--viz., _possessing inertia--at all_, it must be under
the dominion of forces incomparably more energetic than gravitation, and
quite of a different nature."[291]
On the 17th of March a silvery ray, some 40° long and slightly curved at
its extremity, shone out above the sunset clouds in this country. No
previous intimation had been received of the possibility of such an
apparition, and even astronomers--no lightning messages across the seas
being as yet possible--were perplexed. The nature of the phenomenon,
indeed, soon became evident, but the wonder of it did not diminish with
the study of its attendant circumstances. Never before, within
astronomical memory, had our system been traversed by a body pursuing
such an adventurous career. The closest analogy was offered by the great
comet of 1680 (Newton's), which rushed past the sun at a distance of
only 144,000 miles; but even this--on the cosmical scale--scarcely
perceptible interval was reduced nearly one-half in the case we are now
concerned with. The centre of the comet of 1843 approached the
formidable luminary within 78,000 miles, leaving, it is estimated, a
clear space of not more than 32,000 between the surfaces of the bodies
brought into such perilous proximity. The escape of the wanderer was,
however, secured by the extraordinary rapidity of its flight. It swept
past perihelion at a rate--366 miles a second--which, if continued,
would have carried it right round the sun in _two hours_; and in only
eleven minutes more than that short period it actually described half
the _curvature_ of its orbit--an arc of 180°--although in travelling
over the remaining half many hundreds of sluggish years will doubtless
be consumed.
The behaviour of this comet may be regarded as an _experimentum crucis_
as to the nature of tails. For clearly no fixed appendage many millions
of miles in length could be whirled like a brandished sabre from one
side of the sun to the other in 131 minutes. Cometary trains are then,
as Olbers rightly conceived them to be, emanations, not
appendages--inconceivably rapid outflows of highly rarefied matter, the
greater part, if not all, of which becomes permanently detached from the
nucleus.
That of the comet of 1843 reached, about the time that it became visible
in this country, the extravagant length of 200 millions of miles.[292]
It was narrow, and bounded by nearly parallel and nearly rectilinear
lines, resembling--to borrow a comparison of Aristotle's--a "road"
through the constellations; and after the 3rd of March showed no trace
of hollowness, the axis being, in fact, rather brighter than the edges.
Distinctly perceptible in it were those singular aurora-like
coruscations which gave to the "tresses" of Charles V.'s comet the
appearance--as Cardan described it--of "a torch agitated by the wind,"
and have not unfrequently been observed to characterise other similar
objects. A consideration first adverted to by Olbers proves these to
originate in our own atmosphere. For owing to the great difference in
the distances from the earth of the origin and extremity of such vast
effluxes, the light proceeding from their various parts is transmitted
to our eyes in notably different intervals of time. Consequently a
luminous undulation, even though propagated instantaneously from end to
end of a comet's tail, would appear to us to occupy many minutes in its
progress. But the coruscations in question pass as swiftly as a falling
star. They are, then, of terrestrial production.
Periods of the utmost variety were by different computators assigned to
the body, which arrived at perihelion, February 27, 1843, at 9.47 p.m.
Professor Hubbard of Washington found that it required 533 years to
complete a revolution; MM. Laugier and Mauvais of Paris considered the
true term to be 35;[293] Clausen looked for its return at the end of
between six and seven. A recent discussion[294] by Professor Kreutz of
all the available data gives a probable period of 512 years for this
body, and precludes its hypothetical identity with the comet of 1668,
known as the "Spina" of Cassini.
It may now be asked, what were the conclusions regarding the nature of
comets drawn by astronomers from the considerable amount of novel
experience accumulated during the first half of this century? The first
and best assured was that the matter composing them is in a state of
extreme tenuity. Numerous and trustworthy observations showed that the
feeblest rays of light might traverse some hundreds of thousands of
miles of their substance, even where it was apparently most condensed,
without being perceptibly weakened. Nay, instances were recorded in
which stars were said to have gained in brightness from the
process![295] On the 24th of June, 1825, Olbers[296] saw the comet then
visible all but obliterated by the central passage of a star too small
to be distinguished with the naked eye, its own light remaining wholly
unchanged. A similar effect was noted December 1, 1811, when the great
comet of that year approached so close to Altair, the _lucida_ of the
Eagle, that the star seemed to be transformed into the nucleus of the
comet.[297] Even the central blaze of Halley's comet in 1835 was
powerless to impede the passage of stellar rays. Struve[298] observed at
Dorpat, on September 17, an all but central occultation; Glaisher[299]
one (so far as he could ascertain) absolutely so eight days later at
Cambridge. In neither case was there any appreciable diminution of the
star's light. Again, on the 11th of October, 1847, Mr. Dawes,[300] an
exceptionally keen observer, distinctly saw a star of the tenth
magnitude through the exact centre of a comet discovered on the first of
that month by Maria Mitchell of Nantucket.
Examples, on the other hand, are not wanting of the diminution of
stellar light under similar circumstances;[301] and we meet two alleged
instances of the vanishing of a star behind a comet. Wartmann of Geneva
observed the first, November 28, 1828;[302] but his instrument was
defective, and the eclipsing body, Encke's comet, has shown itself
otherwise perfectly translucent. The second case of occultation occurred
September 13, 1890, when an eleventh magnitude star was stated to have
completely disappeared during the transit over it of Denning's
comet.[303]
From the failure to detect any effects of refraction in the light of
stars occulted by comets, it was inferred (though, as we know now,
erroneously) that their composition is rather that of dust than that of
vapour; that they consist not of any continuous substance, but of
discrete solid particles, very finely divided and widely scattered. In
conformity with this view was the known smallness of their masses.
Laplace had shown that if the amount of matter forming Lexell's comet
had been as much as 1/5000 of that contained in our globe, the effect of
its attraction, on the occasion of its approach within 1,438,000 miles
of the earth, July 1, 1770, must have been apparent in the lengthening
of the year. And that some comets, at any rate, possess masses
immeasurably below this maximum value was clearly proved by the
undisturbed parallel march of the two fragments of Biela's in 1846.
But the discovery in this branch most distinctive of the period under
review is that of "short period" comets, of which four[304] were known
in 1850. These, by the character of their movements, serve as a link
between the planetary and cometary worlds, and by the nature of their
construction, seem to mark a stage in cometary decay. For that comets
are rather transitory agglomerations, than permanent products of
cosmical manufacture, appeared to be demonstrated by the division and
disappearance of one amongst their number, as well as by the singular
and rapid changes in appearance undergone by many, and the seemingly
irrevocable diffusion of their substance visible in nearly all. They
might then be defined, according to the ideas respecting them prevalent
fifty years ago, as bodies unconnected by origin with the solar system,
but encountered, and to some extent appropriated, by it in its progress
through space, owing their visibility in great part, if not altogether,
to light reflected from the sun, and their singular and striking forms
to the action of repulsive forces emanating from him, the penalty of
their evanescent splendour being paid in gradual waste and final
dissipation and extinction.
FOOTNOTES:
[Footnote 241: _Allgemeine Geographische Ephemeriden_, vol. iv., p.
287.]
[Footnote 242: _Astr. Jahrbuch_, 1823, p. 217. The period (1,208 days)
of this body is considerably shorter than that of any other known
comet.]
[Footnote 243: "Sicut bombyces filo fundendo, sic cometas cauda
exspiranda consumi et denique mori."--_De Cometis_, Op., vol. vii., p.
110.]
[Footnote 244: Considerable uncertainty, however, still prevails on the
point. The inverse relation assumed by Lagrange to exist between
distance from the sun and density brought out the Mercurian mass
1/2025810 that of the sun (Laplace, _Exposition du Syst. du Monde_, t.
ii., p. 50, ed. 1824). Von Asten deduced from the movements of Encke's
comet, 1818-48, a value of 1/7636440; while Backlund from its seven
returns, 1871-1891, derived 1/9647000 (_Comptes Rendus_, Oct. 1, 1894).]
[Footnote 245: Arago, _Annuaire_ (1832), p. 218.]
[Footnote 246: Hind, _The Comets_, p. 20.]
[Footnote 247: _Phil. Trans._, vol. xlvi., p. 204.]
[Footnote 248: _Astr. Nach._, No. 2,134.]
[Footnote 249: _Comptes Rendus_, t. cvii., p. 588.]
[Footnote 250: _Mém. de St. Pétersbourg_, t. xxxii., No. 3, 1884; _Astr.
Nach._, No. 2,727.]
[Footnote 251: _Month. Not._, vol. xix., p. 72.]
[Footnote 252: _Mécanique Céleste_, t. ii., p. 197.]
[Footnote 253: See Berberich, _Astr. Nach._, Nos. 2,836-7, 3,125;
Deichmüller, _Ibid._, No. 3,123.]
[Footnote 254: _Month. Not._, vol. ii., p. 117.]
[Footnote 255: _Astr. Nach._, No. 128.]
[Footnote 256: _Annuaire_, 1832, p. 186.]
[Footnote 257: _Am. Journ. of Science_, vol. i. (2nd series), p. 293.
Prof. Hubbard's calculations indicated a probability that the definitive
separation of the two nuclei occurred as early as September 30, 1884.
_Astronomical Journal_ (Gould's), vol. iv., p. 5. See also, on the
subject of this comet, W. T. Lynn, _Intellectual Observer_, vol. xi., p.
208; E. Ledger, _Observatory_, August, 1883, p. 244; and H. A. Newton,
_Am. Journ. of Science_, vol. xxxi., p. 81, February, 1886.]
[Footnote 258: _Month. Not._, vol. vii., p. 73.]
[Footnote 259: _Bulletin Ac. Imp. de St. Pétersbourg_, t. vi., col. 77.
The latest observation of the parent nucleus was that of Argelander,
April 27, at Bonn.]
[Footnote 260: D'Arrest, _Astr. Nach._, No. 1,624.]
[Footnote 261: _Der Brorsen'sche Comet._ Von Dr. E. Lamp, Kiel, 1892;
Plummer, _Knowledge_, vol. xix., p. 41.]
[Footnote 262: Schulhof, _Astr. Nach._, No. 3,267; _Observatory_, vol.
xviii., p. 64; F. H. Seares, _Astr. Nach._, Nos. 3,606-7; Plummer,
_Knowledge_, vol. xix., p. 156.]
[Footnote 263: _Comptes Rendus_, t. xxv., p. 570.]
[Footnote 264: _Month. Not._, vol. xii., p. 248.]
[Footnote 265: _Bull. Astr._, t. vi., pp. 241, 289.]
[Footnote 266: _Étude sur la Théorie des Comètes périodiques. Annales de
l'Observatoíre_, t. xx., Paris, 1891.]
[Footnote 267: _Amer. Journ. of Science_, vol. xlii., pp. 183, 482,
1891.]
[Footnote 268: _Observatory_, vol. xiv., p. 194.]
[Footnote 269: _Phil. Trans._, vol. cii., pp. 118-124.]
[Footnote 270: _Ueber den Schweif des grossen Cometen von 1811, Monat.
Corr._, vol. xxv., pp. 3-22. Reprinted by Zöllner. _Ueber die Natur der
Cometen_, pp. 3-15.]
[Footnote 271: _Natur der Cometen_, p. 148.]
[Footnote 272: The subject of a classical memoir by Bessel, published in
1810.]
[Footnote 273: A fresh investigation of its orbit has been published by
N. Herz of Vienna. See _Bull. Astr._, t. ix., p. 427.]
[Footnote 274: _Astr. Jahrbuch_ (Bode's), 1823, p. 134.]
[Footnote 275: Reproduced in Webb's _Celestial Objects_, 4th ed.]
[Footnote 276: _Month. Not._, vol. xxxvi., p. 309.]
[Footnote 277: _Celestial Objects_, p. 40, note.]
[Footnote 278: See Airy's Address, _Mem. R. A. S._, vol. x., p. 376.
Rosenberger calculated no more, though he lived until 1890. W. T. Lynn,
_Observatory_, vol. xiii., p. 125.]
[Footnote 279: Hind, _The Comets_, p. 47.]
[Footnote 280: Arago, _Annuaire_, 1836, p. 228.]
[Footnote 281: _Astr. Nach._, No. 300.]
[Footnote 282: It deserves to be recorded that Robert Hooke drew a very
similar inference from his observations of the comets of 1680 and 1682.
_Month. Not._, vol. xiv., pp. 77-83.]
[Footnote 283: _Briefwechsel zwischen Olbers und Bessel_, Bd. ii., p.
390.]
[Footnote 284: Herschel, _Results_, p. 405.]
[Footnote 285: _Mem. R. A. S._, vol. x., p. 92,]
[Footnote 286: _Results_, p. 401.]
[Footnote 287: Pontécoulant, _Comptes Rendus_, t. lviii., p. 825.]
[Footnote 288: _Annuaire_, 1836, p. 233.]
[Footnote 289: _Cosmos_, vol. i., p. 90, _note_ (Otté's trans.).]
[Footnote 290: Herschel, _Outlines of Astronomy_, p. 399, 9th ed.]
[Footnote 291: _Outlines_, p. 398.]
[Footnote 292: Boguslawski calculated that it extended on the 21st of
March to 581 millions.--_Report. Brit. Ass._, 1845, p. 89.]
[Footnote 293: _Comptes Rendus_, t. xvi., p. 919.]
[Footnote 294: _Observatory_, vol. xxiv., p. 167; Astr. Nach., No.
3,320.]
[Footnote 295: Piazzi noticed a considerable increase of lustre in a
very faint star of the twelfth magnitude viewed through a comet. Mädler,
_Reden_, etc., p. 248, _note_.]
[Footnote 296: _Astr. Jahrbuch_, 1828, p. 151.]
[Footnote 297: Mädler, _Gesch. d. Astr._, Bd. ii., p. 412.]
[Footnote 298: _Recueil de l'Ac. Imp. de St. Pétersbourg_, 1835, p.
143.]
[Footnote 299: Guillemin's _World of Comets_, trans, by J. Glaisher, p.
294, _note_.]
[Footnote 300: _Month. Not._, vol. viii., p. 9.]
[Footnote 301: A real, though only partial stoppage of light seems
indicated by Herschel's observations on the comet of 1807. Stars seen
through the tail, October 18, lost much of their lustre. One near the
head was only faintly visible by glimpses. _Phil. Trans._, vol. xcvii.,
p. 153.]
[Footnote 302: Arago, _Annuaire_, 1832, p. 205.]
[Footnote 303: _Ibid._, 1891, p. 290.]
[Footnote 304: Viz., Encke's, Biela's, Faye's, and Brorsen's.]
CHAPTER VI
_INSTRUMENTAL ADVANCES_
It is impossible to follow with intelligent interest the course of
astronomical discovery without feeling some curiosity as to the means by
which such surpassing results have been secured. Indeed, the bare
acquaintance with _what_ has been achieved, without any corresponding
knowledge of _how_ it has been achieved, supplies food for barren wonder
rather than for fruitful and profitable thought. Ideas advance most
readily along the solid ground of practical reality, and often find true
sublimity while laying aside empty marvels. Progress is the result, not
so much of sudden flights of genius, as of sustained, patient, often
commonplace endeavour; and the true lesson of scientific history lies in
the close connection which it discloses between the most brilliant
developments of knowledge and the faithful accomplishment of his daily
task by each individual thinker and worker.
It would be easy to fill a volume with the detailed account of the long
succession of optical and mechanical improvements by means of which the
observation of the heavens has been brought to its present degree of
perfection; but we must here content ourselves with a summary sketch of
the chief amongst them. The first place in our consideration is
naturally claimed by the telescope.
This marvellous instrument, we need hardly remind our readers, is of two
distinct kinds--that in which light is gathered together into a focus by
_refraction_, and that in which the same end is attained by
_reflection_. The image formed is in each case viewed through a
magnifying lens, or combination of lenses, called the eye-piece. Not for
above a century after the "optic glasses" invented or stumbled upon by
the spectacle-maker of Middelburg (1608) had become diffused over
Europe, did the reflecting telescope come, even in England, the place of
its birth, into general use. Its principle (a sufficiently obvious one)
had indeed been suggested by Mersenne as early as 1639;[305] James
Gregory in 1663[306] described in detail a mode of embodying that
principle in a practical shape; and Newton, adopting an original system
of construction, actually produced in 1668 a tiny speculum, one inch
across, by means of which the apparent distance of objects was reduced
thirty-nine times. Nevertheless, the exorbitantly long tubeless
refractors, introduced by Huygens, maintained their reputation until
Hadley exhibited to the Royal Society, January 12, 1721,[307] a
reflector of six inches aperture, and sixty-two in focal length, which
rivalled in performance, and of course indefinitely surpassed in
manageability, one of the "aerial" kind of 123 feet.
The concave-mirror system now gained a decided ascendant, and was
brought to unexampled perfection by James Short of Edinburgh during the
years 1732-68. Its resources were, however, first fully developed by
William Herschel. The energy and inventiveness of this extraordinary man
marked an epoch wherever they were applied. His ardent desire to measure
and gauge the stupendous array of worlds which his specula revealed to
him, made him continually intent upon adding to their "space-penetrating
power" by increasing their light-gathering surface. These, as he was the
first to explain,[308] are in a constant proportion one to the other.
For a telescope with twice the linear aperture of another will collect
four times as much light, and will consequently disclose an object four
times as faint as could be seen with the first, or, what comes to the
same, an object equally bright at twice the distance. In other words, it
will possess double the space-penetrating power of the smaller
instrument. Herschel's great mirrors--the first examples of the giant
telescopes of modern times--were then primarily engines for extending
the bounds of the visible universe; and from the sublimity of this
"final cause" was derived the vivid enthusiasm which animated his
efforts to success.
It seems probable that the seven-foot telescope constructed by him in
1775--that is within little more than a year after his experiments in
shaping and polishing metal had begun--already exceeded in effective
power any work by an earlier optician; and both his skill and his
ambition rapidly developed. His efforts culminated, after mirrors of
ten, twenty, and thirty feet focal length had successively left his
hands, in the gigantic forty-foot, completed August 28, 1789. It was the
first reflector in which only a single mirror was employed. In the
"Gregorian" form, the focussed rays are, by a second reflection from a
small concave[309] mirror, thrown _straight back_ through a central
aperture in the larger one, behind which the eye-piece is fixed. The
object under examination is thus seen in the natural direction. The
"Newtonian," on the other hand, shows the object in a line of sight at
right angles to the true one, the light collected by the speculum being
diverted to one side of the tube by the interposition of a small plane
mirror, situated at an angle of 45° to the axis of the instrument. Upon
these two systems Herschel worked until 1787, when, becoming convinced
of the supreme importance of economising light (necessarily wasted by
the second reflection), he laid aside the small mirror of his forty-foot
then in course of construction, and turned it into a "front-view"
reflector. This was done--according to the plan proposed by Lemaire in
1732--by slightly inclining the speculum so as to enable the image
formed by it to be viewed with an eye-glass fixed at the upper margin of
the tube. The observer thus stood with his back turned to the object he
was engaged in scrutinising.
The advantages of the increased brilliancy afforded by this modification
were strikingly illustrated by the discovery, August 28 and September
17, 1789, of the two Saturnian satellites nearest the ring.
Nevertheless, the monster telescope of Slough cannot be said to have
realised the sanguine expectations of its constructor. The occasions on
which it could be usefully employed were found to be extremely rare. It
was injuriously affected by every change of temperature. The great
weight (25 cwt.) of a speculum four feet in diameter rendered it
peculiarly liable to distortion. With all imaginable care, the delicate
lustre of its surface could not be preserved longer than two years,[310]
when the difficult process of repolishing had to be undertaken. It was
accordingly never used after 1811, when, having _gone blind_ from damp,
it lapsed by degrees into the condition of a museum inmate.
The exceedingly high magnifying powers employed by Herschel constituted
a novelty in optical astronomy, to which he attached great importance.
The work of ordinary observation would, however, be hindered rather than
helped by them. The attempt to increase in this manner the efficacy of
the telescope is speedily checked by atmospheric, to say nothing of
other difficulties. Precisely in the same proportion as an object is
magnified, the disturbances of the medium through which it is seen are
magnified also. Even on the clearest and most tranquil nights, the air
is never for a moment really still. The rays of light traversing it are
continually broken by minute fluctuations of refractive power caused by
changes of temperature and pressure, and the currents which these
engender. With such luminous quiverings and waverings the astronomer has
always more or less to reckon; their absence is simply a question of
degree; if sufficiently magnified, they are at all times capable of
rendering observation impossible.
Thus, such powers as 3,000, 4,000, 5,000, even 6,652,[311] which
Herschel now and again applied to his great telescopes, must, save on
the rarest occasions, prove an impediment rather than an aid to vision.
They were, however, used by him only for special purposes,
experimentally, not systematically, and with the clearest discrimination
of their advantages and drawbacks. It is obvious that perfectly
different ends are subserved by increasing the _aperture_ and by
increasing the _power_ of a telescope. In the one case, a larger
quantity of light is captured and concentrated; in the other, the same
amount is distributed over a wider area. A diminution of brilliancy in
the image accordingly attends, _coeteris paribus_, upon each
augmentation of its apparent size. For this reason, such faint objects
as nebulæ are most successfully observed with moderate powers applied to
instruments of a great capacity for light, the details of their
structure actually disappearing when highly magnified. With stellar
groups the reverse is the case. Stars cannot be magnified, simply
because they are too remote to have any sensible dimensions; but the
space between them can. It was thus for the purpose of dividing very
close double stars that Herschel increased to such an unprecedented
extent the magnifying capabilities of his instruments; and to this
improvement incidentally the discovery of Uranus, March 13, 1781,[312]
was due. For by the examination with strong lenses of an object which,
even with a power of 227, presented a suspicious appearance, he was able
at once to pronounce its disc to be real, not merely "spurious," and so
to distinguish it unerringly from the crowd of stars amidst which it was
moving.
While the reflecting telescope was astonishing the world by its rapid
development in the hands of Herschel, its unpretending rival was slowly
making its way towards the position which the future had in store for
it. The great obstacle which long stood in the way of the improvement of
refractors was the defect known as "chromatic aberration." This is due
to no other cause than that which produces the rainbow and the
spectrum--the separation, or "dispersion" in their passage through a
refracting medium, of the variously coloured rays composing a beam of
white light. In an ordinary lens there is no common point of
concentration; each colour has its own separate focus; and the resulting
image, formed by the superposition of as many images as there are hues
in the spectrum, is indefinitely terminated with a tinted border,
eminently baffling to exactness of observation.
The extravagantly long telescopes of the seventeenth century were
designed to _avoid_ this evil (as well as another source of indistinct
vision in the spherical shape of lenses); but no attempt to _remedy_ it
was made until an Essex gentleman succeeded, in 1733, in so combining
lenses of flint and crown glass as to produce refraction without
colour.[313] Mr. Chester More Hall was, however, equally indifferent to
fame and profit, and took no pains to make his invention public. The
_effective_ discovery of the achromatic telescope was, accordingly,
reserved for John Dollond, whose method of correcting at the same time
chromatic and spherical aberration was laid before the Royal Society in
1758. Modern astronomy may be said to have been thereby rendered
possible. Refractors have always been found better suited than
reflectors to the ordinary work of observatories. They are, so to speak,
of a more robust, as well as of a more plastic nature. They suffer less
from vicissitudes of temperature and climate. They retain their
efficiency with fewer precautions and under more trying circumstances.
Above all, they co-operate more readily with mechanical appliances, and
lend themselves with far greater facility to purposes of exact
measurement.
A practical difficulty, however, impeded the realisation of the
brilliant prospects held out by Dollond's invention. It was found
impossible to procure flint-glass, such as was needed for optical
use--that is, of perfectly homogeneous quality--except in fragments of
insignificant size. Discs of more than two or three inches in diameter
were of extreme rarity; and the crushing excise duty imposed upon the
article by the financial unwisdom of the Government, both limited its
production, and, by rendering experiments too costly for repetition,
barred its improvement.
Up to this time, Great Britain had left foreign competitors far behind
in the instrumental department of astronomy. The quadrants and circles
of Bird, Cary and Ramsden were unapproached abroad. The reflecting
telescope came into existence and reached maturity on British soil. The
refracting telescope was cured of its inherent vices by British
ingenuity. But with the opening of the nineteenth century, the almost
unbroken monopoly of skill and contrivance which our countrymen had
succeeded in establishing was invaded, and British workmen had to be
content to exchange a position of supremacy for one of at least partial
temporary inferiority.
Somewhat about the time that Herschel set about polishing his first
speculum, Pierre Louis Guinand, a Swiss artisan, living near
Chaux-de-Fonds, in the canton of Neuchâtel, began to grind spectacles
for his own use, and was thence led on to the rude construction of
telescopes by fixing lenses in pasteboard tubes. The sight of an England
achromatic stirred a higher ambition, and he took the first opportunity
of procuring some flint glass from England (then the only source of
supply), with the design of imitating an instrument the full
capabilities of which he was destined to be the humble means of
developing. The English glass proving of inferior quality, he conceived
the possibility, unaided and ignorant of the art as he was, of himself
making better, and spent seven years (1784-90) in fruitless experiments
directed to that end. Failure only stimulated him to enlarge their
scale. He bought some land near Les Brenets, constructed upon it a
furnace capable of melting two quintals of glass, and reducing himself
and his family to the barest necessaries of life, he poured his earnings
(he at this time made bells for repeaters) unstintingly into his
crucibles.[314] His undaunted resolution triumphed. In 1799 he carried
to Paris and there showed to Lalande several discs of flawless crystal
four to six inches in diameter. Lalande advised him to keep his secret,
but in 1805 he was induced to remove to Munich, where he became the
instructor of the immortal Fraunhofer. His return to Les Brenets in 1814
was signalised by the discovery of an ingenious mode of removing
striated portions of glass by breaking and re-soldering the product of
each melting, and he eventually attained to the manufacture of perfect
discs up to 18 inches in diameter. An object-glass for which he had
furnished the material to Cauchoix, procured him, in 1823, a royal
invitation to settle in Paris; but he was no longer equal to the change,
and died at the scene of his labours, February 13 following.
This same lens (12 inches across) was afterwards purchased by Sir James
South, and the first observation made with it, February 13, 1830,
disclosed to Sir John Herschel the sixth minute star in the central
group of the Orion nebula, known as the "trapezium."[315] Bequeathed by
South to Trinity College, Dublin, it was employed at the Dunsink
Observatory by Brünnow and Ball in their investigations of stellar
parallax. A still larger objective (of nearly 14 inches) made of
Guinand's glass was secured in Paris, about the same time, by Mr. Edward
Cooper of Markree Castle, Ireland. The peculiarity of the method
discovered at Les Brenets resided in the manipulation, not in the
quality of the ingredients; the secret, that is to say, was not
chemical, but mechanical.[316] It was communicated by Henry Guinand (a
son of the inventor) to Bontemps, one of the directors of the glassworks
at Choisy-le-Roi, and by him transmitted to Messrs. Chance of
Birmingham, with whom he entered into partnership when the revolutionary
troubles of 1848 obliged him to quit his native country. The celebrated
American opticians, Alvan Clark & Sons, derived from the Birmingham firm
the materials for some of their early telescopes, notably the 19-inch
Chicago and 26-inch Washington equatoreals; but the discs for the great
Lick refractor, and others shaped by them in recent years, have been
supplied by Feil of Paris.
Two distinguished amateurs, meanwhile, were preparing to reassert on
behalf of reflecting instruments their claim to the place of honour in
the van of astronomical discovery. Of Mr. Lassell's specula something
has already been said.[317] They were composed of an alloy of copper and
tin, with a minute proportion of arsenic (after the example of
Newton[318]), and were remarkable for perfection of figure and
brilliancy of surface.
The capabilities of the Newtonian plan were developed still more
fully--it might almost be said to the uttermost--by the enterprise of an
Irish nobleman. William Parsons, known as Lord Oxmantown until 1841,
when, on his father's death, he succeeded to the title of Earl of Rosse,
was born at York, June 17, 1800. His public duties began before his
education was completed. He was returned to Parliament as member for
King's County while still an undergraduate at Oxford, and continued to
represent the same constituency for thirteen years (1821-34). From 1845
until his death, which took place, October 31, 1867, he sat, silent but
assiduous, in the House of Lords as an Irish representative peer; he
held the not unlaborious post of President of the Royal Society from
1849 to 1854; presided over the meeting of the British Association at
Cork in 1843, and was elected Vice-Chancellor of Dublin University in
1862. In addition to these extensive demands upon his time and thoughts,
were those derived from his position as practically the feudal chief of
a large body of tenantry in times of great and anxious responsibility,
to say nothing of the more genial claims of an unstinted hospitality.
Yet, while neglecting no public or private duty, this model nobleman
found leisure to render to science services so conspicuous as to entitle
his name to a lasting place in its annals.
He early formed the design of reaching the limits of the attainable in
enlarging the powers of the telescope, and the qualities of his mind
conspired with the circumstances of his fortune to render the design a
feasible one. From refractors it was obvious that no such vast and rapid
advance could be expected. English glass-manufacture was still in a
backward state. So late as 1839, Simms (successor to the distinguished
instrumentalist Edward Troughton) reported a specimen of crystal
scarcely 7-1/2 inches in diameter, and perfect only over six, to be
unique in the history of English glass-making.[319] Yet at that time the
fifteen-inch achromatic of Pulkowa had already left the workshop of
Fraunhofer's successors at Munich. It was not indeed until 1845, when
the impost which had so long hampered their efforts was removed, that
the optical artists of these islands were able to compete on equal terms
with their rivals on the Continent. In the case of reflectors, however,
there seemed no insurmountable obstacle to an almost unlimited increase
of light-gathering capacity; and it was here, after some unproductive
experiments with fluid lenses, that Lord Oxmantown concentrated his
energies.
He had to rely entirely on his own invention, and to earn his own
experience. James Short had solved the problem of giving to metallic
surfaces a perfect parabolic figure (the only one by which parallel
incident rays can be brought to an exact focus); but so jealous was he
of his secret, that he caused all his tools to be burnt before his
death;[320] nor was anything known of the processes by which Herschel
had achieved his astonishing results. Moreover, Lord Oxmantown had no
skilled workmen to assist him. His implements, both animate and
inanimate, had to be formed by himself. Peasants taken from the plough
were educated by him into efficient mechanics and engineers. The
delicate and complex machinery needed in operations of such hairbreadth
nicety as his enterprise involved, the steam-engine which was to set it
in motion, at times the very crucibles in which his specula were cast,
issued from his own workshops.
In 1827 experiments on the composition of speculum-metal were set on
foot, and the first polishing-machine ever driven by steam-power was
contrived in 1828. But twelve arduous years of struggle with recurring
difficulties passed before success began to dawn. A material less
tractable than the alloy selected, of four chemical equivalents of
copper to one of tin,[321] can scarcely be conceived. It is harder than
steel, yet brittle as glass, crumbling into fragments with the slightest
inadvertence of handling or treatment;[322] and the precision of figure
requisite to secure good definition is almost beyond the power of
language to convey. The quantities involved are so small as not alone to
elude sight, but to confound imagination. Sir John Herschel tells us
that "the _total_ thickness to be abraded from the edge of a spherical
speculum 48 inches in diameter and 40 feet focus, to convert it into a
paraboloid, is only 1/21333 of an inch;"[323] yet upon this minute
difference of form depends the clearness of the image, and, as a
consequence, the entire efficiency of the instrument. "Almost infinite,"
indeed (in the phrase of the late Dr. Robinson), must be the exactitude
of the operation adapted to bring about so delicate a result.
At length, in 1839, two specula, each three feet in diameter, were
turned out in such perfection as to prompt a still bolder experiment.
The various processes needed to insure success were now ascertained and
under control; all that was necessary was to repeat them on a larger
scale. A gigantic mirror, six feet across and fifty-four in focal
length, was accordingly cast on the 13th of April, 1842; in two months
it was ground down to figure by abrasion with emery and water, and
daintily polished with rouge; and by the month of February, 1845, the
"leviathan of Parsonstown" was available for the examination of the
heavens.
The suitable mounting of this vast machine was a problem scarcely less
difficult than its construction. The shape of a speculum needs to be
maintained with an elaborate care equal to that used in imparting it. In
fact, one of the most formidable obstacles to increasing the size of
such reflecting surfaces consists in their liability to bend under their
own weight. That of the great Rosse speculum was no less than four tons.
Yet, although six inches in thickness, and composed of a material only a
degree inferior in rigidity to wrought iron, the strong pressure of a
man's hand at its back produced sufficient flexure to distort
perceptibly the image of a star reflected in it.[324] Thus the delicacy
of its form was perishable equally by the stress of its own gravity, and
by the slightest irregularity in the means taken to counteract that
stress. The problem of affording a perfectly equable support in all
possible positions was solved by resting the speculum upon twenty-seven
platforms of cast iron, felt-covered, and carefully fitted to the shape
of the areas they were to carry, which platforms were themselves borne
by a complex system of triangles and levers, ingeniously adapted to
distribute the weight with complete uniformity.[325]
A tube which resembled, when erect, one of the ancient round towers of
Ireland,[326] served as the habitation of the great mirror. It was
constructed of deal staves bound together with iron hoops, was
fifty-eight feet long (including the speculum-box), and seven in
diameter. A reasonably tall man may walk through it (as Dean Peacock
once did) with umbrella uplifted. Two piers of solid masonry, about
fifty feet high, seventy long, and twenty-three apart, flanked the huge
engine on either side. Its lower extremity rested on a universal joint
of cast iron; above, it was slung in chains, and even in a gale of wind
remained perfectly steady. The weight of the entire, although amounting
to fifteen tons, was so skilfully counterpoised, that the tube could
with ease be raised or depressed by two men working a windlass. Its
horizontal range was limited by the lofty walls erected for its support
to about ten degrees on each side of the meridian; but it moved
vertically from near the horizon through the zenith as far as the pole.
Its construction was of the Newtonian kind, the observer looking into
the side of the tube near its upper end, which a series of galleries and
sliding stages enabled him to reach in any position. It has also, though
rarely, been used without a second mirror, as a "Herschelian" reflector.
The splendour of the celestial objects as viewed with this vast
"light-grasper" surpassed all expectation. "Never in my life," exclaimed
Sir James South, "did I see such glorious sidereal pictures."[327] The
orb of Jupiter produced an effect compared to that of the introduction
of a coach-lamp into the telescope;[328] and certain star-clusters
exhibited an appearance (we again quote Sir James South) "such as man
before had never seen, and which for its magnificence baffles all
description." But it was in the examination of the nebulæ that the
superiority of the new instrument was most strikingly displayed. A large
number of these misty objects, which the utmost powers of Herschel's
specula had failed to resolve into stars, yielded at once to the
Parsonstown reflector; while many others showed under entirely changed
forms through the disclosure of previously unseen details of structure.
One extremely curious result of the increase of light was the abolition
of any sharp distinction between the two classes of "annular" and
"planetary" nebulæ. Up to that time, only four ring-shaped systems--two
in the northern and two in the southern hemisphere--were known to
astronomers; they were now reinforced by five of the planetary kind, the
discs of which were observed to be centrally perforated; while the
definite margins visible in weaker instruments were replaced by ragged
edges or filamentous fringes.
Still more striking was the discovery of an entirely new and most
remarkable species of nebulæ. These were termed "spiral," from the more
or less regular convolutions, resembling the whorls of a shell, in which
the matter composing them appeared to be distributed. The first and most
conspicuous specimen of this class was met with in April, 1845; it is
situated in Canes Venatici, close to the tail of the Great Bear, and
wore, in Sir J. Herschel's instruments, the aspect of a split ring
encompassing a bright nucleus, thus presenting, as he supposed, a
complete analogue to the system of the Milky Way. In the Rosse mirror it
shone out as a vast whirlpool of light--a stupendous witness to the
presence of cosmical activities on the grandest scale, yet regulated by
laws as to the nature of which we are profoundly ignorant. Professor
Stephen Alexander of New Jersey, however, concluded, from an
investigation (necessarily founded on highly precarious data) of the
mechanical condition of these extraordinary agglomerations, that we see
in them "the partially scattered fragments of enormous masses once
rotating in a state of dynamical equilibrium." He further suggested
"that the separation of these fragments may still be in progress,"[329]
and traced back their origin to the disruption, through its own
continually accelerated rotation, of a "primitive spheroid" of
inconceivably vast dimensions. Such also, it was added (the curvilinear
form of certain outliers of the Milky Way giving evidence of a spiral
structure), is probably the history of our own cluster; the stars
composing which, no longer held together in a delicately adjusted system
like that of the sun and planets, are advancing through a period of
seeming confusion towards an appointed goal of higher order and more
perfect and harmonious adaptation.[330]
The class of spiral nebulæ included, in 1850, fourteen members, besides
several in which the characteristic arrangement seemed partial or
dubious.[331] A tendency in the exterior stars of other clusters to
gather into curved branches (as in our Galaxy) was likewise noted; and
the existence of unsuspected analogies was proclaimed by the significant
combination in the "Owl" nebula (a large planetary in Ursa Major)[332]
of the twisted forms of a spiral with the perforated effect distinctive
of an annular nebula.
Once more, by the achievements of the Parsonstown reflector, the
supposition of a "shining fluid" filling vast regions of space was
brought into (as it has since proved) undeserved discredit. Although
Lord Rosse himself rejected the inference, that because many nebulæ had
been resolved, all were resolvable, very few imitated his truly
scientific caution; and the results of Bond's investigations[333] with
the Harvard College refractor quickened and strengthened the current of
prevalent opinion. It is now certain that the evidence furnished on both
sides of the Atlantic as to the stellar composition of some conspicuous
objects of this class (notably the Orion and "Dumb-bell" nebulæ) was
delusive; but the spectroscope alone was capable of meeting it with a
categorical denial. Meanwhile there seemed good ground for the
persuasion, which now, for the last time, gained the upper hand, that
nebulæ are, without exception, true "island-universes," or assemblages
of distant suns.
Lord Rosse's telescope possesses a nominal power of 6,000--that is, it
shows the moon as if viewed with the naked eye at a distance of forty
miles. But this seeming advantage is neutralised by the weakening of the
available light through excessive diffusion, as well as by the troubles
of the surging sea of air through which the observation must necessarily
be made. Professor Newcomb, in fact, doubts whether with _any_ telescope
our satellite has ever been seen to such advantage as it would be if
brought within 500 miles of the unarmed eye.[334]
The French opticians' rule of doubling the number of millimetres
contained in the aperture of an instrument to find the highest
magnifying power usually applicable to it, would give 3,600 as the
maximum for the leviathan of Birr Castle; but in a climate like that of
Ireland the occasions must be rare when even that limit can be reached.
Indeed, the experience acquired by its use plainly shows that
atmospheric rather than mechanical difficulties impede a still further
increase of telescopic power. Its construction may accordingly be said
to mark the _ne plus ultra_ of effort in one direction, and the
beginning of its conversion towards another. It became thenceforward
more and more obvious that the conditions of observation must be
ameliorated before any added efficacy could be given to it. The full
effect of an uncertain climate in nullifying optical improvements was
recognised, and the attention of astronomers began to be turned towards
the advantages offered by more tranquil and more translucent skies.
Scarcely less important for the practical uses of astronomy than the
optical qualities of the telescope is the manner of its mounting. The
most admirable performance of the optician can render but unsatisfactory
service if its mechanical accessories are ill-arranged or inconvenient.
Thus the astronomer is ultimately dependent upon the mechanician; and so
excellently have his needs been served, that the history of the
ingenious contrivances by which discoveries have been prepared would
supply a subject (here barely glanced at) not far inferior in extent and
instruction to the history of those discoveries themselves.
There are two chief modes of using the telescope, to which all others
may be considered subordinate.[335] Either it may be invariably directed
towards the south, with no motion save in the plane of the meridian, so
as to intercept the heavenly bodies at the moment of transit across that
plain; or it may be arranged so as to follow the daily revolution of the
sky, thus keeping the object viewed permanently in sight instead of
simply noting the instant of its flitting across the telescopic field.
The first plan is that of the "transit instrument," the second that of
the "equatoreal." Both were, by a remarkable coincidence, introduced
about 1690[336] by Olaus Römer, the brilliant Danish astronomer who
first measured the velocity of light.
The uses of each are entirely different. With the transit, the really
fundamental task of astronomy--the determination of the movements of the
heavenly bodies--is mainly accomplished; while the investigation of
their nature and peculiarities is best conducted with the equatoreal.
One is the instrument of mathematical, the other of descriptive
astronomy. One furnishes the materials with which theories are
constructed and the tests by which they are corrected; the other
registers new facts, takes note of new appearances, sounds the depths
and peers into every nook of the heavens.
The great improvement of giving to a telescope equatoreally mounted an
automatic movement by connecting it with clockwork, was proposed in 1674
by Robert Hooke. Bradley in 1721 actually observed Mars with a telescope
"moved by a machine that made it keep pace with the stars;"[337] and Von
Zach relates[338] that he had once followed Sirius for twelve hours with
a "heliostat" of Ramsden's construction. But these eighteenth-century
attempts were of no practical effect. Movement by clockwork was
virtually a complete novelty when it was adopted by Fraunhofer in 1824
to the Dorpat refractor. By simply giving to an axis unvaryingly
directed towards the celestial pole an equable rotation with a period of
twenty-four hours, a telescope attached to it, and pointed in _any_
direction, will trace out on the sky a parallel of declination, thus
necessarily accompanying the movement of any star upon which it may be
fixed. It accordingly forms part of the large sum of Fraunhofer's merits
to have secured this inestimable advantage to observers.
Sir John Herschel considered that Lassell's application of equatoreal
mounting to a nine-inch Newtonian in 1840 made an epoch in the history
of "that eminently British instrument, the reflecting telescope."[339]
Nearly a century earlier,[340] it is true, Short had fitted one of his
Gregorians to a complicated system of circles in such a manner that, by
moving a handle, it could be made to follow the revolution of the sky;
but the arrangement did not obtain, nor did it deserve, general
adoption. Lassell's plan was a totally different one; he employed the
crossed axes of the true equatoreal, and his success removed, to a great
extent, the fatal objection of inconvenience in use, until then
unanswerably urged against reflectors. The very largest of these can now
be mounted equatoreally; even the Rosse, within its limited range, has
been for some years provided with a movement by clockwork along
declination-parallels.
The art of accurately dividing circular arcs into the minute equal parts
which serve as the units of astronomical measurement, remained, during
the whole of the eighteenth century, almost exclusively in English
hands. It was brought to a high degree of perfection by Graham, Bird and
Ramsden, all of whom, however, gave the preference to the old-fashioned
mural quadrant and zenith-sector over the entire circle, which Römer had
already found the advantage of employing. The five-foot vertical circle,
which Piazzi with some difficulty induced Ramsden to complete for him in
1789, was the first divided instrument constructed in what may be called
the modern style. It was provided with magnifiers for reading off the
divisions (one of the neglected improvements of Römer), and was set up
above a smaller horizontal circle, forming an "altitude and azimuth"
combination (again Römer's invention), by which both the elevation of a
celestial object above the horizon and its position as referred to the
horizon could be measured. In the same year, Borda invented the
"repeating circle" (the principle of which had been suggested by Tobias
Mayer in 1756[341]), a device for exterminating, so far as possible,
errors of graduation by _repeating_ an observation with different parts
of the limb. This was perhaps the earliest systematic effort to correct
the imperfections of instruments by the manner of their use.
The manufacture of astronomical circles was brought to a very refined
state of excellence early in the nineteenth century by Reichenbach at
Munich, and after 1818 by Repsold at Hamburg. Bessel states[342] that
the "reading-off" on an instrument of the kind by the latter artist was
accurate to about 1/80th of a human hair. Meanwhile the traditional
reputation of the English school was fully sustained; and Sir George
Airy did not hesitate to express his opinion that the new method of
graduating circles, published by Troughton in 1809,[343] was the
"greatest improvement ever made in the art of instrument-making."[344]
But a more secure road to improvement than that of mere mechanical
exactness was pointed out by Bessel. His introduction of a regular
theory of instrumental errors might almost be said to have created a new
art of observation. Every instrument, he declared in memorable
words,[345] must be twice made--once by the artist, and again by the
observer. Knowledge is power. Defects that are ascertained and can be
allowed for are as good as non-existent. Thus the truism that the best
instrument is worthless in the hands of a careless or clumsy observer,
became supplemented by the converse maxim, that defective appliances
may, through skilful use, be made to yield valuable results. The
Königsberg observations--of which the first instalment was published in
1815--set the example of regular "reduction" for instrumental errors.
Since then, it has become an elementary part of an astronomer's duty to
study the _idiosyncrasy_ of each one of the mechanical contrivances at
his disposal, in order that its inevitable, but now certified deviations
from ideal accuracy may be included amongst the numerous corrections by
which the pure essence of even approximate truth is distilled from the
rude impressions of sense.
Nor is this enough; for the casual circumstances attending each
observation have to be taken into account with no less care than the
inherent or _constitutional_ peculiarities of the instrument with which
it is made. There is no "once for all" in astronomy. Vigilance can never
sleep; patience can never tire. Variable as well as constant sources of
error must be anxiously heeded; one infinitesimal inaccuracy must be
weighed against another; all the forces and vicissitudes of
nature--frosts, dews, winds, the interchanges of heat, the disturbing
effects of gravity, the shiverings of the air, the tremors of the earth,
the weight and vital warmth of the observer's own body, nay, the rate at
which his brain receives and transmits its impressions, must all enter
into his calculations, and be sifted out from his results.
It was in 1823 that Bessel drew attention to discrepancies in the times
of transits given by different astronomers.[346] The quantities involved
were far from insignificant. He was himself nearly a second in advance
of all his contemporaries, Argelander lagging behind him as much as a
second and a quarter. Each individual, in fact, was found to have a
certain definite _rate of perception_, which, under the name of
"personal equation," now forms so important an element in the correction
of observations that a special instrument for accurately determining its
amount in each case is in actual use at Greenwich.
Such are the refinements upon which modern astronomy depends for its
progress. It is a science of hairbreadths and fractions of a second. It
exists only by the rigid enforcement of arduous accuracy and unwearying
diligence. Whatever secrets the universe still has in store for man will
only be communicated on these terms. They are, it must be acknowledged,
difficult to comply with. They involve an unceasing struggle against the
infirmities of his nature and the instabilities of his position. But the
end is not unworthy the sacrifices demanded. One additional ray of light
thrown on the marvels of creation--a single, minutest encroachment upon
the strongholds of ignorance--is recompense enough for a lifetime of
toil. Or rather, the toil is its own reward, if pursued in the lofty
spirit which alone becomes it. For it leads through the abysses of space
and the unending vistas of time to the very threshold of that infinity
and eternity of which the disclosure is reserved for a life to come.
FOOTNOTES:
[Footnote 305: Grant, _Hist. Astr._, p. 527.]
[Footnote 306: _Optica Promota_, p. 93.]
[Footnote 307: _Phil. Trans._, vol. xxxii., p. 383.]
[Footnote 308: _Ibid._, vol. xc., p. 65.]
[Footnote 309: Cassegrain, a Frenchman, substituted in 1672 a _convex_
for a _concave_ secondary speculum. The tube was thereby enabled to be
shortened by twice the focal length of the mirror in question. The great
Melbourne reflector (four feet aperture, by Grubb) is constructed upon
this plan.]
[Footnote 310: _Phil. Trans._, vol. civ., p. 275, _note_.]
[Footnote 311: _Phil. Trans._, vol. xc., p. 70. With the forty-foot,
however, only very moderate powers seemed to have been employed, whence
Dr. Robinson argued a deficiency of defining power. _Proc. Roy. Irish
Ac._, vol. ii., p. 11.]
[Footnote 312: _Phil. Trans._, vol. lxxi., p. 492.]
[Footnote 313: It is remarkable that, as early as 1695, the possibility
of an achromatic combination was inferred by David Gregory from the
structure of the human eye. See his _Catoptricæ et Dioptricæ Sphericæ
Elementa_, p. 98.]
[Footnote 314: Wolf, _Biographien_, Bd. ii., p. 301.]
[Footnote 315: _Month. Not._, vol. i., p. 153. _note_.]
[Footnote 316: Henrivaux, _Encyclopédie Chimique_, t. v., fasc. 5, p.
363.]
[Footnote 317: See _ante_, p. 83.]
[Footnote 318: _Phil. Trans._, vol. vii., p. 4007.]
[Footnote 319: J. Herschel, _The Telescope_, p. 39.]
[Footnote 320: _Month. Not._, vol. xxix., p. 125.]
[Footnote 321: A slight excess of copper renders the metal easier to
work, but liable to tarnish. Robinson, _Proc. Roy. Irish Ac._, vol. ii.,
p. 4.]
[Footnote 322: _Brit. Ass._, 1843, Dr. Robinson's closing Address.
_Athenæum_, Sept. 23, p. 866.]
[Footnote 323: _The Telescope_, p. 82.]
[Footnote 324: Lord Rosse in _Phil. Trans._, vol. cxl., p. 302.]
[Footnote 325: This method is the same in principle with that applied by
Grubb in 1834 to a 15-inch speculum for the observatory of Armagh.
_Phil. Trans._, vol. clix., p. 145.]
[Footnote 326: Robinson, _Proc. Roy. Ir. Ac._, vol. iii., p. 120.]
[Footnote 327: _Astr. Nach._, No. 536.]
[Footnote 328: Airy, _Month. Not._, vol. ix., p. 120.]
[Footnote 329: _Astronomical Journal_ (Gould's), vol. ii., p. 97.]
[Footnote 330: _Ibid._, p. 160.]
[Footnote 331: Lord Rosse in _Phil. Trans._, vol. cxl., p. 505.]
[Footnote 332: No. 2343 of Herschel's (1864) Catalogue. Before 1850 a
star was visible in each of the two larger openings by which it is
pierced; since then, one only. Webb, _Celestial Objects_ (4th ed.), p.
409.]
[Footnote 333: _Mem. Am. Ac._, vol. iii., p. 87; _Astr. Nach._, No.
611.]
[Footnote 334: _Pop. Astr._, p. 145.]
[Footnote 335: This statement must be taken in the most general sense.
Supplementary observations of great value are now made at Greenwich with
the altitude and azimuth instrument, which likewise served Piazzi to
determine the places of his stars; while a "prime vertical instrument"
is prominent at Pulkowa.]
[Footnote 336: As early as 1620, according to R. Wolf (_Ges. der Astr._,
p. 587), Father Scheiner made the experiment of connecting a telescope
with an axis directed to the pole, while Chinese "equatoreal armillæ,"
dating from the thirteenth century, existed at Pekin until 1900, when
they were carried off as "loot" to Berlin. J. L. E. Dreyer,
_Copernicus_, vol. i., p. 134.]
[Footnote 337: _Miscellaneous Works_, p. 350.]
[Footnote 338: _Astr. Jahrbuch_, 1799 (published 1796), p. 115.]
[Footnote 339: _Month. Not._, vol. xli., p. 189.]
[Footnote 340: _Phil. Trans._, vol. xlvi., p. 242.]
[Footnote 341: Grant, _Hist. of Astr._, p. 487.]
[Footnote 342: _Pop. Vorl._, p. 546.]
[Footnote 343: _Phil. Trans._, vol. xcix., p. 105.]
[Footnote 344: _Report Brit. Ass._, 1832, p. 132.]
[Footnote 345: _Pop. Vorl._, p. 432.]
[Footnote 346: C. T. Anger, _Grundzüge der neucren astronomischen
Beobachtungs-Kunst_, p. 3.]
PART II
RECENT PROGRESS OF ASTRONOMY
CHAPTER I
_FOUNDATION OF ASTRONOMICAL PHYSICS_
In the year 1826, Heinrich Schwabe of Dessau, elated with the hope of
speedily delivering himself from the hereditary incubus of an
apothecary's shop,[347] obtained from Munich a small telescope and began
to observe the sun. His choice of an object for his researches was
instigated by his friend Harding of Göttingen. It was a peculiarly happy
one. The changes visible in the solar surface were then generally
regarded as no less capricious than the changes in the skies of our
temperate regions. Consequently, the reckoning and registering of
sun-spots was a task hardly more inviting to an astronomer than the
reckoning and registering of summer clouds. Cassini, Keill, Lemonnier,
Lalande, were unanimous in declaring that no trace of regularity could
be detected in their appearances or effacements.[348] Delambre
pronounced them "more curious than really useful."[349] Even Herschel,
profoundly as he studied them, and intimately as he was convinced of
their importance as symptoms of solar activity, saw no reason to suspect
that their abundance and scarcity were subject to orderly alternation.
One man alone in the eighteenth century, Christian Horrebow of
Copenhagen, divined their periodical character, and foresaw the time
when the effects of the sun's vicissitudes upon the globes revolving
round him might be investigated with success; but this prophetic
utterance was of the nature of a soliloquy rather than of a
communication, and remained hidden away in an unpublished journal until
1859, when it was brought to light in a general ransacking of
archives.[350]
Indeed, Schwabe himself was far from anticipating the discovery which
fell to his share. He compared his fortune to that of Saul, who, seeking
his father's asses, found a kingdom.[351] For the hope which inspired
his early resolution lay in quite another direction. His patient ambush
was laid for a possible intramercurial planet, which, he thought, must
sooner or later betray its existence in crossing the face of the sun. He
took, however, the most effectual measures to secure whatever new
knowledge might be accessible. During forty-three years his
"imperturbable telescope"[352] never failed, weather and health
permitting, to bring in its daily report as to how many, or if any,
spots were visible on the sun's disc, the information obtained being day
by day recorded on a simple and unvarying system. In 1843 he made his
first announcement of a probable decennial period,[353] but it met with
no general attention; although Julius Schmidt of Bonn (afterwards
director of the Athens Observatory) and Gautier of Geneva were impressed
with his figures, and Littrow had himself, in 1836,[354] hinted at the
likelihood of some kind of regular recurrence. Schwabe, however, worked
on, gathering each year fresh evidence of a law such as he had
indicated; and when Humboldt published in 1851, in the third volume of
his _Kosmos_,[355] a table of the sun-spot statistics collected by him
from 1826 downwards, the strength of his case was perceived with, so to
speak, a start of surprise; the reality and importance of the discovery
were simultaneously recognised, and the persevering Hofrath of Dessau
found himself famous among astronomers. His merit--recognised by the
bestowal of the Astronomical Society's Gold Medal in 1857--consisted in
his choice of an original and appropriate line of work, and in the
admirable tenacity of purpose with which he pursued it. His resources
and acquirements were those of an ordinary amateur; he was distinguished
solely by the unfortunately rare power of turning both to the best
account. He died where he was born and had lived, April 11, 1875, at the
ripe age of eighty-six.
Meanwhile an investigation of a totally different character, and
conducted by totally different means, had been prosecuted to a very
similar conclusion. Two years after Schwabe began his solitary
observations, Humboldt gave the first impulse, at the Scientific
Congress of Berlin in 1828, to a great international movement for
attacking simultaneously, in various parts of the globe, the complex
problem of terrestrial magnetism. Through the genius and energy of
Gauss, Göttingen became its centre. Thence new apparatus, and a new
system for its employment, issued; there, in 1833, the first regular
magnetic observatory was founded, whilst at Göttingen was fixed the
universal time-standard for magnetic observations. A letter addressed by
Humboldt in April, 1836, to the Duke of Sussex as President of the Royal
Society, enlisted the co-operation of England. A network of magnetic
stations was spread all over the British dominions, from Canada to Van
Diemen's Land; measures were concerted with foreign authorities, and an
expedition was fitted out, under the able command of Captain (afterwards
Sir James) Clark Ross, for the special purpose of bringing intelligence
on the subject from the dismal neighbourhood of the South Pole. In 1841,
the elaborate organisation created by the disinterested efforts of
scientific "agitators" was complete; Gauss's "magnetometers" were
vibrating under the view of attentive observers in five continents, and
simultaneous results began to be recorded.
Ten years later, in September, 1851, Dr. John Lamont, the Scotch
director of the Munich Observatory, in reviewing the magnetic
observations made at Göttingen and Munich from 1835 to 1850, perceived
with some surprise that they gave unmistakable indications of a period
which he estimated at 10-1/3 years.[356] The manner in which this
periodicity manifested itself requires a word of explanation. The
observations in question referred to what is called the "declination" of
the magnetic needle--that is, to the position assumed by it with
reference to the points of the compass when moving freely in a
horizontal plane. Now this position--as was discovered by Graham in
1722--is subject to a small daily fluctuation, attaining its maximum
towards the east about 8 A.M., and its maximum towards the west shortly
before 2 P.M. In other words, the direction of the needle approaches (in
these countries at the present time) nearest to the true north some four
hours before noon, and departs farthest from it between one and two
hours after noon. It was the _range_ of this daily variation that Lamont
found to increase and diminish once in every 10-1/3 years.
In the following winter, Sir Edward Sabine, ignorant as yet of Lamont's
conclusion, undertook to examine a totally different set of
observations. The materials in his hands had been collected at the
British colonial stations of Toronto and Hobarton from 1843 to 1848, and
had reference, not to the regular diurnal swing of the needle, but to
those curious spasmodic vibrations, the inquiry into the laws of which
was the primary object of the vast organisation set on foot by Humboldt
and Gauss. Yet the upshot was practically the same. Once in about ten
years, magnetic disturbances (termed by Humboldt "storms") were
perceived to reach a maximum of violence and frequency. Sabine was the
first to note the coincidence between this unlooked-for result and
Schwabe's sun-spot period. He showed that, so far as observation had yet
gone, the two cycles of change agreed perfectly both in duration and
phase, maximum corresponding to maximum, minimum to minimum. What the
nature of the connection could be that bound together by a common law
effects so dissimilar as the rents in the luminous garment of the sun,
and the swayings to and fro of the magnetic needle, was and still
remains beyond the reach of well-founded theory; but the fact was from
the first undeniable.
The memoir containing this remarkable disclosure was presented to the
Royal Society, March 18, and read May 6, 1852.[357] On the 31st of July
following, Rudolf Wolf at Berne,[358] and on the 18th of August, Alfred
Gautier at Sion,[359] announced, separately and independently, perfectly
similar conclusions. This triple event is perhaps the most striking
instance of the successful employment of the Baconian method of
co-operation in discovery, by which "particulars" are amassed by one set
of investigators--corresponding to the "Depredators" and "Inoculators"
of Solomon's House--while inductions are drawn from them by another and
a higher class--the "Interpreters of Nature." Yet even here the
convergence of two distinct lines of research was wholly fortuitous, and
skilful combination owed the most brilliant part of its success to the
unsought bounty of what we call Fortune.
The exactness of the coincidence thus brought to light was fully
confirmed by further inquiries. A diligent search through the scattered
records of sun-spot observations, from the time of Galileo and Scheiner
onwards, put Wolf[360] in possession of materials by which he was
enabled to correct Schwabe's loosely-indicated decennial period to one
of slightly over eleven (11.11) years; and he further showed that this
fell in with the ebb and flow of magnetic change even better than
Lamont's 10-1/3 year cycle. The analogy was also pointed out between the
"light-curve," or zig-zagged line representing on paper the varying
intensity in the lustre of certain stars, and the similar delineation of
spot-frequency; the ascent from minimum to maximum being, in both cases,
usually steeper than the descent from maximum to minimum; while an
additional point of resemblance was furnished by the irregularities in
height of the various maxima. In other words, both the number of spots
on the sun and the brightness of variable stars increase, as a rule,
more rapidly than they decrease; nor does the amount of that increase,
in either instance, show any approach to uniformity.
The endeavour, suggested by the very nature of the phenomenon, to
connect sun-spots with weather was less successful. The first attempt of
the kind was made by Sir William Herschel in 1801, and a very notable
one it was. Meteorological statistics, save of the scantiest and most
casual kind, did not then exist; but the price of corn from year to year
was on record, and this, with full recognition of its inadequacy, he
adopted as his criterion. Nor was he much better off for information
respecting the solar condition. What little he could obtain, however,
served, as he believed, to confirm his surmise that a copious emission
of light and heat accompanies an abundant formation of "openings" in the
dazzling substance whence our supply of those indispensable commodities
is derived.[361] He gathered, in short, from his inquiries very much
what he had expected to gather, namely, that the price of wheat was high
when the sun showed an unsullied surface, and that food and spots became
plentiful together.[362]
Yet this plausible inference was scarcely borne out by a more exact
collocation of facts. Schwabe failed to detect any reflection of the
sun-spot period in his meteorological register. Gautier[363] reached a
provisional conclusion the reverse--though not markedly the reverse--of
Herschel's. Wolf, in 1852, derived from an examination of Vogel's
collection of Zürich Chronicles (1000-1800 A.D.) evidence showing (as he
thought) that minimum years were usually wet and stormy, maximum years
dry and genial;[364] but a subsequent review of the subject in 1859
convinced him that no relation of any kind between the two kinds of
effects was traceable.[365] With the singular affection of our
atmosphere known as the Aurora Borealis (more properly Aurora Polaris)
the case was different. Here the Zürich Chronicles set Wolf on the right
track in leading him to associate such luminous manifestations with a
disturbed condition of the sun; since subsequent detailed observation
has exhibited the curve of auroral frequency as following with such
fidelity the jagged lines figuring to the eye the fluctuations of solar
and magnetic activity, as to leave no reasonable doubt that all three
rise and sink together under the influence of a common cause. As long
ago as 1716,[366] Halley had conjectured that the Northern Lights were
due to magnetic "effluvia," but there was no evidence on the subject
forthcoming until Hiorter observed at Upsala in 1741 their agitating
influence upon the magnetic needle. That the effect was no casual one
was made superabundantly clear by Arago's researches in 1819 and
subsequent years. Now both were perceived to be swayed by the same
obscure power of cosmical disturbance.
The sun is not the only one of the heavenly bodies by which the
magnetism of the earth is affected. Proofs of a similar kind of lunar
action were laid by Kreil in 1841 before the Bohemian Society of
Sciences, and with minor corrections were fully substantiated by
Sabine's more extended researches. It was thus ascertained that each
lunar day, or the interval of twenty-four hours and about fifty-four
minutes between two successive meridian passages of our satellite, is
marked by a perceptible, though very small, double oscillation of the
needle--two progressive movements from east to west, and two returns
from west to east.[367] Moreover, the lunar, like the solar influence
(as was proved in each case by Sabine's analysis of the Hobarton and
Toronto observations), extends to all three "magnetic elements,"
affecting not only the position of the horizontal or _declination_
needle, but also the dip and intensity. It seems not unreasonable to
attribute some portion of the same subtle power to the planets and even
to the stars, though with effects rendered imperceptible by distance.
We have now to speak of the discovery and application to the heavenly
bodies of a totally new method of investigation. Spectrum analysis may
be shortly described as a mode of distinguishing the various species of
matter by the kind of light proceeding from each. This definition at
once explains how it is that, unlike every other system of chemical
analysis, it has proved available in astronomy. Light, so far as
_quality_ is concerned, ignores distance. No intrinsic change, that we
yet know of, is produced in it by a journey from the farthest bounds of
the visible universe; so that, provided only that in _quantity_ it
remain sufficient for the purpose, its peculiarities can be equally well
studied whether the source of its vibrations be one foot or a hundred
billion miles distant. Now the most obvious distinction between one kind
of light and another resides in colour. But of this distinction the eye
takes cognisance in an æsthetic, not in a scientific sense. It finds
gladness in the "thousand tints" of nature, but can neither analyse nor
define them. Here the refracting prism--or the combination of prisms
known as the "spectroscope"--comes to its aid, teaching it to measure as
well as to perceive. It furnishes, in a word, an accurate scale of
colour. The various rays which, entering the eye together in a confused
crowd, produce a compound impression made up of undistinguishable
elements, are, by the mere passage through a triangular piece of glass,
separated one from the other, and ranged side by side in orderly
succession, so that it becomes possible to tell at a glance what kinds
of light are present, and what absent. Thus, if we could only be assured
that the various chemical substances when made to glow by heat, emit
characteristic rays--rays, that is, occupying a place in the spectrum
reserved for them, and for them _only_--we should at once be in
possession of a mode of identifying such substances with the utmost
readiness and certainty. This assurance, which forms the solid basis of
spectrum analysis, was obtained slowly and with difficulty.
The first to employ the prism in the examination of various flames (for
it is only in a state of vapour that matter emits distinctive light) was
a young Scotchman named Thomas Melvill, who died in 1753, at the age of
twenty-seven. He studied the spectrum of burning spirits, into which
were successively introduced sal ammoniac, potash, alum, nitre, and
sea-salt, and observed the singular predominance, under almost all
circumstances, of a particular shade of yellow light, perfectly definite
in its degree of refrangibility[368]--in other words, taking up a
perfectly definite position in the spectrum. His experiments were
repeated by Morgan,[369] Wollaston, and--with far superior precision and
diligence--by Fraunhofer.[370] The great Munich optician, whose work was
completely original, rediscovered Melvill's deep yellow ray and measured
its place in the colour-scale. It has since become well known as the
"sodium line," and has played a very important part in the history of
spectrum analysis. Nevertheless, its ubiquity and conspicuousness long
impeded progress. It was elicited by the combustion of a surprising
variety of substances--sulphur, alcohol, ivory, wood, paper; its
persistent visibility suggesting the accomplishment of some universal
process of nature rather than the presence of one individual kind of
matter. But if spectrum analysis were to exist as a science at all, it
could only be by attaining certainty as to the unvarying association of
one special substance with each special quality of light.
Thus perplexed, Fox Talbot[371] hesitated in 1826 to enounce this
fundamental principle. He was inclined to believe that the presence in
the spectrum of any individual ray told unerringly of the volatilisation
in the flame under scrutiny of some body as whose badge or distinctive
symbol that ray might be regarded; but the continual prominence of the
yellow beam staggered him. It appeared, indeed, without fail where
sodium _was_; but it also appeared where it might be thought only
reasonable to conclude that sodium _was not_. Nor was it until thirty
years later that William Swan,[372] by pointing out the extreme delicacy
of the spectral test, and the singularly wide dispersion of sodium, made
it appear probable (but even then only probable) that the questionable
yellow line was really due invariably to that substance. Common salt
(chloride of sodium) is, in fact, the most diffusive of solids. It
floats in the air; it flows with water; every grain of dust has its
attendant particle; its absolute exclusion approaches the impossible.
And withal, the light that it gives in burning is so intense and
concentrated, that if a single grain be divided into 180 million parts,
and one alone of such inconceivably minute fragments be present in a
source of light, the spectroscope will show unmistakably its
characteristic beam.
Amongst the pioneers of knowledge in this direction were Sir John
Herschel[373]--who, however, applied himself to the subject in the
interests of optics, not of chemistry--W. A. Miller,[374] and
Wheatstone. The last especially made a notable advance when, in the
course of his studies on the "prismatic decomposition" of the electric
light, he reached the significant conclusion that the rays visible in
its spectrum were different for each kind of metal employed as
"electrodes."[375] Thus indications of a wider principle were to be
found in several quarters, but no positive certainty on any single point
was obtained, until, in 1859, Gustav Kirchhoff, professor of physics in
the University of Heidelberg, and his colleague, the eminent chemist
Robert Bunsen, took the matter in hand. By them the general question as
to the necessary and invariable connection of certain rays in the
spectrum with certain kinds of matter, was first resolutely confronted,
and first definitely answered. It was answered affirmatively--else there
could have been no science of spectrum analysis--as the result of
experiments more numerous, more stringent, and more precise than had
previously been undertaken.[376] And the assurance of their conclusion
was rendered doubly sure by the discovery, through the peculiarities of
their light alone, of two new metals, named from the blue and red rays
by which they were respectively distinguished, "cæsium," and
"rubidium."[377] Both were immediately afterwards actually obtained in
small quantities by evaporation of the Durckheim mineral waters.
The link connecting this important result with astronomy may now be
indicated. In the year 1802 it occurred to William Hyde Wollaston to
substitute for the round hole used by Newton and his successors for the
admittance of light to be examined with the prism, an elongated
"crevice" 1/20th of an inch in width. He thereupon perceived that the
spectrum, thus formed of light, as it were, _purified_ by the abolition
of overlapping images, was traversed by seven dark lines. These he took
to be natural boundaries of the various colours,[378] and satisfied with
this quasi-explanation, allowed the subject to drop. It was
independently taken up after twelve years by a man of higher genius. In
the course of experiments on light, directed towards the perfecting of
his achromatic lenses, Fraunhofer, by means of a slit and a telescope,
made the surprising discovery that the solar spectrum is crossed, not by
seven, but by thousands of obscure transverse streaks.[379] Of these he
counted some 600, and carefully mapped 324, while a few of the most
conspicuous he set up (if we may be permitted the expression) as
landmarks, measuring their distances apart with a theodolite, and
affixing to them the letters of the alphabet, by which they are still
universally known. Nor did he stop here. The same system of examination
applied to the rest of the heavenly bodies showed the mild effulgence of
the moon and planets to be deficient in precisely the same rays as
sunlight; while in the stars it disclosed the differences in likeness
which are always an earnest of increased knowledge. The spectra of
Sirius and Castor, instead of being delicately ruled crosswise
throughout, like that of the sun, were seen to be interrupted by three
massive bars of darkness--two in the blue and one in the green;[380] the
light of Pollux, on the other hand, seemed precisely similar to sunlight
attenuated by distance or reflection, and that of Capella, Betelgeux,
and Procyon to share some of its peculiarities. One solar line
especially--that marked in his map with the letter D--proved common to
all the four last-mentioned stars; and it was remarkable that it exactly
coincided in position with the conspicuous yellow beam (afterwards, as
we have said, identified with the light of glowing sodium) which he had
already found to accompany most kinds of combustion. Moreover, both the
_dark_ solar and the _bright_ terrestrial "D lines" were displayed by
the refined Munich appliances as double.
In this striking correspondence, discovered by Fraunhofer in 1815, was
contained the very essence of solar chemistry; but its true significance
did not become apparent until long afterwards. Fraunhofer was by
profession, not a physicist, but a practical optician. Time pressed; he
could not and would not deviate from his appointed track; all that was
possible to him was to indicate the road to discovery, and exhort others
to follow it.[381]
Partially and inconclusively at first this was done. The "fixed lines"
(as they were called) of the solar spectrum took up the position of a
standing problem, to the solution of which no approach seemed possible.
Conjectures as to their origin were indeed rife. An explanation put
forward by Zantedeschi[382] and others, and dubiously favoured by Sir
David Brewster and Dr. J. H. Gladstone,[383] was that they resulted from
"interference"--that is, a destruction of the motion producing in our
eyes the sensation of light, by the superposition of two light-waves in
such a manner that the crests of one exactly fill up the hollows of the
other. This effect was supposed to be brought about by imperfections in
the optical apparatus employed.
A more plausible view was that the atmosphere of the earth was the agent
by which sunlight was deprived of its missing beams. For a few of them
this is actually the case. Brewster found in 1832 that certain dark
lines, which were invisible when the sun stood high in the heavens,
became increasingly conspicuous as he approached the horizon.[384] These
are the well-known "atmospheric lines;" but the immense majority of
their companions in the spectrum remain quite unaffected by the
thickness of the stratum of air traversed by the sunlight containing
them. They are then obviously due to another cause.
There remained the true interpretation--absorption in the _sun's_
atmosphere; and this, too, was extensively canvassed. But a remarkable
observation made by Professor Forbes of Edinburgh[385] on the occasion
of the annular eclipse of May 15, 1836, appeared to throw discredit upon
it. If the problematical dark lines were really occasioned by the
stoppage of certain rays through the action of a vaporous envelope
surrounding the sun, they ought, it seemed, to be strongest in light
proceeding from his edges, which, cutting that envelope obliquely,
passed through a much greater depth of it. But the circle of light left
by the interposing moon, and of course derived entirely from the rim of
the solar disc, yielded to Forbes's examination precisely the same
spectrum as light coming from its central parts. This circumstance
helped to baffle inquirers, already sufficiently perplexed. It still
remains an anomaly, of which no satisfactory explanation has been
offered.
Convincing evidence as to the true nature of the solar lines was however
at length, in the autumn of 1859, brought forward at Heidelburg.
Kirchhoff's _experimentum crucis_ in the matter was a very simple one.
He threw bright sunshine across a space occupied by vapour of sodium,
and perceived with astonishment that the dark Fraunhofer line D, instead
of being effaced by flame giving a luminous ray of the same
refrangibility, was deepened and thickened by the superposition.
He tried the same experiment, substituting for sunbeams light from a
Drummond lamp, and with similar result. A dark furrow, corresponding in
every respect to the solar D-line, was instantly seen to interrupt the
otherwise unbroken radiance of its spectrum. The inference was
irresistible, that the effect thus produced artificially was brought
about naturally in the same way, and that sodium formed an ingredient in
the glowing atmosphere of the sun.[386] This first discovery was quickly
followed up by the identification of numerous bright rays in the spectra
of other metallic bodies with others of the hitherto mysterious
Fraunhofer lines. Kirchhoff was thus led to the conclusion that (besides
sodium) iron, magnesium, calcium, and chromium, are certainly solar
constituents, and that copper, zinc, barium, and nickel are also
present, though in smaller quantities.[387] As to cobalt, he hesitated
to pronounce, but its existence in the sun has since been established.
These memorable results were founded upon a general principle first
enunciated by Kirchhoff in a communication to the Berlin Academy,
December 15, 1859, and afterwards more fully developed by him.[388] It
may be expressed as follows: Substances of every kind are opaque to the
precise rays which they emit at the same temperature; that is to say,
they stop the kinds of light or heat which they are then actually in a
condition to radiate. But it does not follow that _cool_ bodies absorb
the rays which they would give out if sufficiently heated. Hydrogen at
ordinary temperatures, for instance, is almost perfectly transparent,
but if raised to the glowing point--as by the passage of electricity--it
_then_ becomes capable of arresting, and at the same time of displaying
in its own spectrum light of four distinct colours.
This principle is fundamental to solar chemistry. It gives the key to
the hieroglyphics of the Fraunhofer lines. The identical characters
which are written _bright_ in terrestrial spectra are written _dark_ in
the unrolled sheaf of sun-rays; the meaning remains unchanged. It must,
however, be remembered that they are only _relatively_ dark. The
substances stopping those particular tints in the neighbourhood of the
sun are at the same time vividly glowing with the very same. Remove the
dazzling solar background, by contrast with which they show as obscure,
and they will be seen, and, at critical moments, actually have been
seen, in all their native splendour. It is because the atmosphere of the
sun is cooler than the globe it envelops that the different kinds of
vapour constituting that atmosphere take more than they give, absorb
more light than they are capable of emitting; raise them to the same
temperature as the sun itself, and their powers of emission and
absorption being brought exactly to the same level, the thousands of
dusky rays in the solar spectrum will be at once obliterated.
The establishment of the terrestrial science of spectrum analysis was
due, as we have seen, equally to Kirchhoff and Bunsen, but its celestial
application to Kirchhoff alone. He effected this object of the
aspirations, more or less dim, of many other thinkers and workers, by
the union of two separate, though closely related lines of research--the
study of the different kinds of light _emitted_ by various bodies, and
the study of the different kinds of light _absorbed_ by them. The latter
branch appears to have been first entered upon by Dr. Thomas Young in
1803;[389] it was pursued by the younger Herschel,[390] by William Allen
Miller, Brewster, and Gladstone. Brewster indeed made, in 1833,[391] a
formal attempt to found what might be called an inverse system of
analysis with the prism based upon absorption; and his efforts were
repeated, just a quarter of a century later, by Gladstone.[392] But no
general point of view was attained; nor, it may be added, was it by this
path attainable.
Kirchhoff's map of the solar spectrum, drawn to scale with exquisite
accuracy, and printed in three shades of ink to convey the graduated
obscurity of the lines, was published in the Transactions of the Berlin
Academy for 1861 and 1862.[393] Representations of the principal lines
belonging to various elementary bodies formed, as it were, a series of
marginal notes accompanying the great solar scroll, enabling the veriest
tiro in the new science to decipher its meaning at a glance. Where the
dark solar and bright metallic rays agreed in position, it might safely
be inferred that the metal emitting them was a solar constituent; and
such coincidences were numerous. In the case of iron alone, no less than
sixty occurred in one-half of the spectral area, rendering the
chances[394] absolutely overwhelming against mere casual conjunction.
The preparation of this elaborate picture proved so trying to the eyes
that Kirchhoff was compelled by failing vision to resign the latter half
of the task to his pupil Hofmann. The complete map measured nearly eight
feet in length.
The conclusions reached by Kirchhoff were no sooner announced than they
took their place, with scarcely a dissenting voice, among the
established truths of science. The broad result, that the dark lines in
the spectrum of the sun afford an index to its chemical composition no
less reliable than any of the tests used in the laboratory, was equally
captivating to the imagination of the vulgar, and authentic in the
judgment of the learned; and, like all genuine advances in the knowledge
of Nature, it stimulated curiosity far more than it gratified it. Now
the history of how discoveries were missed is often quite as instructive
as the history of how they were made; it may then be worth while to
expend a few words on the thoughts and trials by which, in the present
case, the actual event was heralded.
Three times it seemed on the verge of being anticipated. The experiment,
which in Kirchhoff's hands proved decisive, of passing sunlight through
glowing vapours and examining the superposed spectra, was performed by
Professor W. A. Miller of King's College in 1845.[395] Nay, more, it was
performed with express reference to the question, then already (as has
been noted) in debate, of the possible production of Fraunhofer's lines
by absorption in a solar atmosphere. Yet it led to nothing.
Again, at Paris in 1849, with a view to testing the asserted coincidence
between the solar D-line and the bright yellow beam in the spectrum of
the electric arc (really due to the unsuspected presence of sodium),
Léon Foucault threw a ray of sunshine across the arc and observed its
spectrum.[396] He was surprised to see that the D-line was rendered more
intensely dark by the combination of lights. To assure himself still
further, he substituted a reflected image of one of the white-hot
carbon-points for the sunbeam, with an identical result. _The same ray
was missing._ It needed but another step to have generalised this
result, and thus laid hold of a natural truth of the highest importance;
but that step was not taken. Foucault, keen and brilliant though he was,
rested satisfied with the information that the _voltaic arc_ had the
power of stopping the kind of light emitted by it; he asked no further
question, and was consequently the bearer of no further intelligence on
the subject.
The truth conveyed by this remarkable experiment was, however, divined
by one eminent man. Professor Stokes of Cambridge stated to Sir William
Thomson (now Lord Kelvin), shortly after it had been made, his
conviction that an absorbing atmosphere of sodium surrounded the sun.
And so forcibly was his hearer impressed with the weight of the argument
based upon the absolute agreement of the D-line in the solar spectrum
with the yellow ray of burning sodium (then freshly certified by W. H.
Miller), combined with Foucault's "reversal" of that ray, that he
regularly inculcated, in his public lectures on natural philosophy at
Glasgow, five or six years before Kirchhoff's discovery, not only the
_fact_ of the presence of sodium in the solar neighbourhood, but also
the _principle_ of the study of solar and stellar chemistry in the
spectra of flames.[397] Yet it does not appear to have occurred to
either of these two distinguished professors--themselves among the
foremost of their time in the successful search for new truths--to
verify practically a sagacious conjecture in which was contained the
possibility of a scientific revolution. It is just to add, that
Kirchhoff was unacquainted, when he undertook his investigation, either
with the experiment of Foucault or the speculation of Stokes.
For C. J. Ångström, on the other hand, perhaps somewhat too much has
been claimed in the way of anticipation. His _Optical Researches_
appeared at Upsala in 1853, and in their English garb two years
later.[398] They were undoubtedly pregnant with suggestion, yet made no
epoch in discovery. The old perplexities continued to prevail after, as
before their publication. To Ångström, indeed, belongs the great merit
of having revived Euler's principle of the equivalence of emission and
absorption; but he revived it in its original crude form, and without
the qualifying proviso which alone gave it value as a clue to new
truths. According to his statement, a body absorbs all the series of
vibrations it is, under any circumstances, capable of emitting, as well
as those connected with them by simple harmonic relations. This is far
too wide. To render it either true or useful, it had to be reduced to
the cautious terms employed by Kirchhoff. Radiation strictly and
necessarily corresponds with absorption only _when the temperature is
the same_. In point of fact, Ångström was still, in 1853, divided
between adsorption and interference as the mode of origin of the
Fraunhofer dark rays. Very important, however, was his demonstration of
the compound nature of the spark-spectrum, which he showed to be made up
of the spectrum of the metallic electrodes superposed upon that of the
gas or gases across which the discharge passed.
It may here be useful--since without some clear ideas on the subject no
proper understanding of recent astronomical progress is possible--to
take a cursory view of the elementary principles of spectrum analysis.
To many of our readers they are doubtless already familiar; but it is
better to appear trite to some than obscure even to a few.
The spectrum, then, of a body is simply the light proceeding from it
_spread out_ by refraction[399] into a brilliant variegated band,
passing from brownish-red through crimson, orange, yellow, green, and
azure into dusky violet. The reason of this spreading-out or
"dispersion" is that the various colours have different wave-lengths,
and consequently meet with different degrees of retardation in
traversing the denser medium of the prism. The shortest and quickest
vibrations (producing the sensation we call "violet") are thrown
farthest away from their original path--in other words, suffer the
widest "deviation;" the longest and slowest (the red) travel much nearer
to it. Thus the sheaf of rays which would otherwise combine into a patch
of white light are separated through the divergence of their tracks
after refraction by a prism, so as to form a tinted riband. This
_visible_ spectrum is prolonged _invisibly_ at both ends by a long range
of vibrations, either too rapid or too sluggish to affect the eye as
light, but recognisable through their chemical and heating effects.
Now all incandescent solid or liquid substances, and even gases ignited
under great pressure, give what is called a "continuous spectrum;" that
is to say, the light derived from them is of every conceivable hue.
Sorted out with the prism, its tints merge imperceptibly one into the
other, uninterrupted by any dark spaces. No colours, in short, are
missing. But gases and vapours rendered luminous by heat emit rays of
only a few tints, which accordingly form an interrupted spectrum,
usually designated as one of lines or bands. And since these rays are
perfectly definite and characteristic--not being the same for any two
substances--it is easy to tell what kind of matter is concerned in
producing them. We may suppose that the inconceivably minute particles
which by their rapid thrilling agitate the ethereal medium so as to
produce light, are free to give out their peculiar tone of vibration
only when floating apart from each other in gaseous form; but when
crowded together into a condensed mass, the clear ring of the
distinctive note is drowned, so to speak, in a universal molecular
clang. Thus prismatic analysis has no power to identify individual kinds
of matter, except when they present themselves as glowing vapours.
A spectrum is said to be "reversed" when lines previously seen bright on
a dark background appear dark on a bright background. In this form it is
equally characteristic of chemical composition with the "direct"
spectrum, being due to _absorption_, as the latter is to _emission_. And
absorption and emission are, by Kirchhoff's law, strictly correlative.
This is easily understood by the analogy of sound. For just as a
tuning-fork responds to sound-waves of its own pitch, but remains
indifferent to those of any other, so those particles of matter whose
nature it is, when set swinging by heat, to vibrate a certain number of
times in a second, thus giving rise to light of a particular shade of
colour, appropriate those same vibrations, and those only, when
transmitted past them,--or, phrasing it otherwise, are opaque to them,
and transparent to all others.
It should further be explained that the _shape_ of the bright or dark
spaces in the spectrum has nothing whatever to do with the nature of the
phenomena. The "lines" and "bands" so frequently spoken of are seen as
such for no other reason than because the light forming them is admitted
through a narrow, straight opening. Change that opening into a fine
crescent or a sinuous curve, and the "lines" will at once appear as
crescents or curves.
Resuming in a sentence what has been already explained, we find that the
prismatic analysis of the heavenly bodies was founded upon three classes
of facts: First, the unmistakable character of the light given by each
different kind of glowing vapour; secondly, the identity of the light
absorbed with the light emitted by each; thirdly, the coincidence
observed between rays missing from the solar spectrum and rays absorbed
by various terrestrial substances. Thus, a realm of knowledge,
pronounced by Morinus[400] in the seventeenth century, and no less
dogmatically by Auguste Comte[401] in the nineteenth, hopelessly out of
reach of the human intellect, was thrown freely open, and the chemistry
of the sun and stars took at once a leading place among the experimental
sciences.
The immediate increase of knowledge was not the chief result of
Kirchhoff's labours; still more important was the change in the scope
and methods of astronomy, which, set on foot in 1852 by the detection of
a common period affecting at once the spots on the sun and the magnetism
of the earth, was extended and accelerated by the discovery of spectrum
analysis. The nature of that change is concisely indicated by the
heading of the present chapter; we would now ask our readers to
endeavour to realise somewhat distinctly what is implied by the
"foundation of astronomical physics."
Just three centuries ago, Kepler drew a forecast of what he called a
"physical astronomy"--a science treating of the efficient causes of
planetary motion, and holding the "key to the inner astronomy."[402]
What Kepler dreamed of and groped after, Newton realized. He showed the
beautiful and symmetrical revolutions of the solar system to be governed
by a uniformly acting cause, and that cause no other than the familiar
force of gravity, which gives stability to all our terrestrial
surroundings. The world under our feet was thus for the first time
brought into physical connection with the worlds peopling space, and a
very tangible relationship was demonstrated as existing between what
used to be called the "corruptible" matter of the earth and the
"incorruptible" matter of the heavens.
This process of unification of the cosmos--this levelling of the
celestial with the sublunary--was carried no farther until the fact
unexpectedly emerged from a vast and complicated mass of observations,
that the magnetism of the earth is subject to subtle influences,
emanating, certainly from some, and presumably from all of the heavenly
bodies; the inference being thus rendered at least plausible, that a
force not less universal than gravity itself, but with whose modes of
action we are as yet unacquainted, pervades the universe, and forms, it
might be said, an intangible bond of sympathy between its parts. Now for
the investigation of this influence two roads are open. It may be
pursued by observation either of the bodies from which it proceeds, or
of the effects which it produces--that is to say, either by the
astronomer or by the physicist, or, better still, by both concurrently.
Their acquisitions are mutually profitable; nor can either be considered
as independent of the other. Any important accession to knowledge
respecting the sun, for example, may be expected to cast a reflected
light on the still obscure subject of terrestrial magnetism; while
discoveries in magnetism or its _alter ego_ electricity must profoundly
affect solar inquiries.
The establishment of the new method of spectrum analysis drew far closer
this alliance between celestial and terrestrial science. Indeed, they
have come to merge so intimately one into the other, that it is no
easier to trace their respective boundaries than it is to draw a clear
dividing-line between the animal and vegetable kingdoms. Yet up to the
middle of the last century, astronomy, while maintaining her strict
union with mathematics, looked with indifference on the rest of the
sciences; it was enough that she possessed the telescope and the
calculus. Now the materials for her inductions are supplied by the
chemist, the electrician, the inquirer into the most recondite mysteries
of light and the molecular constitution of matter. She is concerned with
what the geologist, the meteorologist, even the biologist, has to say;
she can afford to close her ears to no new truth of the physical order.
Her position of lofty isolation has been exchanged for one of community
and mutual aid. The astronomer has become, in the highest sense of the
term, a physicist; while the physicist is bound to be something of an
astronomer.
This, then, is what is designed to be conveyed by the "foundation of
astronomical or cosmical physics." It means the establishment of a
science of Nature whose conclusions are not only presumed by analogy,
but are ascertained by observation, to be valid wherever light can
travel and gravity is obeyed--a science by which the nature of the stars
can be studied upon the earth, and the nature of the earth can be made
better known by study of the stars--a science, in a word, which is, or
aims at being, one and universal, even as Nature--the visible reflection
of the invisible highest Unity--is one and universal.
It is not too much to say that a new birth of knowledge has ensued. The
astronomy so signally promoted by Bessel[403]--the astronomy placed by
Comte[404] at the head of the hierarchy of the physical sciences--was
the science of the _movements_ of the heavenly bodies. And there were
those who began to regard it as a science which, from its very
perfection, had ceased to be interesting--whose tale of discoveries was
told, and whose farther advance must be in the line of minute technical
improvements, not of novel and stirring disclosures. But the science of
the _nature_ of the heavenly bodies is one only in the beginning of its
career. It is full of the audacities, the inconsistencies, the
imperfections, the possibilities of youth. It promises everything; it
has already performed much; it will doubtless perform much more. The
means at its disposal are vast and are being daily augmented. What has
so far been secured by them it must now be our task to extricate from
more doubtful surroundings and place in due order before our readers.
FOOTNOTES:
[Footnote 347: Wolf, _Gesch. der Astr._, p. 655.]
[Footnote 348: Manuel Johnson, _Mem. R.A.S._, vol. xxvi., p. 197.]
[Footnote 349: _Astronomie Théorique et Pratique_, t. iii., p. 20.]
[Footnote 350: Wolf, _Gesch. der Astr._, p. 654.]
[Footnote 351: _Month. Not._, vol. xvii., p. 241.]
[Footnote 352: _Mem. R.A.S._, vol. xxvi., p. 200.]
[Footnote 353: _Astr. Nach._, No. 495.]
[Footnote 354: Gehler's _Physikalisches Wörterbuch_, art.
_Sonnenflecken_, p. 851.]
[Footnote 355: _Zweite Abth._, p. 401.]
[Footnote 356: _Annalen der Physik_ (Poggendorff's), Bd. lxxxiv., p.
580.]
[Footnote 357: _Phil. Trans._, vol. cxlii., p. 103.]
[Footnote 358: _Mittheilungen der Naturforschenden Gesellschaft_, 1852,
p. 183.]
[Footnote 359: _Archives des Sciences_, t. xxi., p. 194.]
[Footnote 360: _Neue Untersuchungen, Mitth. Naturf. Ges._, 1852, p.
249.]
[Footnote 361: _Phil. Trans._, vol. xci., p. 316.]
[Footnote 362: Evidence of an eleven-yearly fluctuation in the price of
food-grains in India was collected some years ago by Mr. Frederick
Chambers. _Nature_, vol. xxxiv., p. 100.]
[Footnote 363: _Bibl. Un. de Genève_, t. li., p. 336.]
[Footnote 364: _Neue Untersuchungen_, p. 269.]
[Footnote 365: _Die Sonne und ihre Flecken_, p. 30. Arago was the first
who attempted to decide the question by keeping, through a series of
years, a parallel register of sun-spots and weather; but the data
regarding the solar condition amassed at the Paris Observatory from 1822
to 1830 were not sufficiently precise to support any inference.]
[Footnote 366: _Phil. Trans._, vol. xxix., p. 421.]
[Footnote 367: _Ibid._, vols. cxliii., p. 558, cxlvi., p. 505.]
[Footnote 368: _Observations on Light and Colours_, p. 35.]
[Footnote 369: _Phil. Trans._, vol. lxxv., p. 190.]
[Footnote 370: _Denkschriften_ (Munich. Ac. of Sc.), 1814, 1815, Bd. v.,
p. 197.]
[Footnote 371: _Edinburgh Journal of Science_, vol. v., p. 77. See also
_Phil. Mag._, Feb., 1834, vol. iv., p. 112.]
[Footnote 372: _Ed. Phil. Trans.,_ vol. xxi., p. 411.]
[Footnote 373: _On the Absorption of Light by Coloured Media, Ed. Phil.
Trans._, vol. ix., p. 445 (1823).]
[Footnote 374: _Phil. Mag._, vol. xxvii, (ser. iii.), p. 81.]
[Footnote 375: _Report Brit. Ass._, 1835, p. 11 (pt. ii.). _Electrodes_
are the terminals from one to the other of which the electric spark
passes, volatilising and rendering incandescent in its transit some
particles of their substance, the characteristic light of which
accordingly flashes out in the spectrum.]
[Footnote 376: _Phil. Mag._, vol. xx., p. 93.]
[Footnote 377: _Annalen der Physik_, Bd. cxiii., p. 357.]
[Footnote 378: _Phil. Trans._, vol. xcii., p. 378.]
[Footnote 379: _Denkschriften_, Bd. v., p. 202.]
[Footnote 380: _Ibid._, p. 220; _Edin. Jour. of Science_, vol. viii., p.
9.]
[Footnote 381: _Denkschriften_, Bd. v., p. 222.]
[Footnote 382: _Arch. des Sciences_, 1849, p. 43.]
[Footnote 383: _Phil. Trans._, vol. cl., p. 159, _note_.]
[Footnote 384: _Ed. Phil. Trans._, vol. xii., p. 528.]
[Footnote 385: _Phil. Trans._, vol. cxxvi., p. 453. "I conceive," he
says, "that this result proves decisively that the sun's atmosphere has
nothing to do with the production of this singular phenomenon" (p. 455).
And Brewster's well-founded opinion that it had much to do with it was
thereby, in fact, overthrown.]
[Footnote 386: _Monatsberichte_, Berlin, 1859, p. 664.]
[Footnote 387: _Abhandlungen_, Berlin, 1861, pp. 80, 81.]
[Footnote 388: _Ibid._, 1861, p. 77; _Annalen der Physik_, Bd. cxix., p.
275. A similar conclusion, reached by Balfour Stewart in 1858, for
heat-rays (_Ed. Phil. Trans._, vol. xxii., p. 13), was, in 1860, without
previous knowledge of Kirchhoff's work, extended to light (_Phil. Mag._,
vol. xx., p. 534); but his experiments wanted the precision of those
executed at Heidelburg.]
[Footnote 389: _Miscellaneous Works_, vol. i., p. 189.]
[Footnote 390: _Ed. Phil. Trans._, vol. ix., p. 458.]
[Footnote 391: _Ibid._, vol. xii., p. 519.]
[Footnote 392: _Quart. Jour. Chem. Soc._, vol. x. p. 79.]
[Footnote 393: A facsimile accompanied Sir H. Roscoe's translation of
Kirchhoff's "Researches on the Solar Spectrum" (London, 1862-63).]
[Footnote 394: Estimated by Kirchhoff's at a _trillion to one_.
_Abhandl._, 1861, p. 79.]
[Footnote 395: _Phil. Mag._, vol. xxvii. (3rd series), p. 90.]
[Footnote 396: _L'Institut_, Feb. 7, 1849, p. 45; _Phil. Mag._, vol.
xix. (4th series), p. 193.]
[Footnote 397: _Ann. d. Phys._, vol. cxviii., p. 110.]
[Footnote 398: _Phil. Mag._, vol. ix. (4th series), p. 327.]
[Footnote 399: Spectra may be produced by _diffraction_ as well as by
_refraction_; but we are here only concerned with the subject in its
simplest aspect.]
[Footnote 400: _Astrologia Gallica_ (1661), p. 189.]
[Footnote 401: _Pos. Phil._, vol. i., pp. 114, 115 (Martineau's
trans.).]
[Footnote 402: _Proem Astronomiæ Pars Optica_ (1640), _Op._, t. ii.]
[Footnote 403: _Pop. Vorl._, pp. 14, 19, 408.]
[Footnote 404: _Pos. Phil._, p. 115.]
CHAPTER II
_SOLAR OBSERVATIONS AND THEORIES_
The zeal with which solar studies have been pursued during the last half
century has already gone far to redeem the neglect of the two preceding
ones. Since Schwabe's discovery was published in 1851, observers have
multiplied, new facts have been rapidly accumulated, and the previous
comparative quiescence of thought on the great subject of the
constitution of the sun, has been replaced by a bewildering variety of
speculations, conjectures, and more or less justifiable inferences. It
is satisfactory to find this novel impulse not only shared, but to a
large extent guided, by our countrymen.
William Rutter Dawes, one of many clergymen eminent in astronomy,
observed, in 1852, with the help of a solar eye-piece of his own
devising, some curious details of spot-structure.[405] The
umbra--heretofore taken for the darkest part of the spot--was seen to be
suffused with a mottled, nebulous illumination, in marked contrast with
the striated appearance of the penumbra; while through this "cloudy
stratum" a "black opening" permitted the eye to divine farther
unfathomable depths beyond. The _hole_ thus disclosed--evidently the
true nucleus--was found to be present in all considerable, as well as in
many small maculæ.
Again, the whirling motions of some of these objects were noticed by
him. The remarkable form of one sketched at Wateringbury, in Kent,
January 17, 1852, gave him the means of detecting and measuring a
rotatory movement of the whole spot round the black nucleus at the rate
of 100 degrees in six days. "It appeared," he said, "as if some
prodigious ascending force of a whirlwind character, in bursting through
the cloudy stratum and the two higher and luminous strata, had given to
the whole a movement resembling its own."[406] An interpretation
founded, as is easily seen, on the Herschelian theory, then still in
full credit.
An instance of the same kind was observed by Mr. W. R. Birt in
1860,[407] and cyclonic movements are now a recognised feature of
sun-spots. They are, however, as Father Secchi[408] concluded from his
long experience, but temporary and casual. Scarcely three per cent. of
all spots visible exhibit the spiral structure which should invariably
result if a conflict of opposing, or the friction of unequal, currents
were essential, and not merely incidental to their origin. A whirlpool
phase not unfrequently accompanies their formation, and may be renewed
at periods of recrudescence or dissolution; but it is both partial and
inconstant, sometimes affecting only one side of a spot, sometimes
slackening gradually its movement in one direction, to resume it, after
a brief pause, in the opposite. Persistent and uniform notions, such as
the analogy of terrestrial storms would absolutely require, are not to
be found. So that the "cyclonic theory" of sun-spots, suggested by
Herschel in 1847,[409] and urged, from a different point of view, by
Faye in 1872, may be said to have completely broken down.
The drift of spots over the sun's surface was first systematically
investigated by Carrington, a self-constituted astronomer, gifted with
the courage and the instinct of thoughtful labour.
Born at Chelsea in May, 1826, Richard Christopher Carrington entered
Trinity College, Cambridge, in 1844. He was intended for the Church, but
Professor Challis's lectures diverted him to astronomy, and he resolved,
as soon as he had taken his degree, to prepare, with all possible
diligence, to follow his new vocation. His father, who was a brewer on a
large scale at Brentford, offered no opposition; ample means were at his
disposal; nevertheless, he chose to serve an apprenticeship of three
years as observer in the University of Durham, as though his sole object
had been to earn a livelihood. He quitted the post only when he found
that its restricted opportunities offered no farther prospect of
self-improvement.
He now built an observatory of his own at Redhill in Surrey, with the
design of completing Bessel's and Argelander's survey of the northern
heavens by adding to it the circumpolar stars omitted from their view.
This project, successfully carried out between 1854 and 1857, had
another and still larger one superposed upon it before it had even begun
to be executed. In 1852, while the Redhill Observatory was in course of
erection, the discovery of the coincidence between the sun-spot and
magnetic periods was announced. Carrington was profoundly interested,
and devoted his enforced leisure to the examination of records, both
written and depicted, of past solar observations. Struck with their
fragmentary and inconsistent character, he resolved to "appropriate," as
he said, by "close and methodical research," the eleven-year period next
ensuing.[410] He calculated rightly that he should have the field pretty
nearly to himself; for many reasons conspire to make public
observatories slow in taking up new subjects, and amateurs with freedom
to choose, and means to treat them effectually, were scarcer then than
they are now.
The execution of this laborious task was commenced November 9, 1853. It
was intended to be merely a _parergon_--a "second subject," upon which
daylight energies might be spent, while the hours of night were reserved
for cataloguing those stars that "are bereft of the baths of ocean." Its
results, however, proved of the highest interest, although the
vicissitudes of life barred the completion, in its full integrity, of
the original design. By the death, in 1858, of the elder Carrington, the
charge of the brewery devolved upon his son; and eventually absorbed so
much of his care that it was found advisable to bring the solar
observations to a premature close, on March 24, 1861.
His scientific life may be said to have closed with them. Attacked four
years later with severe, and, in its results, permanent illness, he
disposed of the Brentford business, and withdrew to Churt, near Farnham,
in Surrey. There, in a lonely spot, on the top of a detached conical
hill known as the "Devil's Jump," he built a second observatory, and
erected an instrument which he was no longer able to use with pristine
effectiveness; and there, November 27, 1875, he died of the rupture of a
blood vessel on the brain, before he had completed his fiftieth
year.[411]
His observations of sun-spots were of a geometrical character. They
concerned positions and movements, leaving out of sight physical
peculiarities. Indeed, the prudence with which he limited his task to
what came strictly within the range of his powers to accomplish, was one
of Carrington's most valuable qualities. The method of his observations,
moreover, was chosen with the same practical sagacity as their objects.
As early as 1847, Sir John Herschel had recommended the daily
self-registration of sun-spots,[412] and he enforced the suggestion,
with more immediate prospect of success, in 1854.[413] The art of
celestial photography, however, was even then in a purely tentative
stage, and Carrington wisely resolved to waste no time on dubious
experiments, but employ the means of registration and measurement
actually at his command. These were very simple, yet very effective. To
the "helioscope" employed by Father Scheiner[414] two centuries and a
quarter earlier, a species of micrometer was added. The image of the sun
was projected upon a screen by means of a firmly-clamped telescope, in
the focus of which were placed two cross-wires forming angles of 45°
with the meridian. The six instants were then carefully noted at which
these were met by the edges of the disc as it traversed the screen, and
by the nucleus of the spot to be measured.[415] A short process of
calculation then gave the exact position of the spot as referred to the
sun's centre.
From a series of 5,290 observations made in this way, together with a
great number of accurate drawings, Carrington derived conclusions of
great importance on each of the three points which he had proposed to
himself to investigate. These were: the law of the sun's rotation, the
existence and direction of systematic currents, and the distribution of
spots on the solar surface.
Grave discrepancies were early perceived to exist between determinations
of the sun's rotation by different observers. Galileo, with "comfortable
generality," estimated the period at "about a lunar month";[416]
Scheiner, at twenty-seven days.[417] Cassini, in 1678, made it 25·58;
Delambre, in 1775, no more than twenty-five days. Later inquiries
brought these divergences within no more tolerable limits. Laugier's
result of 25·34 days--obtained in 1841--enjoyed the highest credit, yet
it differed widely in one direction from that of Böhm (1852), giving
25·52 days, and in the other from that of Kysæus (1846), giving 25·09
days. Now the cause of these variations was really obvious from the
first, although for a long time strangely overlooked. Scheiner pointed
out in 1630 that different spots gave different periods, adding the
significant remark that one at a distance from the solar equator
revolved more slowly than those nearer to it.[418] But the hint was
wasted. For upwards of two centuries ideas on the subject were either
retrograde or stationary. What were called the "proper motions" of spots
were, however, recognised by Schröter,[419] and utterly baffled
Laugier,[420] who despaired of obtaining any concordant result as to the
sun's rotation except by taking the mean of a number of discordant ones.
At last, in 1855, a valuable course of observations made at Capo di
Monte, Naples, in 1845-6, enabled C. H. F. Peters[421] to set in the
clearest light the insecurity of determinations based on the assumption
of fixity in objects plainly affected by movements uncertain both in
amount and direction.
Such was the state of affairs when Carrington entered upon his task.
Everything was in confusion; the most that could be said was that the
confusion had come to be distinctly admitted and referred to its true
source. What he discovered was this: that the sun, or at least the outer
shell of the sun visible to us, has _no single period of rotation_, but
drifts round, carrying the spots with it, at a rate continually
accelerated from the poles to the equator. In other words, the time of
axial revolution is shortest at the equator and lengthens with increase
of latitude. Carrington devised a mathematical formula by which the rate
or "law" of this lengthening was conveniently expressed; but it was a
purely empirical one. It was a concise statement, but implied no
physical interpretation. It summarised, but did not explain the facts.
An assumed "mean period" for the solar rotation of 25·38 days
(twenty-five days nine hours, very nearly), was thus found to be
_actually_ conformed to only in two parallels of solar latitude (14°
north and south), while the equatorial period was slightly less than
twenty-five, and that of latitude 50° rose to twenty-seven days and a
half.[422] These curious results gave quite a new direction to ideas on
solar physics.
The other two "elements" of the sun's rotation were also ascertained by
Carrington with hitherto unattained precision. He fixed the inclination
of its axis to the ecliptic at 82° 45'; the longitude of the ascending
node at 73° 40' (for the epoch 1850 A.D.). These data--which have
scarcely yet been improved upon--suffice to determine the position in
space of the sun's equator. Its north pole is directed towards a star in
the coils of the Dragon, midway between Vega and the Pole-star; its
plane intersects that of the earth's orbit in such a way that our planet
finds itself in the same level on or about the 3rd of June and the 5th
of December, when any spots visible on the disc cross it in apparently
straight lines. At other times, the paths pursued by them seem
curved--downward (to an observer in the northern hemisphere) between
June and December, upward between December and June.
A singular peculiarity in the distribution of sun-spots emerged from
Carrington's studies at the time of the minimum of 1856. Two broad belts
of the solar surface, as we have seen, are frequented by them, of which
the limits may be put at 6° and 35° of north and south latitude.
Individual equatorial spots are not uncommon, but nearer to the poles
than 35° they are a rare exception. Carrington observed--as an extreme
instance--in July, 1858, one in south latitude 44°; and Peters, in June,
1846, watched, during several days, a spot in 50° 24' north latitude.
But beyond this no true macula has ever been seen; for Lahire's reported
observation of one in latitude 70° is now believed to have had its place
on the solar globe erroneously assigned; and the "veiled spots"
described by Trouvelot in 1875[423] as occurring within 10° of the pole
can only be regarded as, at the most, the same kind of disturbance in an
undeveloped form.
But the novelty of Carrington's observations consisted in the detection
of certain changes in distribution concurrent with the progress of the
eleven-year period. As the minimum approached, the spot-zones contracted
towards the equator, and there finally vanished; then, as if by a fresh
impulse, spots suddenly reappeared in high latitude, and spread
downwards with the development of the new phase of activity. Scarcely
had this remark been made public,[424] when Wolf[425] found a
confirmation of its general truth in Böhm's observations during the
years 1833-36; and a perfectly similar behaviour was noted both by
Spörer and Secchi at the minimum epoch of 1867. The ensuing period gave
corresponding indications; and it may now be looked upon as established
that the spot-zones close in towards the equator with the advance of
each cycle, their activity culminating, as a rule, in a mean latitude of
about 16°, and expiring when it is reduced to 6°. Before this happens,
however, a completely new disturbance will have manifested itself some
35° north and south of the equator, and will have begun to travel over
the same course as its predecessor. Each series of sun-spots is thus, to
some extent, overlapped by the succeeding one; so that while the average
interval from one maximum to the next is eleven years, the period of
each distinct wave of agitation is twelve or fourteen.[426] Curious
evidence of the retarded character of the maximum of 1883-4 was to be
found in the unusually low latitude of the spot-zones when it occurred.
Their movement downward having gone on regularly while the crisis was
postponed, its final symptoms were hence displaced locally as well as in
time. The "law of zones" was duly obeyed at the minima of 1890[427] and
1901, and Spörer found evidence of conformity to it so far back as
1619.[428] His researches, however, also showed that it was in abeyance
during some seventy years previously to 1716, during which period
sun-spots remained persistently scarce, and auroral displays were feeble
and infrequent even in high northern latitudes. An unaccountable
suspension of solar activity is, in fact, indicated.[429]
Gustav Spörer, born at Berlin in 1822, began to observe sun-spots with
the view of assigning the law of solar rotation in December, 1860. His
assiduity and success with limited means attracted attention, and a
Government endowment was procured for his little solar observatory at
Anclam, in Pomerania, the Crown Prince (afterwards Emperor Frederick)
adding a five-inch refractor to its modest equipment. Unaware of
Carrington's discovery (not made known until January, 1859), he arrived
at and published, in June, 1861,[430] a similar conclusion as to the
equatorial quickening of the sun's movement on its axis. Appointed
observer in the new Astrophysical establishment at Potsdam in 1874, he
continued his sun-spot determinations there for twenty years, and died
July 7, 1895.
The time had now evidently come for a fundamental revision of current
notions respecting the nature of the sun. Herschel's theory of a cool,
dark, habitable globe, surrounded by, and protected against, the
radiations of a luminous and heat-giving envelope, was shattered by the
first _dicta_ of spectrum analysis. Traces of it may be found for a few
years subsequent to 1859,[431] but they are obviously survivals from an
earlier order of ideas, doomed to speedy extinction. It needs only a
moment's consideration of the meaning at last found for the Fraunhofer
lines to see the incompatibility of the new facts with the old
conceptions. They implied not only the presence near the sun, as glowing
vapours, of bodies highly refractory to heat, but that these glowing
vapours formed the relatively cool envelope of a still hotter internal
mass. Kirchhoff, accordingly, included in his great memoir "On the Solar
Spectrum," read before the Berlin Academy of Sciences, July 11, 1861, an
exposition of the views on the subject to which his memorable
investigations had led him. They may be briefly summarised as follows:
Since the body of the sun gives a continuous spectrum, it must be either
solid or liquid,[432] while the interruptions in its light prove it to
be surrounded by a complex atmosphere of metallic vapours, somewhat
cooler than itself. Spots are simply clouds due to local depressions of
temperature, differing in no respect from terrestrial clouds except as
regards the kinds of matter composing them. These _sun-clouds_ take
their origin in the zones of encounter between polar and equatorial
currents in the solar atmosphere.
This explanation was liable to all the objections urged against the
"cumulus theory" on the one hand, and the "trade-wind theory" on the
other. Setting aside its propounder, it was consistently upheld perhaps
by no man eminent in science except Spörer; and his advocacy of it
proved ineffective to secure its general adoption.
M. Faye, of the Paris Academy of Sciences, was the first to propose a
coherent scheme of the solar constitution covering the whole range of
new discovery. The fundamental ideas on the subject now in vogue here
made their first connected appearance. Much, indeed, remained to be
modified and corrected; but the transition was finally made from the old
to the new order of thought. The essence of the change may be conveyed
in a single sentence. The sun was thenceforth regarded, not as a mere
heated body, or--still more remotely from the truth--as a cool body
unaccountably spun round with a cocoon of fire, but as a vast
_heat-radiating machine_. The terrestrial analogy was abandoned in one
more particular besides that of temperature. The solar system of
circulation, instead of being adapted, like that of the earth, to the
distribution of heat received from without, was seen to be directed
towards the transportation towards the surface of the heat contained
within. Polar and equatorial currents, tending to a purely superficial
equalisation of temperature, were replaced by vertical currents bringing
up successive portions of the intensely heated interior mass, to
contribute their share in turn to the radiation into space which might
be called the proper function of a sun.
Faye's views, which were communicated to the Academy of Sciences,
January 16, 1865,[433] were avowedly based on the anomalous mode of
solar rotation discovered by Carrington. This may be regarded either as
an acceleration increasing from the poles to the equator, or as a
retardation increasing from the equator to the poles, according to the
rate of revolution we choose to assume for the unseen nucleus. Faye
preferred to consider it a retardation produced by ascending currents
continually left behind as the sphere widened in which the matter
composing them was forced to travel. He further supposed that the depth
from which these vertical currents rose, and consequently the amount of
retardation effected by their ascent to the surface, became
progressively greater as the poles were approached, owing to the
considerable flattening of the spheroidal surface from which they
started;[434] but the adoption of this expedient has been shown to
involve inadmissible consequences.
The extreme internal mobility betrayed by Carrington's and Spörer's
observations led to the inference that the matter composing the sun was
mainly or wholly gaseous. This had already been suggested by Father
Secchi[435] a year earlier, and by Sir John Herschel in April,
1864;[436] but it first obtained general currency through Faye's more
elaborate presentation. A physical basis was afforded for the view by
Cagniard de la Tour's experiments in 1822,[437] proving that, under
conditions of great heat and pressure, the vaporous state was compatible
with a very considerable density. The position was strengthened when
Andrews showed, in 1869,[438] that above a fixed limit of temperature,
varying for different bodies, true liquefaction is impossible, even
though the pressure be so tremendous as to retain the gas within the
same space that enclosed the liquid. The opinion that the mass of the
sun is gaseous now commands a very general assent; although the gaseity
admitted is of such a nature as to afford the consistence rather of
honey or pitch than of the aeriform fluids with which we are familiar.
On another important point the course of subsequent thought was
powerfully influenced by Faye's conclusions in 1865. Arago somewhat
hastily inferred from experiments with the polariscope the wholly
gaseous nature of the visible disc of the sun. Kirchhoff, on the
contrary, believed (erroneously, as we now know) that the brilliant
continuous spectrum derived from it proved it to be a white-hot solid or
liquid. Herschel and Secchi[439] indicated a cloud-like structure as
that which would best harmonise the whole of the evidence at command.
The novelty introduced by Faye consisted in regarding the photosphere no
longer "as a defined surface, in the mathematical sense, but as a limit
to which, in the general fluid mass, ascending currents carry the
physical or chemical phenomena of incandescence."[440] Uprushing floods
of mixed vapours with strong affinities--say of calcium or sodium and
oxygen--at last attain a region cool enough to permit their combination;
a fine dust of solid or liquid compound particles (of lime or soda, for
example) there collects into the photospheric clouds, and descending by
its own weight in torrents of incandescent rain, is dissociated by the
fierce heat below, and replaced by ascending and combining currents of
similar constitution.
This first attempt to assign the part played in cosmical physics by
chemical affinities was marked by the importation into the theory of the
sun of the now familiar phrase _dissociation_. It is indeed tolerably
certain that no such combinations as those contemplated by Faye occur at
the photospheric level, since the temperature there must be enormously
higher than would be needed to reduce all metallic earths and oxides;
but molecular changes of some kind, dependent perhaps in part upon
electrical conditions, in part upon the effects of radiation into space,
most likely replace them. The conjecture was emitted by Dr. Johnstone
Stoney in 1867[441] that the photospheric clouds are composed of
carbon-particles precipitated from their mounting vapour just where the
temperature is lowered by expansion and radiation to the boiling-point
of that substance. But this view, though countenanced by Ångström,[442]
and advocated by Hastings of Baltimore,[443] and other authorities,[444]
is open to grave objections.[445]
In Faye's theory, sun-spots were regarded as simply breaks in the
photospheric clouds, where the rising currents had strength to tear them
asunder. It followed that they were regions of increased heat--regions,
in fact, where the temperature was too high to permit the occurrence of
the precipitations to which the photosphere is due. Their obscurity was
attributed, as in Dr. Brester's more recent _Théorie du Soleil_, to
deficiency of emissive power. Yet here the verdict of the spectroscope
is adverse and irreversible.
After every deduction, however, has been made, we still find that
several ideas of permanent value were embodied in this comprehensive
sketch of the solar constitution. The principal of these were; first,
that the sun is a mainly gaseous body; secondly, that its stores of heat
are rendered available at the surface by means of vertical
convection-currents--by the bodily transport, that is to say, of
intensely hot matter upward, and of comparatively cool matter downward;
thirdly, that the photosphere is a surface of condensation, forming the
limit set by the cold of space to this circulating process, and that a
similar formation must attend, at a certain stage, the cooling of every
cosmical body.
To Warren de la Rue belongs the honour of having obtained the earliest
results of substantial value in celestial photography. What had been
done previously was interesting in the way of promise, but much could
not be claimed for it as actual performance. Some "pioneering
experiments" were made by Dr. J. W. Draper of New York in 1840,
resulting in the production of a few "moon-pictures" one inch in
diameter;[446] but slight encouragement was derived from them, either to
himself or others. Bond of Cambridge (U.S.), however, secured in 1850
with the Harvard 15-inch refractor that daguerreotype of the moon with
which the career of extra-terrestrial photography may be said to have
formally opened. It was shown in London at the Great Exhibition of 1851,
and determined the direction of De la Rue's efforts. Yet it did little
more than prove the art to be a possible one.
Warren de la Rue was born in Guernsey in 1815, and died in London April
19, 1889. Educated at the École Sainte-Barbe in Paris, he made a large
fortune as a paper manufacturer in England, and thus amply and early
provided the material supplies for his scientific campaign. Towards the
end of 1853 he took some successful lunar photographs. They were
remarkable as the first examples of the application to astronomical
light-painting of the collodion process, invented by Archer in 1851; and
also of the use of reflectors (De la Rue's was one of thirteen inches,
constructed by himself) for that kind of work. The absence of a driving
apparatus was, however, very sensibly felt; the difficulty of moving the
instrument by hand so as accurately to follow the moon's apparent motion
being such as to cause the discontinuance of the experiments until 1857,
when the want was supplied. De la Rue's new observatory, built in that
year at Cranford, was expressly dedicated to celestial photography; and
there he applied to the heavenly bodies the stereoscopic method of
obtaining relief, and turned his attention to the delicate business of
photographing the sun.
A solar daguerreotype was taken at Paris, April 2, 1845,[447] by
Foucault and Fizeau, acting on a suggestion from Arago. But the attempt,
though far from being unsuccessful, does not, at that time, seem to have
been repeated. Its great difficulty consisted in the enormous
light-power of the object to be represented, rendering an inconceivably
short period of exposure indispensable, under pain of getting completely
"burnt-up" plates. In 1857 De la Rue was commissioned by the Royal
Society to construct an instrument specially adapted to the purpose for
the Kew Observatory. The resulting "photoheliograph" may be described as
a small telescope (of 3-1/2 inches aperture and 50 focus), with a
plate-holder at the eye-end, guarded in front by a spring-slide, the
rapid movement of which across the field of view secured for the
sensitive plate a virtually instantaneous exposure. By its means the
first solar light-pictures of real value were taken, and the autographic
record of the solar condition recommended by Sir John Herschel was
commenced and continued at Kew during fourteen years--1858-72. The work
of photographing the sun is now carried on in every quarter of the
globe, from Mauritius to Massachusetts, and the days are few indeed on
which the self-betrayal of the camera can be evaded by our chief
luminary. In the year 1883 the incorporation of Indian with Greenwich
pictures afforded a record of the state of the solar surface on 340
days; and 364 were similarly provided for in 1897 and 1899.
The conclusions arrived at by photographic means at Kew were
communicated to the Royal Society in a series of papers drawn up jointly
by De la Rue, Balfour Stewart, and Benjamin Loewy, in 1865 and
subsequent years. They influenced materially the progress of thought on
the subject they were concerned with.
By its rotation the sun itself offers opportunities for bringing the
stereoscope to bear upon it. Two pictures, taken at an interval of
twenty-six minutes, show just the amount of difference needed to give,
by their combination, the maximum effect of solidity.[448] De la Rue
thus obtained, in 1861, a stereoscopic view of a sun-spot and
surrounding faculæ, representing the various parts in their true mutual
relations. "I have ascertained in this way," he wrote,[449] "that the
faculæ occupy the highest portions of the sun's photosphere, the spots
appearing like holes in the penumbræ, which appeared lower than the
regions surrounding them; in one case, parts of the faculæ were
discovered to be sailing over a spot apparently at some considerable
height above it." Thus Wilson's inference as to the depressed nature of
spots received, after the lapse of not far from a century, proof of the
most simple, direct, and convincing kind. A careful application of
Wilson's own geometrical test gave results only a trifle less decisive.
Of 694 spots observed, 78 per cent. showed, as they traversed the disc,
the expected effects of perspective;[450] and their absence in the
remaining 22 per cent. might be explained by internal commotions
producing irregularities of structure. The absolute depth of
spot-cavities--at least of their sloping sides--was determined by Father
Secchi through measurement of the "parallax of profundity"[451]--that
is, of apparent displacements attendant on the sun's rotation, due to
depression below the sun's surface. He found that in every case it fell
short of 4,000 miles, and averaged not more than 1,321, corresponding,
on the terrestrial scale, to an excavation in the earth's crust of 1-1/5
miles. Of late, however, the reality of even this moderate amount of
depression has been denied. Mr. Howlett's persevering observations,
extending over a third of a century, the results of which were presented
to the Royal Astronomical Society in December, 1894,[452] availed to
shatter the consensus of opinion which had so long been maintained on
the subject of spot-structure.[453] It has become impossible any longer
to hold that it is uniformly cavernous; and what seem like actually
protruding umbræ are occasionally vouched for on unimpeachable
authority.[454] We can only infer that the forms of sun-spots are really
more various than had been supposed; that they are peculiarly subject to
disturbance; and that the level of the nuclei may rise and fall during
the phases of commotion, like lavas within volcanic craters.
The opinion of the Kew observers as to the nature of such disturbances
was strongly swayed by another curious result of the "statistical
method" of inquiry. They found that of 1,137 instances of spots
accompanied by faculæ, 584 had those faculæ chiefly or entirely on the
left, 508 showed a nearly equal distribution, while 45 only had faculous
appendages mainly on the right side.[455] Now the rotation of the sun,
as we see it, is performed from left to right; so that the marked
tendency of the faculæ was a lagging one. This was easily accounted for
by supposing the matter composing them to have been flung upwards from a
considerable depth, whence it would reach the surface with the lesser
_absolute_ velocity belonging to a smaller circle of revolution, and
would consequently fall behind the cavities or "spots" formed by its
abstraction. An attempt, it is true, made by M. Wilsing at Potsdam in
1888[456] to determine the solar rotation from photographs of faculæ had
an outcome inconsistent with this view of their origin. They
unexpectedly gave a uniform period. No trace of the retardation poleward
from the equator, shown by the spots, could be detected in their
movements. But the experiment was obviously inconclusive;[457] and M.
Stratonoff's[458] repetition of it with ampler materials gave a full
assurance that faculæ rotate like spots in periods lengthening as
latitude augments.
The ideas of M. Faye were, on two fundamental points, contradicated by
the Kew investigators. He held spots to be regions of _uprush_ and of
heightened temperature; they believed their obscurity to be due to a
_downrush_ of comparatively cool vapours. Now M. Chacornac, observing,
at Ville-Urbanne, March 6, 1865, saw floods of photospheric matter
visibly precipitating themselves into the abyss opened by a great spot,
and carrying with them small neighbouring maculæ.[459] Similar instances
were repeatedly noted by Father Secchi, who considered the existence of
a kind of _suction_ in spots to be quite beyond question.[460] The
tendency in their vicinity, to put it otherwise, is _centripetal_, not
_centrifugal_; and this alone seems to negative the supposition of a
central uprush.
A fresh witness was by this time at hand. The application of the
spectroscope to the direct examination of the sun's surface dates from
March 4, 1866, when Sir Norman Lockyer (to give him his present title)
undertook an inquiry into the cause of the darkening in spots.[461] It
was made possible by the simple device of throwing upon the slit of the
spectroscope an _image_ of the sun, any part of which could be subjected
to special scrutiny, instead of, as had hitherto been done, admitting
rays from every portion of his surface indiscriminately. The answer to
the inquiry was prompt and unmistakable, and was again, in this case,
adverse to the French theorist's view. The obscurations in question were
found to be produced by no deficiency of emissive power, but by an
increase of absorptive action. The background of variegated light
remains unchanged, but more of it is stopped by the interposition of a
dense mass of relatively cool vapours. The spectrum of a sun-spot is
crossed by the same set of multitudinous dark lines, with some minor
differences, visible in the ordinary solar spectrum. We must then
conclude that the same vapours (speaking generally) which are dispersed
over the unbroken solar surface are accumulated in the umbral cavity,
the compression incident to such accumulation being betrayed by the
thickening of certain lines of absorption. But there is also a general
absorption, extending almost continuously from one end of the
spot-spectrum to the other. Using, however, a spectroscope of
exceptionally high dispersive power, Professor Young of Princeton, New
Jersey, succeeded in 1883 in "resolving" the supposed continuous
obscurity of spot-spectra into a countless multitude of fine dark lines
set very close together.[462] Their structure was seen still more
perfectly, about five years later, by M. Dunér,[463] Director of the
Upsala Observatory, who traced besides some shadowy vestiges of the
crowded doublets and triplets forming the array, from the spots on to
the general solar surface. They cease to be separable in the blue part
of the spectrum; and the ultra-violet radiations of spots show nothing
distinctive.[464]
As to the movements of the constipated vapours forming spots, the
spectroscope is also competent to supply information. The principle of
the method by which it is procured will be explained farther on. Suffice
it here to say that the transport, at any considerable velocity, to or
from the eye of the gaseous material giving bright or dark lines, can be
measured by the displacement of such lines from their previously known
normal positions. In this way movements have been detected in or above
spots of enormous rapidity, ranging up to 320 _miles per second_. But
the result, so far, has been to negative the ascription to them of any
systematic direction. Uprushes and downrushes are doubtless, as Father
Cortie remarks,[465] "correlated phenomena in the production of a
sun-spot"; but neither seem to predominate as part of its regular
internal economy.
The same kind of spectroscopic evidence tells heavily against a theory
of sun-spots started by Faye in 1872. He had been foremost in pointing
out that the observations of Carrington and Spörer absolutely forbade
the supposition that any phenomenon at all resembling our trade-winds
exists in the sun. They showed, indeed, that beyond the parallels of 20°
there is a general tendency in spots to a slow poleward displacement,
while within that zone they incline to approach the equator; but their
"proper movements" gave no evidence of uniformly flowing currents in
latitude. The systematic drift of the photosphere is strictly a drift in
longitude; its direction is everywhere parallel to the equator. This
fact being once clearly recognised, the "solar tornado" hypothesis at
once fell to pieces; but M. Faye[466] perceived another source of
vorticose motion in the unequal rotating velocities of contiguous
portions of the photosphere. The "pores" with which the whole surface of
the sun is studded he took to be the smaller eddies resulting from these
inequalities; the spots to be such eddies developed into whirlpools. It
only needs to thrust a stick into a stream to produce the kind of effect
designated. And it happens that the differences of angular movement
adverted to attain a maximum just about the latitudes where spots are
most frequent and conspicuous.
There are, however, grave difficulties in identifying the two kinds of
phenomena. One (already mentioned) is the total absence of the regular
swirling motion--in a direction contrary to that of the hands of a watch
north of the solar equator, in the opposite sense south of it--which
should impress itself upon every lineament of a sun-spot if the cause
assigned were a primary producing, and not merely (as it possibly may
be) a secondary determining one. The other, pointed out by Young,[467]
is that the cause is inadequate to the effect. The difference of
movement, or _relative drift_, supposed to occasion such prodigious
disturbances, amounts, at the utmost, for two portions of the
photosphere 123 miles apart, to about five yards a minute. Thus the
friction of contiguous sections must be quite insignificant.
A view better justified by observation was urged by Secchi in and after
the year 1872, and was presented in an improved form by Professor Young
in his excellent little book on _The Sun_, published in 1882.[468] Spots
are manifestly associated with violent eruptive action, giving rise to
the faculæ and prominences which usually garnish their borders. It is
accordingly contended that upon the withdrawal of matter from below by
the flinging up of a prominence must ensue a sinking-in of the surface,
into which the partially cooled erupted vapours rush and settle,
producing just the kind of darkening by increased absorption told of by
the spectroscope. Round the edges of the cavity the rupture of the
photospheric shell will form lines of weakness provocative of further
eruptions, which will, in their turn, deepen and enlarge the cavity. The
phenomenon thus tends to perpetuate itself, until equilibrium is at last
restored by internal processes. A sun-spot might then be described as an
inverted terrestrial volcano, in which the outbursts of heated matter
take place on the borders instead of at the centre of the crater, while
the cooled products gather in the centre instead of at the borders.
But on the earth, the solid crust forcibly represses the steam gathering
beneath until it has accumulated strength for an explosion, while there
is no such restraining power that we know of in the sun. Zöllner,
indeed, adapted his theory of the solar constitution to the special
purpose of procuring it; yet with very partial success, since almost
every new fact has proved adverse to his assumptions. Volcanic action is
essentially spasmodic. It implies habitual constraint varied by
temporary outbreaks, inconceivable in a gaseous globe, such as we
believe the sun to be.
If the "volcanic hypothesis" represented the truth, no spot could
possibly appear without a precedent eruption. The real order of the
phenomenon, however, is exceedingly difficult to ascertain; nor is it
perhaps invariable. Although, in most cases, the "opening" shows first,
that may be simply because it is more easily seen. According to Father
Sidgreaves,[469] the disturbance has then already passed the incipient
stage. He considers it indeed "highly probable that the preparatory sign
of a new spot is always a small, bright patch of facula."
This sequence, if established, would be fatal to Lockyer's theory of
sun-spots, communicated to the Royal Society, May 6, 1886,[470] and
further developed some months later in his work on _The Chemistry of the
Sun_. Spots are represented in it as incidental to a vast system of
solar atmospheric circulation, starting with the polar out- and up-flows
indicated by observations during some total eclipses, and eventuating in
the plunge downward from great heights upon the photosphere of
prodigious masses of condensed materials. From these falls result,
primarily, spots; secondarily, through the answering uprushes in which
chemical and mechanical forces co-operate, their girdles of
flame-prominences. The evidence is, however, slight that such a
circulatory flow as would be needed to maintain this supposed cycle of
occurrences really prevails in the sun's atmosphere; and a similar
objection applies to an "anticyclonic theory" (so to designate it)
elaborated by Egon von Oppolzer in 1893.[471] August Schmidt's optical
rationale of solar phenomena[472] was, on the other hand, a complete
novelty, both in principle and development. Attractive to speculators
from its recondite nature and far-reaching scope, it by no means
commended itself to practical observers, intolerant of finding the all
but palpable realities of their daily experience dealt with as illusory
products of "circular refraction."
A singular circumstance has now to be recounted. On the 1st of
September, 1859, while Carrington was engaged in his daily work of
measuring the positions of sun-spots, he was startled by the sudden
appearance of two patches of peculiarly intense light within the area of
the largest group visible. His first idea was that a ray of unmitigated
sunshine had penetrated the screen employed to reduce the brilliancy of
the image; but, having quickly convinced himself to the contrary, he ran
to summon an additional witness of an unmistakably remarkable
occurrence. On his return he was disappointed to find the strange
luminous outburst already on the wane; shortly afterwards the last trace
vanished. Its entire duration was five minutes--from 11.18 to 11.23
A.M., Greenwich time; and during those five minutes it had traversed a
space estimated at 35,000 miles! No perceptible change took place in the
details of the group of spots visited by this transitory conflagration,
which, it was accordingly inferred, took place at a considerable height
above it.[473]
Carrington's account was precisely confirmed by an observation made at
Highgate. Mr. R. Hodgson described the appearance seen by him as that
"of a very brilliant star of light, much brighter than the sun's
surface, most dazzling to the protected eye, illuminating the upper
edges of the adjacent spots and streaks, not unlike in effect the edging
of the clouds at sunset."[474]
This unique phenomenon seemed as if specially designed to accentuate
the inference of a sympathetic relation between the earth and the sun.
From the 28th of August to the 4th of September, 1859, a magnetic
storm of unparalleled intensity, extent, and duration, was in progress
over the entire globe. Telegraphic communication was everywhere
interrupted--except, indeed, that it was, in some cases, found
practicable to work the lines _without batteries_, by the agency of
the earth-currents alone:[475] sparks issued from the wires; gorgeous
auroræ draped the skies in solemn crimson over both hemispheres, and
even within the tropics; the magnetic needle lost all trace of
continuity in its movements, and darted to and fro as if stricken with
inexplicable panic. The coincidence was drawn even closer. _At the
very instant_[476] of the solar outburst witnessed by Carrington and
Hodgson, the photographic apparatus at Kew registered a marked
disturbance of all the three magnetic elements; while, shortly after
the ensuing midnight, the electric agitation culminated, thrilling the
earth with subtle vibrations, and lighting up the atmosphere from pole
to pole with the coruscating splendours which, perhaps, dimly recall
the times when our ancient planet itself shone as a star.
Here then, at least, the sun was--in Professor Balfour Stewart's
phrase--"taken in the act"[477] of stirring up terrestrial commotions.
Nor have instances since been wanting of an indubitable connection
between outbreaks of individual spots and magnetic disturbances. Four
such were registered in 1882; and symptoms of the same kind, including
the beautiful "Rose Aurora," marked the progress across the sun of the
enormous spot-group of February, 1892--the largest ever recorded at
Greenwich. This extraordinary formation, which covered about 1/300 of
the sun's disc, survived through five complete rotations.[478] It was
remarkable for a persistent drift in latitude, its place altering
progressively from 17° to 30° south of the solar equator.
Again, the central passage of an enormous spot on September 9, 1898,
synchronised with a sharp magnetic disturbance and brilliant
aurora;[479] and the coincidence was substantially repeated in March,
1899,[480] when it was emphasised by the prevalent cosmic calm. The
theory of the connection is indeed far from clear. Lord Kelvin, in
1892,[481] pronounced against the possibility of any direct magnetic
action by the sun upon the earth, on the ground of its involving an
extravagant output of energy; but the fact is unquestionable that--in
Professor Bigelow's words--"abnormal agitations affect the sun and the
earth as a whole and at the same time."[482]
The nearer approach to the event of September 1, 1859, was
photographically observed by Professor George E. Hale at Chicago, July
15, 1892.[483] An active spot in the southern hemisphere was the scene
of this curiously sudden manifestation. During an interval of 12m.
between two successive exposures, a bridge of dazzling light was found
to have spanned the boundary-line dividing the twin-nuclei of the spot;
and these, after another 27m., were themselves almost obliterated by an
overflow of far-spreading brilliancy. Yet two hours later, no trace of
the outburst remained, the spot and its attendant faculæ remaining just
as they had been previously to its occurrence. Unlike that seen by
Carrington, it was accompanied by no exceptional magnetic phenomena,
although a "storm" set in next day.[484] Possibly a terrestrial analogue
to the former might be discovered in the "auroral beam" which traversed
the heavens during a vivid display of polar lights, November 17, 1882,
and shared, there is every reason to believe, their electrical origin
and character.[485]
Meantime M. Rudolf Wolf, transferred to the direction of the Zürich
Observatory, where he died, December 6, 1893, had relaxed none of his
zeal in the investigation of sun-spot periodicity. A laborious revision
of the entire subject with the aid of fresh materials led him, in
1859,[486] to the conclusion that while the _mean_ period differed
little from that arrived at in 1852 of 11.11 years, very considerable
fluctuations on either side of that mean were rather the rule than the
exception. Indeed, the phrase "sun-spot period" must be understood as
fitting very loosely the great fact it is taken to represent; so
loosely, that the interval between two maxima may rise to sixteen and a
half or sink below seven and a half years.[487] In 1861[488] Wolf
showed, and the remark was fully confirmed at Kew, that the shortest
periods brought the most acute crises, and _vice versâ_; as if for each
wave of disturbance a strictly equal amount of energy were available,
which might spend itself lavishly and rapidly, or slowly and
parsimoniously, but could in no case be exceeded. The further inclusion
of recurring solar commotions within a cycle of fifty-five and a half
years was simultaneously pointed out; and Hermann Fritz showed soon
afterwards that the aurora borealis is subject to an identical double
periodicity.[489] The same inquirer has more recently detected both for
auroræ and sun-spots a "secular period" of 222 years,[490] and the Kew
observations indicate for the latter, oscillations accomplished within
twenty-six and twenty-four days,[491] depending, most likely, upon the
rotation of the sun. This is certainly reflected in magnetic, and
perhaps in auroral periodicity. The more closely, in fact,
spot-fluctuations are looked into, the more complex they prove. Maxima
of one order are superposed upon, or in part neutralised by, maxima of
another order;[492] originating causes are masked by modifying causes;
the larger waves of the commotion are indented with minor undulations,
and these again crisped with tiny ripples, while the whole rises and
falls with the swell of the great secular wave, scarcely perceptible in
its progress because so vast in scale.
The idea that solar maculation depends in some way upon the position of
the planets occurred to Galileo in 1612.[493] It has been industriously
sifted by a whole bevy of modern solar physicists. Wolf in 1859[494]
found reason to believe that the eleven-year curve is determined by the
action of Jupiter, modified by that of Saturn, and diversified by
influences proceeding from the earth and Venus. Its tempting approach to
agreement with Jupiter's period of revolution round the sun, indeed,
irresistibly suggested a causal connection; yet it does not seem that
the most skilful "coaxing" of figures can bring about a fundamental
harmony. Carrington pointed out in 1863, that while, during _eight
successive periods_, from 1770 downwards, there were approximate
coincidences between Jupiter's aphelion passages and sun-spot maxima,
the relation had been almost exactly reversed in the two periods
preceding that date;[495] and Wolf himself finally concluded that the
Jovian origin must be abandoned.[496] M. Duponchel's[497] prediction,
nevertheless, of an abnormal retardation of the maximum due in 1881
through certain peculiarities in the positions of Uranus and Neptune
about the time it fell due, was partially verified by the event, since,
after an abortive phase of agitation in April, 1882, the final outburst
was postponed to January, 1894. The interval was thus 13.5 instead of
11.1 years; and it is noticeable that the delay affected chiefly the
southern hemisphere. Alternations of activity in the solar hemispheres
were indeed a marked feature of the maximum of 1884, which, in M. Faye's
view,[498] derived thence its indecisive character, while sharp, strong
crises arise with the simultaneous advance of agitation north and south
of the solar equator. The curve of magnetic disturbance followed with
its usual strict fidelity the anomalous fluctuations of the sun-spot
curve. The ensuing minimum occurred early in 1889, and was succeeded in
1894 by a maximum slightly less feeble than its predecessor.[499]
It cannot be said that much progress has been made towards the
disclosure of the cause, or causes, of the sun-spot cycle. No external
influence adequate to the effect has, at any rate, yet been pointed out.
Most thinkers on this difficult subject provide a quasi-explanation of
the periodicity in question through certain assumed vicissitudes
affecting internal processes;[500] Sir Norman Lockyer and E. von
Oppolzer reach the same end by establishing self-compensatory
fluctuations in the solar atmospheric circulation; Dr. Schuster resorts
to changes in the electrical conductivity of space near the sun.[501] In
all these theories, however, the course of transition is arbitrarily
arranged to suit a period, which imposes itself as a fact peremptorily
claiming admittance, while obstinately defying explanation.
The question so much discussed, as to the influence of sun-spots on
weather, does not admit of a satisfactory answer. The facts of
meteorology are too complex for easy or certain classification. Effects
owning dependence on one cause often wear the livery of another; the
meaning of observed particulars may be inverted by situation; and yet it
is only by the collection and collocation of particulars that we can
hope to reach any general law. There is, however, a good deal of
evidence to support the opinion--the grounds for which were primarily
derived from the labours of Dr. Meldrum at Mauritius--that increased
rainfall and atmospheric agitation attend spot-maxima; while Herschel's
conjecture of a more copious emission of light and heat about the same
epochs has recently obtained some countenance from Savélieff's measures
showing a gain in the strength of the sun's radiation _pari passu_ with
increase in the number of spots visible on his surface.[502]
The examination of what we may call the _texture_ of the sun's surface
derived new interest from a remarkable announcement made by Mr. James
Nasmyth in 1862.[503] He had made (as he supposed) the discovery that
the entire luminous stratum of the sun is composed of a multitude of
elongated shining objects on a darker background, shaped much like
willow-leaves, of vast size, crossing each other in all possible
directions, and possessed of unceasing relative motions. A lively
controversy ensued. In England and abroad the most powerful telescopes
were directed to a scrutiny encompassed with varied difficulties. Mr.
Dawes was especially emphatic in declaring that Nasmyth's
"willow-leaves" were nothing more than the "nodules" of Sir William
Herschel seen under a misleading aspect of uniformity; and there is
little doubt that he was right. It is, nevertheless, admitted that
something of the kind may be seen in the penumbræ and "bridges" of
spots, presenting an appearance compared by Dawes himself in 1852 to
that of a piece of coarse straw-thatching left untrimmed at the
edges.[504]
The term "granulated," suggested by Dawes in 1864,[505] best describes
the mottled aspect of the solar disc as shown by modern telescopes and
cameras. The grains, or rather the "floccules," with which it is thickly
strewn, have been resolved by Langley, under exceptionally favourable
conditions, into "granules" not above 100 miles in diameter; and from
these relatively minute elements, composing, jointly, about one-fifth of
the visible photosphere,[506] he estimates that three-quarters of the
entire light of the sun are derived.[507] Janssen agrees, so far as to
say that if the whole surface were as bright as its brightest parts, its
luminous emission would be ten to twenty times greater than it actually
is.[508]
The rapid changes in the forms of these solar cloud-summits are
beautifully shown in the marvellous photographs taken by Janssen at
Meudon, with exposures reduced at times to 1/100000 of a second! By
their means, also, the curious phenomenon known as the _réseau
photosphérique_ has been made evident.[509] This consists in the
diffusion over the entire disc of fleeting blurred patches, separated by
a reticulation of sharply-outlined and regularly-arranged granules. The
imperfect definition in the smudged areas may be due to agitations in
the solar or terrestrial atmosphere, unless it be--as Dr. Schemer thinks
possible[510]--merely a photographic effect. M. Janssen considers that
the photospheric cloudlets change their shape and character with the
progress of the sun-spot period;[511] but this is as yet uncertain.
The "grains," or more brilliant parts of the photosphere, are now
generally held to represent the upper termination of ascending and
condensing currents, while the darker interstices (Herschel's "pores")
mark the positions of descending cooler ones. In the penumbræ of spots,
the glowing streams rushing up from the tremendous sub-solar furnace are
bent sideways by the powerful indraught, so as to change their vertical
for a nearly horizontal motion, and are thus taken, as it were, in flank
by the eye, instead of being seen end-on in mamelon-form. This gives a
plausible explanation of the channelled structure of penumbræ which
suggested the comparison to a rude thatch. Accepting this theory as in
the main correct, we perceive that the very same circulatory process
which, in its spasms of activity, gives rise to spots, produces in its
regular course the singular "marbled" appearance, for the recording of
which we are no longer at the mercy of the fugitive or delusive
impressions of the human retina. And precisely this circulatory process
it is which gives to our great luminary its permance as a _sun_, or
warming and illuminating body.
FOOTNOTES:
[Footnote 405: _Mem. R. A. S._, vol. xxi., p. 157.]
[Footnote 406: _Ibid._, p. 160.]
[Footnote 407: _Month. Not._, vol. xxi., p. 144.]
[Footnote 408: _Le Soleil_, t. i., pp. 87-90 (2nd ed., 1871).]
[Footnote 409: See _ante_, p. 58.]
[Footnote 410: _Observations at Redhill (1863)_, Introduction.]
[Footnote 411: _Month. Not._, vol. xxxvi., p. 142.]
[Footnote 412: _Cape Observations_, p. 435, _note_.]
[Footnote 413: _Month. Not._, vol. x., p. 158.]
[Footnote 414: _Rosa Ursina_, lib. iii., p. 348.]
[Footnote 415: _Observations at Redhill_, p. 8.]
[Footnote 416: _Op._, t. iii., p. 402.]
[Footnote 417: _Rosa Ursina_, lib. iv., p. 601. Both Galileo and
Scheiner spoke of the _apparent_ or "synodical" period, which is about
one and a third days longer than the _true_ or "sidereal" one. The
difference is caused by the revolution of the earth in its orbit in the
same direction with the sun's rotation on its axis.]
[Footnote 418: _Rosa Ursina_, lib. iii., p. 260.]
[Footnote 419: Faye, _Comptes Rendus_, t. lx., p. 818.]
[Footnote 420: _Ibid._, t. xii., p. 648.]
[Footnote 421: _Proc. Am. Ass. Adv. of Science_, 1885, p. 85.]
[Footnote 422: _Observations at Redhill_, p. 221.]
[Footnote 423: _Am. Jour. of Science_, vol. xi., p. 169.]
[Footnote 424: _Month. Not._, vol. xix., p. 1.]
[Footnote 425: _Vierteljahrsschrift der Naturfors. Gesellschaft_
(Zürich), 1859, p. 252.]
[Footnote 426: Lockyer, _Chemistry of the Sun_, p. 428.]
[Footnote 427: Maunder, _Knowledge_, vol. xv., p. 130.]
[Footnote 428: _Month. Mon._, vol. l., p. 251.]
[Footnote 429: Maunder, _Knowledge_, vol. xvii., p. 173.]
[Footnote 430: _Astr. Nach._, No. 1,315.]
[Footnote 431: As late as 1866 an elaborate treatise in its support was
written by F. Coyteux, entitled _Qu'est-ce que le Soleil? Peut-il être
habité?_ and answering the question in the affirmative.]
[Footnote 432: The subsequent researches of Plücker, Frankland, Wüllner,
and others, showed that gases strongly compressed give an absolutely
unbroken spectrum.]
[Footnote 433: _Comptes Rendus_, t. lx., pp. 89, 138.]
[Footnote 434: _Ibid._, t. c., p. 595.]
[Footnote 435: _Bull. Meteor. dell Osservatorio dell Coll. Rom._, Jan.
1, 1864, p. 4.]
[Footnote 436: _Quart. Jour. of Science_, vol. i., p. 222.]
[Footnote 437: _Ann. de Chim. et de Phys._, t. xxii., p. 127.]
[Footnote 438: _Phil. Trans._, vol. clix., p. 575.]
[Footnote 439: _Les Mondes_, Dec. 22, 1864, p. 707.]
[Footnote 440: _Comptes Rendus_, t. lx., p. 147.]
[Footnote 441: _Proc. Roy. Society_, vol. xvi., p. 29.]
[Footnote 442: _Recherches sur le Spectre Solaire_, p. 38.]
[Footnote 443: _Am. Jour. of Science_, 1881, vol. xxi., p. 41. Hastings
stipulated only for some member of the triad, carbon, silicon, and
boron.]
[Footnote 444: Ranyard, _Knowledge_, vol. xvi., p. 190.]
[Footnote 445: Young, _The Sun_, p. 337, ed. 1897.]
[Footnote 446: H. Draper, _Quart. Journ. of Sc._, vol. i., p. 381; also
_Phil. Mag._, vol. xvii., 1840, p. 222.]
[Footnote 447: Reproduced in Arago's _Popular Astronomy_, plate xii.,
vol. 1.]
[Footnote 448: _Report Brit. Ass._, 1859, p. 148.]
[Footnote 449: _Phil. Trans._, vol. clii., p. 407.]
[Footnote 450: _Researches in Solar Physics_, part i., p. 20.]
[Footnote 451: Both the phrase and the method were suggested by Faye,
who estimated the average depth of the luminous sheath of spots at 2,160
miles. _Comptes Rendus_, t. lxi., p. 1082; t. xcvi., p. 356.]
[Footnote 452: _Month. Not._, vol. lv., p. 74.]
[Footnote 453: Sidgreaves, _Ibid._, p. 282; Cortie, _Ibid._, vol.
lviii., p. 91.]
[Footnote 454: Explained by East as refraction-effects. _Jour. Brit.
Astr. Ass._, vol. viii., p. 187.]
[Footnote 455: _Proc. Roy. Soc._, vol. xiv., p. 39.]
[Footnote 456: _Potsdam Publicationen_, No. 18; _Astr. Nach._, Nos.
3,000, 3,287.]
[Footnote 457: Faye, _Comptes Rendus_, t. cxi., p. 77; Bélopolsky,
_Astr. Nach._, No. 2,991.]
[Footnote 458: _Ibid._, Nos. 3,275, 3,344.]
[Footnote 459: Lockyer, _Contributions to Solar Physics_, p. 70.]
[Footnote 460: _Le Soleil_, p. 87.]
[Footnote 461: _Proc. Roy. Soc._, vol. xv., p. 256.]
[Footnote 462: _Phil. Mag._, vol. xvi., p. 460.]
[Footnote 463: _Recherches sur la Rotation du Soleil_, p. 12.]
[Footnote 464: Hale, _Astr. and Astrophysics_, vol. xi., p. 814.]
[Footnote 465: _Jour. Brit. Astr. Ass._, vol. i., p. 177.]
[Footnote 466: _Comptes Rendus_, t. lxxv., p. 1664; _Revue
Scientifique_, t. v., p. 359 (1883). Mr. Herbert Spencer had already (in
_The Reader_, Feb. 25, 1865) put forward an opinion that spots were of
the nature of "cyclonic clouds."]
[Footnote 467: _The Sun_, p. 174. For Faye's answer to the objection,
see _Comptes Rendus_, t. xcv., p. 1310.]
[Footnote 468: A revised edition appeared in 1897.]
[Footnote 469: _Astr. and Astrophysics_, vol. xii., p. 832.]
[Footnote 470: _Proc. Roy. Soc._, No. 244.]
[Footnote 471: _Astr. Nach._, No. 3,146; _Astr. and Astrophysics_, vol.
xii., pp. 419, 736.]
[Footnote 472: _Sirius_, Sept., 1893; _ibid._, vol. xxiii., p. 97;
_Astrophy. Jour._, vol. i., p. 112 (Wilczynski), p. 178 (Keeler); vol.
ii., p. 73 (Hale).]
[Footnote 473: _Month. Not._, vol. xx., p. 13.]
[Footnote 474: _Ibid._, p. 15.]
[Footnote 475: _Am. Jour._, vol. xxix. (2nd series), pp. 94, 95.]
[Footnote 476: The magnetic disturbance took place at 11.15 A.M., three
minutes before the solar blaze compelled the attention of Carrington.]
[Footnote 477: _Phil. Trans._, vol. cli., p. 428.]
[Footnote 478: Maunder, _Journal Brit. Astr. Ass._, vol. ii., p. 386;
Miss E. Brown, _Ibid._, p. 210; Month. Not., vol. lii., p. 354.]
[Footnote 479: _Observatory_, vol. xxi., p. 387; Maunder, _Knowledge_,
vol. xxi., p. 228; Fényi, _Astroph. Jour._, vol. x., p. 333.]
[Footnote 480: _Ibid._, p. 336; W. Anderson, Observatory, vol. xxii., p.
196.]
[Footnote 481: _Proc. Roy. Society_, vol. lii., p. 307; Rev. W.
Sidgreaves, _Mem. R. A. S._, vol. liv., p. 85.]
[Footnote 482: _Report on Solar and Terrestrial Magnetism_, Washington,
1898, p. 27.]
[Footnote 483: _Astr. and Astrophysics_, vol. xi., p. 611.]
[Footnote 484: _Ibid._, p. 819 (Sidgreaves).]
[Footnote 485: See J. Rand Capron, _Phil. Mag._, vol. xv., p. 318.]
[Footnote 486: _Mittheilungen über die Sonnenflecken_, No. ix.,
_Vierteljahrsschrift der Naturforschenden Gesellschaft in Zürich_,
Jahrgang 4.]
[Footnote 487: _Mitth._, No. lii., p. 58 (1881).]
[Footnote 488: _Ibid._, No. xii., p. 192. Baxendell, of Manchester,
reached independently a similar conclusion. See _Month. Not._, vol.
xxi., p. 141.]
[Footnote 489: Wolf, _Mitth._, No. xv., p. 107, etc. Olmsted, following
Hansteen, had already, in 1856, sought to establish an auroral period of
sixty-five years. _Smithsonian Contributions_, vol. viii., p. 37.]
[Footnote 490: Hahn, _Ueber die Reziehungen der Sonnenfleckenperiode zu
meteorologischen Erscheinungen_, p. 99 (1877).]
[Footnote 491: _Report Brit. Ass._, 1881, p. 518; 1883, p. 418.]
[Footnote 492: The Rev. A. Cortie (_Month. Not._, vol. lx., p. 538)
detects the influence of a short subsidiary cycle, Dr. W. J. S. Lockyer
that of a thirty-five year period (_Nature_, June 20, 1901). Professor
Newcomb (_Astroph. Jour._, vol. xiii., p. 11) considers that solar
activity oscillates uniformly in 11.13 years, with superposed periodic
variations.]
[Footnote 493: _Opere_, t. iii., p. 412.]
[Footnote 494: _Mitth._, Nos. vii. and xviii.]
[Footnote 495: _Observations at Redhill_, p. 248.]
[Footnote 496: _Comptes Rendus_, t. xcv., p. 1249.]
[Footnote 497: _Ibid._, t. xciii., p. 827; t. xcvi., p. 1418.]
[Footnote 498: _Ibid._, t. c, p. 593.]
[Footnote 499: Ellis, _Proc. Roy. Society_, vol. lxiii., p. 70.]
[Footnote 500: Schultz, _Astr. Nach._, Nos. 2,817-18, 2,847-8; Wilsing,
_Ibid._, No. 3,039; Bélopolsky, _Ibid._, No. 2,722.]
[Footnote 501: _Report Brit. Ass._, 1892, p. 635.]
[Footnote 502: A. W. Augur, _Astroph. Jour._, vol. xiii., p. 346.]
[Footnote 503: _Report Brit. Ass._, 1862, p. 16 (pt. ii.).]
[Footnote 504: _Mem. R. A. S._, vol. xxi., p. 161.]
[Footnote 505: _Month. Not._, vol. xxiv., p. 162.]
[Footnote 506: _Am. Jour. of Science_, vol. vii., 1874, p. 92.]
[Footnote 507: Young, _The Sun_, p. 103.]
[Footnote 508: _Ann. Bur. Long._, 1879, p. 679.]
[Footnote 509: _Ibid._, 1878, p. 689.]
[Footnote 510: _Himmelsphotographie_, p. 273.]
[Footnote 511: Ranyard, _Knowledge_, vols. xiv., p. 14, xvi., p. 189;
see also the accompanying photographs.]
CHAPTER III
_RECENT SOLAR ECLIPSES_
By observations made during a series of five remarkable eclipses,
comprised within a period of eleven years, knowledge of the solar
surroundings was advanced nearly to its present stage. Each of these
events brought with it a fresh disclosure of a definite and unmistakable
character. We will now briefly review this orderly sequence of
discovery.
Photography was first systematically applied to solve the problems
presented by the eclipsed sun, July 18, 1860. It is true that a
daguerreotype,[512] taken by Berkowski with the Königsberg heliometer
during the eclipse of 1851, is still valuable as a record of the corona
of that year; and some subsequent attempts were made to register partial
phases of solar occultation, notably by Professor Bartlett at West Point
in 1854;[513] but the ground remained practically unbroken until 1860.
In that year the track of totality crossed Spain, and thither,
accordingly, Warren de la Rue transported his photo-heliograph, and
Father Secchi his six-inch Cauchoix refractor. The question then
primarily at issue was that relating to the nature of the red
protuberances. Although, as already stated, the evidence collected in
1851 gave a reasonable certainty of their connection with the sun,
objectors were not silenced; and when the side of incredulity was
supported by so considerable an authority as M. Faye, it was impossible
to treat it with contempt. Two crucial tests were available. If it could
be shown that the fantastic shapes suspended above the edge of the dark
moon were seen under an identical aspect from two distant stations, that
fact alone would annihilate the theory of optical illusion or "mirage";
while the certainty that they were progressively concealed by the
advancing moon on one side, and uncovered on the other, would
effectually detach them from dependence on our satellite, and establish
them as solar appendages.
Now both these tests were eminently capable of being applied by
photography. But the difficulty arose that nothing was known as to the
chemical power of the rosy prominence-light, while everything depended
on its right estimation. A shot had to be fired, as it were, in the
dark. It was a matter of some surprise, and of no small congratulation,
that, in both cases, the shot took effect.
De la Rue occupied a station at Rivabellosa, in the Upper Ebro valley;
Secchi set up his instrument at Desierto de las Palmas, about 250 miles
to the south-east, overlooking the Mediterranean. From the totally
eclipsed sun, with its strange garland of flames, each observer derived
several perfectly successful impressions, which were found, on
comparison, to agree in the most minute details. This at once settled
the fundamental question as to the substantial reality of these objects;
while their solar character was demonstrated by the passage of the moon
_in front_ of them, indisputably attested by pictures taken at
successive stages of the eclipse. That forms seeming to defy all laws of
equilibrium were, nevertheless, not wholly evanescent, appeared from
their identity at an interval of seven minutes, during which the lunar
shadow was in transit from one station to the other; and the singular
energy of their actinic rays was shown by the record on the sensitive
plates of some prominences invisible in the telescope. Moreover,
photographic evidence strongly confirmed the inference--previously drawn
by Grant and others, and now with fuller assurance by Secchi--that an
uninterrupted stratum of prominence-matter encompasses the sun on all
sides, forming a reservoir from which gigantic jets issue, and into
which they subside.
Thus, first-fruits of accurate knowledge regarding the solar
surroundings were gathered, while the value of the brief moments of
eclipse gained indefinite increase, by supplementing transient visual
impressions with the faithful and lasting records of the camera.
In the year 1868 the history of eclipse spectroscopy virtually began, as
that of eclipse photography in 1860; that is to say, the respective
methods then first gave definite results. On the 18th of August, 1868,
the Indian and Malayan peninsulas were traversed by a lunar shadow
producing total obscuration during five minutes and thirty-eight
seconds. Two English and two French expeditions were despatched to the
distant regions favoured by an event so propitious to the advance of
knowledge, chiefly to obtain the verdict of the prism as to the
composition of prominences. Nor were they despatched in vain. An
identical discovery was made by nearly all the observers. At Jamkandi,
in the Western Ghauts, where Lieutenant (now Colonel) Herschel was
posted, unremitting bad weather threatened to baffle his eager
expectations; but during the lapse of the critical five and a half
minutes the clouds broke, and across the driving wrack a "long,
finger-like projection" jutted out over the margin of the dark lunar
globe. In another moment the spectroscope was pointed towards it; three
bright lines--red, orange, and blue--flashed out, and the problem was
solved.[514] The problem was solved in this general sense, that the
composition out of glowing vapours of the objects infelicitously termed
"protuberances" or "prominences" was no longer doubtful; although
further inquiry was needed for the determination of the particular
species to which those vapours belonged.
Similar, but more complete observations were made, with less atmospheric
hindrance, by Tennant and Janssen at Guntoor, by Pogson at Masulipatam,
and by Rayet at Wha-Tonne, on the coast of the Malay peninsula, the last
observer counting as many as nine bright lines.[515] Among them it was
not difficult to recognise the characteristic light of hydrogen; and it
was generally, though over-hastily, assumed that the orange ray matched
the luminous emissions of sodium. But fuller opportunities were at hand.
The eclipse of 1868 is chiefly memorable for having taught astronomers
to do without eclipses, so far, at least, as one particular branch of
solar inquiry is concerned. Inspired by the beauty and brilliancy of the
variously tinted prominence-lines revealed to him by the spectroscope,
Janssen exclaimed to those about him, "Je verrai ces lignes-là en dehors
des éclipses!" On the following morning he carried into execution the
plan which formed itself in his brain while the phenomenon which
suggested it was still before his eyes. It rests upon an easily
intelligible principle.
The glare of our own atmosphere alone hides the appendages of the sun
from our daily view. To a spectator on an airless planet, the central
globe would appear attended by all its splendid retinue of crimson
prominences, silvery corona, and far-spreading zodiacal light projected
on the star-spangled black background of an absolutely unilluminated
sky. Now the spectroscope offers the means of indefinitely weakening
atmospheric glare by diffusing a constant amount of it over an area
widened _ad libitum_. But monochromatic or "bright-line" light is, by
its nature, incapable of being so diffused. It can, of course, be
_deviated_ by refraction to any extent desired; but it always remains
equally concentrated, in whatever direction it may be thrown. Hence,
when it is mixed up with continuous light--as in the case of the solar
flames shining through our atmosphere--it derives a _relative_ gain in
intensity from every addition to the dispersive power of the
spectroscope with which the heterogeneous mass of beams is analysed.
Employ prisms enough, and eventually the undiminished rays of persistent
colour will stand out from the continually fading rainbow-tinted band,
by which they were at first effectually veiled.
This Janssen saw by a flash of intuition while the eclipse was in
progress; and this he realised at 10 A.M. next morning, August 19,
1868--the date of the beginning of spectroscopic work at the margin of
the unobscured sun. During the whole of that day and many subsequent
ones, he enjoyed, as he said, the advantage of a prolonged eclipse. The
intense interest with which he surveyed the region suddenly laid bare to
his scrutiny was heightened by evidences of rapid and violent change. On
the 18th of August, during the eclipse, a vast spiral structure, _at
least_ 89,000 miles high, was perceived, planted in surprising splendour
on the rim of the interposed moon. If was formed as General Tennant
judged from its appearance in his photographs, by the encounter of two
mounting torrents of flame, and was distinguished as the "Great Horn."
Next day it was in ruins; hardly a trace remained to show where it had
been.[516] Janssen's spectroscope furnished him besides with the
strongest confirmation of what had already been reported by the
telescope and the camera as to the continuous nature of the scarlet
"sierra" lying at the base of the prominences. Everywhere at the sun's
edge the same bright lines appeared.
It was not until the 19th of September that Janssen thought fit to send
news of his discovery to Europe. It seemed little likely to be
anticipated; yet a few minutes before his despatch was handed to the
Secretary of the Paris Academy of Sciences, a communication similar in
purport had been received from Sir Norman Lockyer. There is no need to
discuss the narrow and wearisome question of priority; each of the
competitors deserves, and has obtained, full credit for his invention.
With noteworthy and confident prescience, Lockyer, in 1866, before
anything was yet known regarding the constitution of the "red flames,"
ordered a strongly dispersive spectroscope for the express purpose of
viewing, apart from eclipses, the bright-line spectrum which he expected
them to give. Various delays, however, supervened, and the instrument
was not in his hands until October 16, 1868. On the 20th he picked up
the vivid rays, of which the presence and (approximately) the positions
had in the interim become known. But there is little doubt that, even
without that previous knowledge, they would have been found; and that
the eclipse of August 18 only accelerated a discovery already assured.
Sir William Huggins, meanwhile, had been tending towards the same goal
during two and a half years in his observatory at Tulse Hill. The
principle of the spectroscopic visibility of prominence-lines at the
edge of an uneclipsed sun was quite explicitly stated by him in
February, 1868,[517] and he devised various apparatus for bringing them
into actual view; but not until he knew where to look did he succeed in
seeing them.
Astronomers, thus liberated, by the acquisition of power to survey them
at any time, from the necessity of studying prominences during eclipses,
were able to concentrate the whole of their attention on the corona. The
first thing to be done was to ascertain the character of its spectrum.
This was seen in 1868 only as a faintly continuous one; for Rayet, who
seems to have perceived its distinctive bright line far above the
summits of the flames, connected it, nevertheless, with those objects.
On the other hand, Lieutenant Campbell ascertained on the same occasion
the polarisation of the coronal light in planes passing through the
sun's centre,[518] thereby showing that light to be, in whole or in
part, reflected sunshine. But if reflected sunshine, it was objected,
the chief at least of the dark Fraunhofer lines should be visible in it,
as they are visible in moonbeams, sky illumination, and all other
sun-derived light. The objection was well founded, but was prematurely
urged, as we shall see.
On the 7th of August, 1869, a track of total eclipse crossed the
continent of North America diagonally, entering at Behring's Straits,
and issuing on the coast of North Carolina. It was beset with observers;
but the most effective work was done in Iowa. At Des Moines, Professor
Harkness of the Naval Observatory, Washington, obtained from the corona
an "absolutely continuous spectrum," slightly less bright than that of
the full moon, but traversed by a single green ray.[519] The same green
ray was seen at Burlington and its position measured by Professor Young
of Dartmouth College.[520] It appeared to coincide with that of a dark
line of iron in the solar spectrum, numbered 1,474 on Kirchhoff's scale.
But in 1876 Young was able, by the use of greatly increased dispersion,
to resolve the Fraunhofer line "1474" into a pair, the more refrangible
member of which he considered to be the reversal of the green coronal
ray.[521] Scarcely called in question for over twenty years, the
identification nevertheless broke down through the testimony of the
eclipse-photographs of 1898. Sir Norman Lockyer derived from them a
position for the line in question notably higher up in the spectrum than
that previously assigned to it. Instead of 5,317, its true wave-length
proved to be 5,303 ten millionths of a millimetre;[522] nor does it make
any show by absorption in dispersed sunlight. The originating substance,
designated "coronium," of which nothing is known to terrestrial
chemistry, continues luminous[523] at least 300,000 miles above the
sun's surface, and is hence presumably much lighter even than hydrogen.
A further trophy was carried off by American skill[524] sixteen months
after the determination due to it of the distinctive spectrum of the
corona. The eclipse of December 22, 1870, though lasting only two
minutes and ten seconds, drew observers from the New, as well as from
the Old World to the shores of the Mediterranean. Janssen issued from
beleaguered Paris in a balloon, carrying with him the _vital parts_ of a
reflector specially constructed to collect evidence about the corona.
But he reached Oran only to find himself shut behind a cloud-curtain
more impervious than the Prussian lines. Everywhere the sky was more or
less overcast. Lockyer's journey from England to Sicily, and shipwreck
in the _Psyche_, were recompensed with a glimpse of the solar aureola
during _one second and a half_! Three parties stationed at various
heights on Mount Etna saw absolutely nothing. Nevertheless important
information was snatched in despite of the elements.
The prominent event was Young's discovery of the "reversing layer." As
the surviving solar crescent narrowed before the encroaching moon, "the
dark lines of the spectrum," he tells us, "and the spectrum itself,
gradually faded away, until all at once, as suddenly as a bursting
rocket shoots out its stars, the whole field of view was filled with
bright lines more numerous than one could count. The phenomenon was so
sudden, so unexpected, and so wonderfully beautiful, as to force an
involuntary exclamation."[525] Its duration was about two seconds, and
the impression produced was that of a complete reversal of the
Fraunhofer spectrum--that is, the substitution of a bright for every
dark line.
Now something of the kind was theoretically necessary to account for the
dusky rays in sunlight which have taught us so much, and have yet much
more to teach us; so that, although surprising from its transitory
splendour, the appearance could not strictly be called "unexpected."
Moreover, its premonitory symptom in the fading out of these rays had
been actually described by Secchi in 1868,[526] and looked for by Young
as the moon covered the sun in August 1869. But with the slit of his
spectroscope placed _normally_ to the sun's limb, the bright lines gave
a flash too thin to catch the eye. In 1870 the position of the slit was
_tangential_--it ran along the shallow bed of incandescent vapours,
instead of cutting across it: hence his success.
The same observation was made at Xerez de la Frontera by Mr. Pye, a
member of Young's party; and, although an exceedingly delicate one, has
since frequently been repeated. The whole Fraunhofer series appeared
bright (omitting other instances) to Maclear, Herschel, and Fyers in
1871, at the beginning or end of totality; to Pogson, at the break-up of
an annual eclipse, June 6, 1872; to Stone at Klipfontein, April 16,
1874, when he saw "the field full of bright lines."[527] But between the
picture presented by the "véritable pluie de lignes brilliantes,"[528]
which descended into M. Trépied's spectroscope for three seconds after
the disappearance of the sun, May 17, 1882, and the familiar one of the
dark-line solar spectrum, certain differences were perceiving, showing
their relation to be not simply that of a positive to a negative
impression.
A "reversing layer," or stratum of mixed vapours, glowing, but at a
lower temperature than that of the actual solar surface, was an integral
part of Kirchhoff's theory of the production of the Fraunhofer lines.
Here it was assumed that the missing rays were stopped, and here also it
was assumed that the missing rays would be seen bright, could they be
isolated from the overpowering splendour of their background. This
isolation is effected by eclipses, with the result--beautifully
confirmatory of theory--of _reversing_, or turning from dark to bright,
the Fraunhofer spectrum. The completeness and precision of the reversal,
however, could not be visually attested; and a quarter of a century
elapsed before a successful "snap-shot" provided photographic evidence
on the subject. It was taken at Novaya Zemlya by Mr. Shackleton, a
member of the late Sir George Baden-Powell's expedition to observe the
eclipse of August 9, 1896;[529] and similar records in abundance were
secured during the Indian eclipse of January 22, 1898,[530] and the
Spanish-American eclipse of May 28, 1900.[531] The result of their
leisurely examination has been to verify the existence of a
"reversing-layer," in the literal sense of the term. It is true that no
single "flash" photograph is an inverted transcript of the Fraunhofer
spectrum. The lines are, indeed, in each case--speaking broadly--the
same; but their relative intensities are widely different. Yet this need
occasion no surprise when we remember that the Fraunhofer spectrum
integrates the absorption of multitudinous strata, various in density
and composition, while only the upper section of the formation comes
within view of the sensitive plates exposed at totalities, the low-lying
vaporous beds being necessarily covered by the moon. The total depth of
this glowing envelope may be estimated at 500 to 600 miles, and its
normal state seems to be one of profound tranquillity, judging from the
imperturbable aspect of the array of dark lines due to its sifting
action upon light.
The last of the five eclipses which we have grouped together for
separate consideration was visible in Southern India and Australia,
December 12, 1871. Some splendid photographs were secured by the English
parties on the Malabar coast, showing, for the first time, the
remarkable branching forms of the coronal emanations; but the most
conspicuous result was Janssen's detection of some of the dark
Fraunhofer lines, long vainly sought in the continuous spectrum of the
corona. Chief among these was the D-line of sodium, the original index,
it might be said, to solar chemistry. No proof could be afforded more
decisive that this faint _echoing back_ of the distinctive notes of the
Fraunhofer spectrum, that the polariscope had spoken the truth in
asserting a large part of the coronal radiance to be reflected sunlight.
But it is usually so drenched in original luminosity, that its special
features are almost obliterated. Janssen's success in seizing them was
due in part to the extreme purity of the air at Sholoor, in the
Neilgherries, where he was stationed; in part to the use of an
instrument adapted by its large aperture and short focus to give an
image of the utmost brilliancy. His observation, repeated during the
Caroline Island eclipse of 1883, was photographically verified ten years
later by M. de la Baume Pluvinel in Senegal.[532]
An instrument of great value for particular purposes was introduced into
eclipse-work in 1871. The "slitless spectroscope" consists simply of a
prism placed outside the object-glass of a telescope or the lens of a
camera, whereby the radiance encompassing the eclipsed sun is separated
into as many differently tinted rings as it contains different kinds of
light. These tinted rings were simultaneously viewed by Respighi at
Poodacottah, and by Lockyer at Baikul. Their photographic registration
by the latter in 1875 initiated the transformation of the slitless
spectroscope into the prismatic camera.[533] Meanwhile, the use of an
ordinary spectroscope by Herschel and Tennant at Dodabetta showed the
green ray of coronium to be just as bright in a rift as in the adjacent
streamer. The visible structure of the corona was thus seen to be
independent of the distribution of the gases which enter into its
composition.
By means, then, of the five great eclipses of 1860-71 it was
ascertained: first, that the prominences, and at least the lower part of
the corona, are genuine solar appurtenances; secondly, that the
prominences are composed of hydrogen and other gases in a state of
incandescence, and rise, as irregular outliers, from a continuous
envelope of the same materials, some thousands of miles in thickness;
thirdly, that the corona is of a highly complex constitution, being made
up in part of glowing vapours, in part of matter capable of reflecting
sunlight. We may now proceed to consider the results of subsequent
eclipses.
These have raised, and have helped to solve, some very curious
questions. Indeed, every carefully watched total eclipse of the sun
stimulates as well as appeases curiosity, and leaves a legacy of
outstanding doubt, continually, as time and inquiry go on, removed, but
continually replaced. It cannot be denied that the corona is a
perplexing phenomenon, and that it does not become less perplexing as we
know more about it. It presented itself under quite a new and strange
aspect on the occasion of the eclipse which visited the Western States
of North America, July 29, 1878. The conditions of observation were
peculiarly favourable. The weather was superb; above the Rocky Mountains
the sky was of such purity as to permit the detection of Jupiter's
satellites with the naked eye on several successive nights. The
opportunity for advancing knowledge was made the most of. Nearly a
hundred astronomers, including several Englishmen, occupied twelve
separate posts, and prepared for an attack in force.
The question had often suggested itself, and was a natural one to ask,
whether the corona sympathises with the general condition of the sun?
whether, either in shape or brilliancy, it varies with the progress of
the sun-spot period? A more propitious moment for getting this question
answered could hardly have been chosen than that at which the eclipse
occurred. Solar disturbance was just then at its lowest ebb. The
development of spots for the month of July, 1878, was represented on
Wolf's system of "relative numbers" by the fraction 0·1, as against
135·4 for December, 1870, an epoch of maximum activity. The
"chromosphere"[534] was, for the most part, shallow and quiescent; its
depth, above the spot zones, had sunk from about 6,000 to 2,000
miles;[535] prominences were few and faint. Obviously, if a type of
corona corresponding to a minimum of sun-spots existed, it should be
seen then or never. It _was_ seen; but while, in some respects, it
agreed with anticipation, in others it completely set it at naught.
The corona of 1878, as compared with those of 1869, 1870, and 1871, was
generally admitted to be shrunken in its main outlines and much reduced
in brilliancy. Lockyer pronounced it ten times fainter than in 1871;
Harkness estimated its light at less than one-seventh that derived from
the mist-blotted aureola of 1870.[536] In shape, too, it was markedly
different. When sun-spots are numerous, the corona appears to be most
fully developed above the spot-zones, thus offering to our eyes a rudely
quadrilateral contour. The four great luminous sheaves forming the
corners of the square are made up of rays curving together from each
side into "synclinal" or ogival groups, each of which may be compared to
the petal of a flower. To Janssen, in 1871, the eclipsing moon seemed
like the dark heart of a gigantic dahlia, painted in light on the sky;
and the similitude to the ornament on a compass-card, used by Airy in
1851, well conveys the decorative effect of the beamy, radiated kind of
aureola, never, it would appear, absent when solar activity is at a
tolerably high pitch. In his splendid volume on eclipses,[537] with
which the systematic study of coronal structure may be said to have
begun, Mr. Ranyard first generalised the synclinal peculiarity by a
comparison of records; but the symmetry of the arrangement, though
frequently striking, is liable to be confused by secondary formations.
He further pointed out, with the help of careful drawings from the
photographs of 1871 made by Mr. Wesley, the curved and branching shapes
assumed by the component filaments of massive bundles of rays. Nothing
of all this, however, was visible in 1878. Instead, there was seen, as
the groundwork of the corona, a ring of pearly light, nebulous to the
eye, but shown by telescopes and in photographs to have a fibrous
texture, as if made up of tufts of fine hairs. North and south, a series
of short, vivid, electrical-looking flame-brushes diverged with
conspicuous regularity from each of the solar poles. Their direction was
not towards the centre of the sun, but towards each summit of his axis,
so that the farther rays on either side started almost tangentially to
the surface.
But the leading, and a truly amazing, characteristic of the phenomenon
was formed by two vast, faintly-luminous _wings_ of light, expanded on
either side of the sun in the direction of the ecliptic. These were
missed by very few careful onlookers; but the extent assigned to them
varied with skill in, and facilities for seeing. By far the most
striking observations were made by Newcomb at Separation (Wyoming), by
Cleveland Abbe from the shoulder of Pike's Peak, and by Langley at its
summit, an elevation of 14,100 feet above the sea. Never before had an
eclipse been viewed from anything approaching that altitude, or under so
translucent a sky. A proof of the great reduction in atmospheric glare
was afforded by the perceptibility of the corona four minutes after
totality was over. For the 165 seconds of its duration, the remarkable
streamers above alluded to continued "persistently visible," stretching
away right and left of the sun to a distance of at least ten million
miles! One branch was traced over an apparent extent of fully twelve
lunar diameters, without sign of a definite termination having been
reached; and there were no grounds for supposing the other more
restricted.
The resemblance to the zodiacal light was striking; and a community of
origin between that enigmatical member of our system and the corona was
irresistibly suggested. We should, indeed, expect to see, under such
exceptionally favourable atmospheric conditions as Professor Langley
enjoyed on Pike's Peak, the _roots_ of the zodiacal light presenting
near the sun just such an appearance as he witnessed; but we can imagine
no reason why their visibility should be associated with a low state of
solar activity. Nevertheless this seems to be the case with the
streamers which astonished astronomers in 1878. For in August, 1867,
when similar equatorial emanations, accompanied by similar symptoms of
polar excitement, were described and depicted by Grosch[538] of the
Santiago Observatory, sun-spots were at a minimum; while the corona of
1715, which appears from the record of it by Roger Cotes[539] to have
been of the same type, preceded by three years the ensuing maximum. The
eclipsed sun was seen by him at Cambridge, May 2, 1715, encompassed with
a ring of light about one-sixth of the moon's diameter in breadth, upon
which was superposed a luminous cross formed of long bright branches
lying very nearly in the plane of the ecliptic, and shorter polar arms
so faint as to be only intermittently visible. The resemblance between
his sketch and Cleveland Abbe's drawing of the corona of 1878 is
extremely striking. It should, nevertheless, be noted that some
conspicuous spots were visible on the sun's disc at the time of Cotes's
eclipse, and that the preceding minimum (according to Wolf) occurred in
1712. Thus, the coincidence of epochs is imperfect.
Professor Cleveland Abbe was fully persuaded that the long rays
carefully observed by him from Pike's Peak were nothing else than
streams of meteorites rushing towards or from perihelion; and it is
quite certain that the solar neighbourhood must be crowded with such
bodies. But the peculiar structure at the base of the streamers
displayed in the photographs, the curved rays meeting in pointed arches
like Gothic windows, the visible upspringing tendency, the filamentous
texture,[540] speak unmistakably of the action of forces proceeding
_from_ the sun, not of extraneous matter circling round him.
A further proof of sympathetic change in the corona is afforded by the
analysis of its light. In 1878 the bright line so conspicuous in the
coronal spectrum in 1870 and 1871 had faded to the very limit of
visibility. Several skilled observers failed to see it at all; but Young
and Eastman succeeded in tracing the green "coronium" ray all round the
sun, to a height estimated at 340,000 miles. The substance emitting it
was thus present, though in a low state of incandescence. The continuous
spectrum was relatively strong; faint traces of the Fraunhofer lines
attested for it an origin, in part by reflection; and polarisation was
undoubted, increasing towards the limb, whereas in 1870 it reached a
maximum at a considerable distance from it. Experiments with Edison's
tasimeter seemed to show that the corona radiates a sensible amount of
heat.
The next promising eclipse occurred May 17, 1882. The concourse of
astronomers which has become usual on such occasions assembled this time
at Sohag, in Upper Egypt. Rarely have seventy-four seconds been turned
to such account. To each observer a special task was assigned, and the
advantages of a strict division of labour were visible in the variety
and amount of the information gained.
The year 1882 was one of numerous sun-spots. On the eve of the eclipse
twenty-three separate maculæ were counted. If there were any truth in
the theory which connected coronal forms with fluctuations in solar
activity, it might be anticipated that the vast equatorial expansions
and polar "brushes" of 1878 would be found replaced by the star-like
structure of 1871. This expectation was literally fulfilled. No lateral
streamers were to be seen. The universal failure to perceive them, after
express search in a sky of the most transparent purity, justifies the
emphatic assertion that _they were not there_. Instead, the type of
corona observed in India eleven years earlier, was reproduced with its
shining aigrettes, complex texture and brilliant radiated aspect.
Concordant testimony was given by the spectroscope. The reflected light
derived from the corona was weaker than in 1878, while its original
emissions were proportionately intensified. Nevertheless, most of the
bright lines recorded as coronal[541] were really due, there can be no
doubt, to diffused chromospheric light. On this occasion, the first
successful attempt was made to photograph the coronal spectrum procured
in the ordinary way with a slit and prisms, while the prismatic camera
was also profitably employed. It served to bring out at least one
important fact--that of the uncommon strength in chromospheric regions
of the twin violet beams of calcium, designated "H" and "K"; and
prominence-photography signalised its improvement by the registration,
in the spectrum of one such object, of twenty-nine rays, including many
of the ultra-violet hydrogen series discovered by Sir William Huggins in
the emission of white stars.[542]
Dr. Schuster's photographs of the corona itself were the most extensive,
as well as the most detailed, of any yet secured. One rift imprinted
itself on the plates to a distance of nearly a diameter and a half from
the limb; and the transparency of the streamers was shown by the
delineation through them of the delicate tracery beyond. The singular
and picturesque feature was added of a bright comet, self-depicted in
all the exquisite grace of swift movement betrayed by the fine curve of
its tail, hurrying away from one of its rare visits to our sun, and
rendered momentarily visible by the withdrawal of the splendour in which
it had been, and was again quickly veiled.
From a careful study of these valuable records Sir William Huggins
derived the idea of a possible mode of photographing the corona _without
an eclipse_.[543] As already stated, its ordinary invisibility is
entirely due to the "glare" or reflected light diffused through our
atmosphere. But Huggins found, on examining Schuster's negatives, that a
large proportion of the light in the coronal spectrum, both continuous
and interrupted, is collected in the violet region between the
Fraunhofer lines G and H. There, then, he hoped that, all other rays
being excluded, it might prove strong enough to vanquish inimical glare,
and stamp on prepared plates, through _local_ superiority in
illuminative power, the forms of the appendage by which it is emitted.
His experiments were begun towards the end of May, 1882, and by
September 28 he had obtained a fair earnest of success. The exclusion of
all other qualities of light save that with which he desired to operate,
was accomplished by using chloride of silver as his sensitive material,
that substance being chemically inert to all other but those precise
rays in which the corona has the advantage.[544] Plates thus sensitised
received impressions which it was hardly possible to regard as spurious.
"Not only the general features," Captain Abney affirmed,[545] "are the
same, but details, such as rifts and streamers, have the same position
and form." It was found, moreover, that the corona photographed during
the total eclipse of May 6, 1883, was intermediate in shape between the
coronas photographed by Sir William Huggins before and after that event,
each picture taking its proper place in a series of progressive
modifications highly interesting in themselves, and full of promise for
the value of the method employed to record them.[546] But experiments on
the subject were singularly interrupted. The volcanic explosion in the
Straits of Sunda in August, 1883, brought to astronomers a peculiarly
unwelcome addition to their difficulties. The magnificent sunglows due
to the diffractive effects on light of the vapours and fine dust flung
in vast volumes into the air, and rapidly diffused all round the globe,
betokened an atmospheric condition of all others the most prejudicial to
delicate researches in the solar vicinity. The filmy coronal forms,
accordingly, which had been hopefully traced on the Tulse Hill plates
ceased to appear there; nor were any substantially better results
obtained by Mr. C. Ray Woods, in the purer air either of the Riffel or
the Cape of Good Hope, during the three ensuing years. Moreover,
attempts to obtain coronal photographs during the partial phases of the
eclipse of August 29, 1886, completely failed. No part of the lunar
globe became visible in relief against circumfluous solar radiance on
any of the plates exposed at Grenada; and what vestiges of "structure"
there were, came out almost better _upon_ the moon than _beside_ her,
thus stamping themselves at once as of atmospheric origin.
That the effect sought is a perfectly possible one is proved by the
distinct appearance of the moon projected on the corona, in photographs
of the partially eclipsed sun in 1858, 1889, and 1890, and very notably
in 1898 and 1900.[547]
In the spring of 1893, Professor Hale[548] attacked the problem of
coronal daylight photography, employing the "double-slit" method so
eminently serviceable for the delineation of prominences.[549] But
neither at Kenwood nor at the summit of Pike's Peak, whither, in the
course of the summer, he removed his apparatus, was any action of the
desired kind secured. Similar ill success attended his and Professor
Riccò's employment, on Mount Etna in July, 1894, of a specially designed
coronagraph. Yet discouragement did not induce despair. The end in view
is indeed too important to be readily abandoned; but it can be reached
only when a more particular acquaintance with the nature of coronal
light than we now possess indicates the appropriate device for giving it
a preferential advantage in self-portraiture. Moreover, the
effectiveness of this device may not improbably be enhanced, through
changes in the coronal spectrum at epochs of sun-spot maximum.
The prosperous result of the Sohag observations stimulated the desire to
repeat them on the first favourable opportunity. This offered itself one
year later, May 6, 1883, yet not without the drawbacks incident to
terrestrial conditions. The eclipse promised was of rare length, giving
no less than five minutes and twenty-three seconds of total obscurity,
but its path was almost exclusively a "water-track." It touched land
only on the outskirts of the Marquesas group in the Southern Pacific,
and presented, as the one available foothold for observers, a coral reef
named Caroline Island, seven and a half miles long by one and a half
wide, unknown previously to 1874, and visited only for the sake of its
stores of guano. Seldom has a more striking proof been given of the
vividness of human curiosity as to the condition of the worlds outside
our own, than in the assemblage of a group of distinguished men from the
chief centres of civilisation, on a barren ridge, isolated in a vast and
tempestuous ocean, at a distance, in many cases, of 11,000 miles and
upwards from the ordinary scene of their labours. And all these
sacrifices--the cost and care of preparation, the transport and
readjustment of delicate instruments, the contrivance of new and more
subtle means of investigating phenomena--on the precarious chance of a
clear sky during one particular five minutes! The event, though
fortunate, emphasised the hazard of the venture. The observation of the
eclipse was made possible only by the happy accident of a serene
interval between two storms.
The American expedition was led by Professor Edward S. Holden, and to it
were courteously permitted to be attached Messrs. Lawrance and Woods,
photographers, sent out by the Royal Society of London. M. Janssen was
chief of the French Academy mission; he was accompanied from Meudon by
Trouvelot, and joined from Vienna by Palisa, and from Rome by Tacchini.
A large share of the work done was directed to assuring or negativing
previous results. The circumstances of an eclipse favour illusion. A
single observation by a single observer, made under unfamiliar
conditions, and at a moment of peculiar excitement, can scarcely be
regarded as offering more than a suggestion for future inquiry. But
incredulity may be carried too far. Janssen, for instance, felt
compelled, by the survival of unwise doubts, to devote some of the
precious minutes of obscurity at Caroline Island to confirming what, in
his own persuasion, needed no confirmation--that is, the presence of
reflected Fraunhofer lines in the spectrum of the corona. Trouvelot and
Palisa, on the other hand, instituted an exhaustive, but fruitless
search for the spurious "intramercurian" planets announced by Swift and
Watson in 1878.
New information, however, was not deficient. The corona proved identical
in type with that of 1882,[550] agreeably to what was expected at an
epoch of protracted solar activity. The characteristic aigrettes were of
even greater brilliancy than in the preceding year, and the chemical
effects of the coronal light proved unusually intense. Janssen's
photographs, owing to the considerable apertures (six and eight inches)
of his object-glasses, and the long exposures permitted by the duration
of totality, were singularly perfect; they gave a greater extension to
the coronal than could be traced with the telescope,[551] and showed its
forms as absolutely fixed and of remarkable complexity.
The English pictures, taken with exposures up to sixty seconds, were
likewise of great value. They exhibited details of structure from the
limb to the tips of the streamers, which terminated definitely, and as
it seemed actually, where the impressions on the plates ceased. The
coronal spectrum was also successfully photographed, and although the
reversing layer in its entirety evaded record, a print was caught of
some of its more prominent rays just before and after totality. The use
of the prismatic camera was baffled by the anomalous scarcity of
prominences.
Using an ingenious apparatus for viewing simultaneously the spectrum
from both sides of the sun, Professor Hastings noticed at Caroline
Island alternations, with the advance of the moon, in the respective
heights above the right and left solar limbs of the coronal green line,
which were thought to imply that the corona, with its rifts and sheaves
and "tangled hanks" of rays, is, after all, merely an illusive
appearance produced by the diffraction of sunlight at the moon's
edge.[552] But the observation was assuredly misleading or
misinterpreted. Atmospheric _diffusion_ may indeed, under favouring
circumstances, be effective in deceptively enlarging solar appendages;
but always to a very limited extent.
The controversy is an old one as to the part played by our air in
producing the radiance visible round the eclipsed sun. In its original
form, it is true, it came to an end when Professor Harkness, in
1869,[553] pointed out that the shadow of the moon falls equally over
the air and on the earth, and that if the sun had no luminous
appendages, a circular space of almost absolute darkness would
consequently surround the apparent places of the superposed sun and
moon. Mr. Proctor,[554] with his usual ability, impressed this
mathematically certain truth upon public attention; and Sir John
Herschel calculated that the diameter of the "negative halo" thus
produced would be, in general, no less than 23°.
But about the same time a noteworthy circumstance relating to the state
of things in the solar vicinity was brought into view. On February 11,
1869, Messrs. Frankland and Lockyer communicated to the Royal Society a
series of experiments on gaseous spectra under varying conditions of
heat and density, leading them to the conclusion that the higher solar
prominences exist in a medium of excessive tenuity, and that even at the
base of the chromosphere the pressure is far below that at the earth's
surface.[555] This inference was fully borne out by the researches of
Wüllner; and Janssen expressed the opinion that the chromospheric gases
are rarefied almost to the degree of an air-pump vacuum.[556] Hence was
derived a general and fully justified conviction that there could be
outside, and incumbent upon the chromosphere, no such vast atmosphere as
the corona appeared to represent. Upon the strength of which conviction
the "glare" theory entered, chiefly under the auspices of Sir Norman
Lockyer, upon the second stage of its existence.
The genuineness of the "inner corona" to the height of 5' or 6' from the
limb was admitted; but it was supposed that by the detailed reflection
of its light in our air the far more extensive "outer corona" was
optically created, the irregularities of the moon's edge being called in
to account for the rays and rifts by which its structure was varied.
This view received some countenance from Admiral Maclear's observation,
during the eclipse of 1870, of bright lines "everywhere"--even at the
centre of the lunar disc. Here, indeed, was an undoubted case of
atmospheric diffusion; but here, also, was a safe index to the extent of
its occurrence. Light scatters equally in all directions; so that when
the moon's face at the time of an eclipse shows (as is the common case)
a blank in the spectroscope, it is quite certain that the corona is not
noticeably enlarged by atmospheric causes. A sky drifted over with thin
cirrus clouds and air changed with aqueous vapour amply accounted for
the abnormal amount of scattering in 1870.
But even in 1870 positive evidence was obtained of the substantial
reality of the radiated outer corona, in the appearance on the
photographic plates exposed by Willard in Spain and by Brothers in
Sicily of identical dark rifts. The truth is, that far from being
developed by misty air, it is peculiarly liable to be effaced by it. The
purer the sky, the more extensive, brilliant, and intricate in the
details of its structure the corona appears. Take as an example General
Myer's description of the eclipse of 1869, as seen from the summit of
White Top Mountain, Virginia, at an elevation above the sea of 5,523
feet, in an atmosphere of peculiar clearness.
"To the unaided eye," he wrote,[557] "the eclipse presented, during the
total obscuration, a vision magnificent beyond description. As a centre
stood the full and intensely black disc of the moon, surrounded by the
aureola of a soft bright light, through which shot out, as if from the
circumference of the moon, straight, massive, silvery rays, seeming
distinct and separate from each other, to a distance of two or three
diameters of the solar disc; the whole spectacle showing as on a
background of diffused rose-coloured light."
On the same day, at Des Moines, Newcomb could perceive, through somewhat
hazy air, no long rays, and the four-pointed outline of the corona
reached at its farthest only a _single semidiameter_ of the moon from
the limb. The plain fact, that our atmosphere acts rather as a veil to
hide the coronal radiance than as the medium through which it is
visually formed, emerges from further innumerable records.
No observations of importance were made during the eclipse of September
9, 1885. The path of total obscurity touched land only on the shores of
New Zealand, and two minutes was the outside limit of available time.
Hence local observers had the phenomenon to themselves; nor were they
even favoured by the weather in their efforts to make the most of it.
One striking appearance was, however, disclosed. It was that of two
"white" prominences of unusual brilliancy, shining like a pair of
electric lamps hung one at each end of a solar diameter, right above the
places of two large spots.[558] This coincidence of diametrically
opposite disturbances is of too frequent occurrence to be accidental. M.
Trouvelot observed at Meudon, June 26, 1885, two active and evanescent
prominences thus situated, each rising to the enormous height of 300,000
miles; and on August 16, one scarcely less remarkable, balanced by an
antipodal spot-group.[559] It towered upward, as if by a process of
_unrolling_, to a quarter of a million of miles; after which, in two
minutes, the light died out of it; it had become completely extinct. The
development, again from the ends of a diameter, of a pair of similar
objects was watched, September 19 and 20, 1893, by Father Fényi,
Director of the Kalocsa Observatory; and the phenomenon has been too
often repeated to be accidental.
The eclipse of August 29, 1886, was total during about four minutes over
tropical Atlantic regions; and an English expedition, led by Sir Norman
Lockyer, was accordingly despatched to Grenada in the West Indies, for
the purpose of using the opportunity it offered. But the rainy season
was just then at its height: clouds and squalls were the order of the
day; and the elaborately planned programme of observation could only in
part be carried through. Some good work, none the less, was done.
Professor Tacchini, who had been invited to accompany the party,
ascertained besides some significant facts about prominences. From a
comparison of their forms and sizes during and after the eclipse, it
appeared that only the growing vaporous cores of these objects are shown
by the spectroscope under ordinary circumstances; their upper sections,
giving a faint continuous spectrum, and composed of presumably cooler
materials, can only be seen when the veil of scattered light usually
drawn over them is removed by an eclipse. Thus all modestly tall
prominences have silvery summits; but all do not appear to possess the
"red heart of flame," by which alone they can be rendered perceptible to
daylight observation. Some prove to be ordinarily invisible, because
silvery throughout--"sheeted ghosts," as it were, met only in the dark.
Specimens of the class had been noted as far back as 1842, but Tacchini
first drew particular attention to them. The one observed by him in 1886
rose in a branching form to a height of 150,000 miles, and gave a
brilliantly continuous spectrum, with bright lines at H and K, but no
hydrogen-lines.[560] Hence the total invisibility of the object before
and after the eclipse. During the eclipse, it was seen framed, as it
were, in a pointed arch of coronal light, the symmetrical arrangement of
which with regard to it was obviously significant. Both its unspringing
shape, and the violet rays of calcium strongly emitted by it,
contradicted the supposition that "white prominences" represent a
downrush of refrigerated materials.
The corona of 1886, as photographed by Dr. Schuster and Mr. Maunder,
showed neither the petals and plumes of 1871, nor the streamers of 1878.
It might be called of a transition type.[561] Wide polar rifts were
filled in with tufted radiations, and bounded on either side by
irregularly disposed, compound luminous masses. In the south-western
quadrant, a triangular ray, conspicuous to the naked eye, represented,
Mr. W. H. Pickering thought, the projection of a huge, hollow cone.[562]
Branched and recurving jets were curiously associated with it. The
intrinsic photographic brightness of the corona proved, from Pickering's
measures, to be about 1/54 that of the average surface of the full moon.
The Russian eclipse of August 19, 1887, can only be remembered as a
disastrous failure. Much was expected of it. The shadow-path ran
overland from Leipsic to the Japanese sea, so that the solar
appurtenances would, it was hoped, be disclosed to observers echeloned
along a line of 6,000 miles. But the incalculable element of weather
rendered all forecasts nugatory. The clouds never parted, during the
critical three minutes, over Central Russia, where many parties were
stationed, and Professor D. P. Todd was equally unfortunate in Japan.
Some good photographs were, nevertheless, secured by Professor Arai,
Director of the Tokio Observatory, as well as by MM. Bélopolsky and
Glasenapp at Petrovsk and Jurjevitch respectively. They showed a corona
of simpler form than that of the year before, but not yet of the
pronounced type first associated by Mr. Ranyard with the lowest stage of
solar activity.
The genuineness of the association was ratified by the duplicate
spectacle of the next-ensuing minimum year. Two total eclipses of the
sun distinguished 1889. The first took place on New Year's Day, when a
narrow shadow-path crossed California, allowing less than two minutes
for the numerous experiments prompted by the varied nature of modern
methods of research. American astronomers availed themselves of the
occasion to the full. The heavens were propitious. Photographic records
were obtained in unprecedented abundance, and of unusual excellence.
Their comparison and study placed it beyond reasonable doubt that the
radiated corona belonging to periods of maximum sun-spots gives place,
at periods of minimum, to the "winged" type of 1878. Professor Holden
perceived further that the equatorial extensions characterising the
latter tend to assume a "trumpet-shape."[563] Their extremities diverge,
as if mutually repellent, instead of flowing together along a medial
plane. The maximum actinic brilliancy of the corona of January 1, 1889,
was determined at Lick to be twenty-one times less than that of the full
moon.[564] Its colour was described as "of an intense luminous silver,
with a bluish tinge, similar to the light of an electric arc."[565] Its
spectrum was comparatively simple. Very few bright lines besides those
of hydrogen and coronium, and apparently no dark ones, stood out from
the prismatic background.
"The marked structural features of the corona, as presented by the
negatives" taken by Professors Nipher and Charroppin, were the filaments
and the streamers. The filaments issued from polar calottes of 20°
radius.
"The impression conveyed to the eye," Professor Pritchett wrote,[566]
"is that the equatorial stream of denser coronal matter extends across
and through the filaments, simply obscuring them by its greater
brightness. The effect is just as if the equatorial belt were superposed
upon, or passed through, the filamentary structure. There is nothing in
the photographs to prove that the filaments do not exist all round the
sun.[567] The testimony from negatives of different lengths of exposure
goes to show that the equatorial streamers are made up of numerous
interlacing parts inclined at varying angles to the sun's equator."
The coronal extensions, perceptible with the naked eye to a distance of
more than 3° from the sun, appeared barely one-third of that length on
the best negatives. Little more could be seen of them either in
Barnard's exquisite miniature pictures, or in the photographs obtained
by W. H. Pickering with a thirteen-inch refractor--the largest
instrument so far used in eclipse-photography.
The total eclipse of December 22, 1889, held out a prospect,
unfortunately not realized, of removing some of the doubts and
difficulties that impeded the progress of coronal photography.[568]
Messrs. Burnham and Schaeberle secured at Cayenne some excellent
impressions, showing enough of the corona to prove its identical
character with that depicted in the beginning of the year, but not
enough to convey additional information about its terminal forms or
innermost structure. Any better result was indeed impossible, the
moisture-laden air having cut down the actinic power of the coronal
light to one-fourth its previous value.
Two English expeditions organized by the Royal Astronomical Society
fared still worse. Mr. Taylor was stationed on the West Coast of Africa,
one hundred miles south of Loanda; Father Perry chose as the scene of
his operations the Salut Islands, off French Guiana. Each was supplied
with a reflector constructed by Dr. Common, endowed, by its extremely
short focal length of forty-five, combined with an aperture of twenty
inches, with a light-concentrating force capable, it was hoped, of
compelling the very filmiest coronal branches to self-registration. Had
things gone well two sets of coronal pictures, absolutely comparable in
every respect, and taken at an interval of two hours and a half, would
have been at the disposal of astronomers. But things went very far from
well. Clouds altogether obscured the sun in Africa; they only separated
to allow of his shining through a saturated atmosphere in South America.
Father Perry's observations were the last heroic effort of a dying man.
Stricken with malaria, he crawled to the hospital as soon as the eclipse
was over, and expired five days later, at sea, on board the _Comus_. He
was buried at Barbados. And the sacrifice of his life had, after all,
purchased no decisive success. Most of the plates exposed by him
suffered deterioration from the climate, or from an inevitably delayed
development. A drawing from the best of them by Miss Violet Common[569]
represented a corona differing from its predecessor of January 1,
chiefly through the oppositely unsymmetrical relations of its parts.
Then the western wing had been broader at its base than the eastern; now
the inequality was conspicuously the other way.[570]
The next opportunity for retrieving the mischances of the past was
offered April 16, 1893. The line of totality charted for that day ran
from Chili to Senegambia. American parties appropriated the Andes; both
shores of the Atlantic were in English occupation; French expeditions,
led by Deslandres and Bigourdan, took up posts south of Cape Verde. A
long totality of more than four minutes was favoured by serene skies;
hence an ample store of photographic data was obtained. Professor
Schaeberle, of the Lick Observatory, took, almost without assistance, at
Mina Bronces, a mining station 6,600 feet above the Pacific, fifty-two
negatives, eight of them with a forty-foot telescope, on a scale of four
and a half inches to the solar diameter. Not only the inner corona, but
the array of prominences then conspicuous, appeared in them to be
composed of fibrous jets and arches, held to be sections of elliptic
orbits described by luminous particles about the sun's centre.[571] One
plate received the impression of a curious object,[572] entangled amidst
coronal streamers, and the belief in its cometary nature was ratified by
the bestowal of a comet-medal in recognition of the discovery. Similiar
paraboloidal forms had, nevertheless, occasionally been seen to make an
integral part of earlier coronas; and it remains extremely doubtful
whether Schaeberle's "eclipse-comet" was justly entitled to the
character claimed for it.
The eclipse of 1893 disclosed a radiated corona such as a year of
spot-maximum was sure to bring. An unexpected fact about it was,
however, ascertained. The coronal has been believed to have much in
common with the chromospheric spectrum; it proved, on investigation with
a large prismatic camera, employed under Sir Norman Lockyer's directions
by Mr. Fowler at Fundium, to be absolutely distinct from it. The
fundamental green ray had, on the West African plates, seven more
refrangible associates;[573] but all alike are of unknown origin. They
may be due to many substances, or to one; future research will perhaps
decide; we can at present only say that the gaseous emission of the
corona include none from hydrogen, helium, calcium, or any other
recognisable terrestrial element. Deslandres' attempt to determine the
rotation of the corona through opposite displacements, east and west of
the interposed moon, of the violet calcium-lines supposed to make part
of the coronal spectrum, was thus rendered nugatory. Yet it gave an
earnest of success, by definitely introducing the subject into the
constantly lengthened programme of eclipse-work. There is, however,
little prospect of its being treated effectively until the green line is
vivified by a fresh access of solar activity.
The flight of the moon's shadow was, on August 9, 1896, dogged by
atrocious weather. It traversed, besides, some of the most inhospitable
regions on the earth's surface, and afforded, at the best, but a brief
interval of obscurity. At Novaya Zemlya, however, of all places, the
conditions were tolerably favourable, and, as we have seen, the trophy
of a "flash-spectrograph" was carried off. Some coronal photographs,
moreover, taken by the late Sir George Baden-Powell[574] and by M.
Hansky, a member of a Russian party, were marked by features of
considerable interest. They made apparent a close connection between
coronal outflows and chromospheric jets, cone-shaped beams serving as
the sheaths, or envelopes, of prominences. M. Hansky,[575] indeed,
thought that every streamer had a chromospheric eruption at its base.
Further, dark veinings of singular shapes unmistakably interrupted the
coronal light, and bordered brilliant prominences,[576] reminding us of
certain "black lines" traced by Swift across the "anvil protuberance"
August 7, 1869.[577] In type the corona of 1896 reproduced that of 1886,
as befitted its intermediate position in the solar cycle.
The eclipse-track on January 22, 1898, crossed the Indian peninsula from
Viziadrug, on the Malabar coast, to Mount Everest in the Himalayas. Not
a cloud obstructed the view anywhere, and an unprecedented harvest of
photographic records was garnered. The flash-spectrum, in its successive
phases, appeared on plates taken by Sir Norman Lockyer, Mr. Evershed,
Professor Campbell,[578] and others; Professor Turner[579] set on foot a
novel mode of research by picturing the corona in the polarised
ingredient of its light; Mrs. Maunder[580] practically solved the
problem of photographing the faint coronal extensions, one ray on her
plates running out to nearly six diameters from the moon's limb. Yet she
used a Dallmeyer lens of only one and a half inches aperture. Her
success accorded perfectly with Professor Wadsworth's conclusion that
effectiveness in delineation by slight contrasts of luminosity varies
inversely with aperture. Triple-coated plates, and a comparatively long
exposure of twenty seconds, contributed to a result unlikely, for some
time, to be surpassed. The corona of 1898 presented a mixed aspect. The
polar plumes due at minimum were combined in it with the quadrilateral
ogives belonging to spot-maxima. A slow course of transformation, in
fact, seemed in progress; and it was found to be completed in 1900, when
the eclipse of May 28 revealed the typical halo of a quiescent sun.
The obscurity on this occasion was short--less than 100 seconds--but was
well observed east and west of the Atlantic. No striking gain in
knowledge, however, resulted. Important experiments were indeed made on
the heat of the corona with Langley's bolometer, but their upshot can
scarcely be admitted as decisive. They indicated a marked deficiency of
thermal radiations, implying for coronal light, in Professor Langley's
opinion,[581] an origin analogous to that of the electric
glow-discharge, which, at low pressures, was found by K. Ångström in
1893 to have no invisible heat-spectrum.[582] The corona was
photographed by Professor Barnard, at Wadesborough, North Carolina, with
a 61-1/2-foot horizontal "coelostat." In this instrument, of a type now
much employed in eclipse operations and first recommended by Professor
Turner, a six-inch photographic objective preserved an invariable
position, while a silvered plane mirror, revolving by clockwork once in
forty-eight hours (since the angle of movement is doubled by
reflection), supplied the light it brought to a focus. A temporary
wooden tube connected the lens with the photographic house where the
plates were exposed. Pictures thus obtained with exposures of from one
to fourteen seconds, were described as "remarkably sharp and perfectly
defined, showing the prominences and inner corona very beautifully. The
polar fans came out magnificently."[583]
The great Sumatra eclipse left behind it manifold memories of foiled
expectations. A totality of above six minutes drew observers to the Far
East from several continents, each cherishing a plan of inquiry which
few were destined to execute. All along the line of shadow, which, on
May 18, 1901, crossed Réunion and Mauritius, and again met land at
Sumatra and Borneo, the meteorological forecast was dubious, and the
meteorological actuality in the main deplorable. Nevertheless, the
corona was seen, and fairly well photographed through drifting clouds,
and proved to resemble in essentials the appendage viewed a year
previously. Negatives taken by members of the Lick Observatory
expedition led by Mr. Perrine[584] disclosed the unique phenomenon of a
violent coronal disturbance, with a small compact prominence as its
apparent focus. Tumbling masses and irregular streamers radiating from a
point subsequently shown by the Greenwich photographs to be the seat of
a conspicuous spot, suggested the recent occurrence of an explosion, the
far-reaching effects of which might be traced in the confused floccular
luminosity of a vast surrounding region. Again, photographs in polarised
light attested the radiance of the outer corona to be in large measure
reflected, while that of the inner ring was original; and the inference
was confirmed by spectrographs, recording many Fraunhofer lines when the
slit lay far from the sun's limb, but none in its immediate vicinity. On
plates exposed by Mr. Dyson and Dr. Humphrys with special apparatus, the
coronal spectrum, continuous and linear, impressed itself more
extensively in the ultra-violet than on any previous occasion; and Dr.
Mitchell succeeded in photographing the reversing layer by means of a
grating spectroscope. Finally, Mrs. Maunder, at Mauritius, despite
mischievous atmospheric tremors, obtained with the Newbegin telescope an
excellent series of coronal pictures.[585]
The principles of explanation applied to the corona may be briefly
described as eruptive and electrical. The first was adopted by Professor
Schaeberle in his "Mechanical Theory," advanced in 1890.[586] According
to this view, the eclipse-halo consists of streams of matter shot out
with great velocity from the spot-zones by forces acting perpendicularly
to the sun's surface. The component particles return to the sun after
describing sections of extremely elongated ellipses, unless their
initial speed happen to equal or exceed the critical rate of 383 miles a
second, in which case they are finally driven off into space. The
perspective overlapping and interlacing of these incandescent outflows
was supposed to occasion the intricacies of texture visible in the
corona; and it should be recorded that a virtually identical conclusion
was reached by Mr. Perrine in 1901,[587] by a different train of
reasoning, based upon a distinct set of facts. A theory on very much the
same lines was, moreover, worked out by M. Bélopolsky in 1897.[588]
Schaeberle, however, had the merit of making the first adequate effort
to deduce the real shape of the corona, as it exists in three
dimensions, from its projection upon the surface of the sphere. He
failed, indeed, to account for the variation in coronal types by the
changes in our situation with regard to the sun's equator. It is only
necessary to remark that, if this were so, they should be subject to an
annual periodicity, of which no trace can be discerned.
Electro-magnetic theories have the charm, and the drawback, of dealing
largely with the unknown. But they are gradually losing the vague and
intangible character which long clung to them; and the improved
definition of their outlines has not, so far, brought them into
disaccord with truth. The most promising hypothesis of the kind is due
to Professor Bigelow of Washington. His able discussion of the eclipse
photographs of January 1, 1889,[589] showed a striking agreement between
the observed coronal forms and the calculated effects of a repulsive
influence obeying the laws of electric potential, also postulated by
Huggins in 1885.[590] Finely subdivided matter, expelled from the sun
along lines of force emanating from the neighbourhood of his poles, thus
tends to accumulate at "equipotential surfaces." In deference, however,
to a doubt more strongly felt then than now, whether the presence of
free electricity is compatible with the solar temperature, he avoided
any express assertion that the coronal structure is an electrical
phenomenon, merely pointing out that, if it were, its details would be
just what they are.
Later, in 1892, Pupin in America,[591] and Ebert in Germany,[592]
imitated the coronal streamers by means of electrical discharges in low
vacua between small conducting bodies and strips of tinfoil placed on
the outside of the containing glass receptacles. Finally, a critical
experiment made by Ebert in 1895 served, as Bigelow justly said, "to
clear up the entire subject, and put the theory on a working basis."
Having obtained coronoidal effects in the manner described, he proceeded
to subject them to the action of a strong magnetic field, with the
result of marshalling the scattered rays into a methodical and highly
suggestive array. They followed the direction of the magnetic lines of
force, and, forsaking the polar collar of the magnetised sphere,
surrounded it like a ruffle. The obvious analogy with the aurora polaris
and the solar corona was insisted upon by Ebert himself, and has been
further developed by Bigelow.[593] According to a recent modification of
his hypothesis, the latter appendage is controlled by two opposing
systems of forces; the magnetic causing the rays to diverge from the
poles towards the equator, and the electrostatic urging their spread,
through the mutual repulsion of the particles accumulated in the
"wings," from the equator towards either pole. The cyclical change in
the corona, he adds, is probably due to a variation in the balance of
power thus established, the magnetic polar influence dominating at
minima, the electrostatic at maxima. And he may well feel encouraged by
the fortunate combination of many experimental details into one
explanatory whole, no less than by the hopeful prospect of further
developments, both practical and theoretical, along the same lines.
What we really know about the corona can be summed up in a few words. It
is certainly _not_ a solar atmosphere. It does not gravitate upon the
sun's surface and share his rotation, as our air gravitates upon and
shares the rotation of the earth; and this for the simple reason that
there is no visible growth of pressure downwards (of which the
spectroscope would infallibly give notice) in its gaseous constituents;
whereas under the sole influence of the sun's attractive power, their
density should be multiplied many million times in the descent through a
mere fraction of their actual depth.[594]
They are apparently in a perpetual state of efflux from, and influx to
our great luminary, under the stress of opposing forces. It is not
unlikely that some part, at least, of the coronal materials are provided
by eruptions from the body of the sun;[595] it is almost certain that
they are organized and arranged round it through electro-magnetic
action. This, however, would seem to be influential only upon their
white-hot or reflective ingredients, out of which the streamers and
aigrettes are composed; since the coronal gases appear, from
observations during eclipses, to form a shapeless envelope, with
condensations above the spot-zones, or at the bases of equatorial
extensions. The corona is undoubtedly affected both in shape and
constitution by the periodic ebb and flow of solar activity, its
low-tide form being winged, its high-tide form stellate; while the rays
emitted by the gases contained in it fade, and the continuous spectrum
brightens, at times of minimum sun-spots. The appendage, as a whole,
must be of inconceivable tenuity, since comets cut their way through it
without experiencing sensible retardation. Not even Sir William
Crookes's vacua can give an idea of the rarefaction which this fact
implies. Yet the observed luminous effects may not in reality bear
witness contradictory of it. One solitary molecule in each cubic inch of
space might, in Professor Young's opinion, produce them; while in the
same volume of ordinary air at the sea-level, the molecules number
(according to Dr. Johnstone Stoney) 20,000 trillions!
The most important lesson, however, derived from eclipses is that of
partial independence of them. Some of its fruits in the daily study of
prominences the next chapter will collect; and the harvest has been
rendered more abundant, as well as more valuable, since it has been
found possible to enlist, in this department too, the versatile aid of
the camera.
FOOTNOTES:
[Footnote 512: _Vierteljahrsschrift Astr. Ges._, Jahrg. xxvi., p. 274.]
[Footnote 513: _Astr. Jour._, vol. iv., p. 33.]
[Footnote 514: _Proc. Roy. Soc._, vol. xvii., p. 116.]
[Footnote 515: _Comptes Rendus_, t. lxvii., p. 757.]
[Footnote 516: _Comptes Rendus_, t. lxvii., p. 839.]
[Footnote 517: _Month. Not._, vol. xxvii., p. 88.]
[Footnote 518: _Proc. Roy. Soc._, vol. xvii., p. 123.]
[Footnote 519: _Washington Observations_, 1867, App. ii., Harkness's
Report, p. 60.]
[Footnote 520: _Am. Jour._, vol. xlviii. (2nd series), p. 377.]
[Footnote 521: _Am. Jour._, vol. xi. (3rd series), p. 429.]
[Footnote 522: Campbell, _Astroph. Jour._, vol. x., p. 186.]
[Footnote 523: Keeler, _Reports on Eclipse of January 1, 1889_, p. 47.]
[Footnote 524: Everything in such observations depends upon the proper
manipulation of the slit of the spectroscope.]
[Footnote 525: _Mem. R. A. S._, vol. xli., p. 435.]
[Footnote 526: _Comptes Rendus_, t. lxvii., p. 1019.]
[Footnote 527: _Mem. R. A. S._, vol. xli., p. 43.]
[Footnote 528: _Comptes Rendus_, t. xciv., p. 1640.]
[Footnote 529: Young, _Pop. Astr._, Oct., 1897, p. 333.]
[Footnote 530: J. Evershed, _Indian Eclipse_, 1898, p. 65; _Month.
Not._, vol. lviii., p. 298; _Proc. Roy. Soc._, Jan. 17, 1901.]
[Footnote 531: Frost, _Astroph. Jour._, vol. xii., p. 85; Lord, _Ibid._,
vol. xiii., p. 149.]
[Footnote 532: _Comptes Rendus_, t. cxvii., No. 1; _Jour. Brit. Astr.
Ass._, vol. iii., p. 532.]
[Footnote 533: Lockyer, _Phil. Trans._, vol. clvii., p. 551.]
[Footnote 534: The rosy envelope of prominence-matter was so named by
Lockyer in 1868 (_Phil. Trans._, vol. clix., p. 430).]
[Footnote 535: According to Trouvelot (_Wash. Obs._, 1876, App. iii., p.
80), the subtracted matter was, at least to some extent, accumulated in
the polar regions.]
[Footnote 536: _Bull. Phil. Soc. Washington_, vol. iii., p. 118.]
[Footnote 537: _Mem. R. A. S._, vol. xli., 1879.]
[Footnote 538: _Astr. Nach._, No. 1,737.]
[Footnote 539: _Correspondence with Newton_, pp. 181-184; Ranyard, _Mem.
Astr. Soc._, vol. xli., p. 501.]
[Footnote 540: S. P. Langley, _Wash. Obs._, 1876, App. iii., p. 209;
_Nature_, vol. lxi., p. 443.]
[Footnote 541: Schuster (_Proc. Roy. Soc._, vol. xxxv., p. 154) measured
and photographed about thirty.]
[Footnote 542: Abney, _Phil. Trans._, vol. clxxv., p. 267.]
[Footnote 543: _Proc. Roy. Soc._, vol. xxxiv., p. 409. Experiments
directed to the same end had been made by Dr. O. Lohse at Potsdam,
1878-80. _Astr. Nach._, No. 2,486.]
[Footnote 544: The sensitiveness of chloride of silver extends from _h_
to H; that is, over the upper or more refrangible half of the space in
which the main part of the coronal light is concentrated.]
[Footnote 545: _Proc. Roy. Soc._, vol. xxxiv., p. 414.]
[Footnote 546: _Report Brit. Assoc._, 1883, p. 351.]
[Footnote 547: Maunder, _Indian Eclipse_, p. 125; _Eclipse of 1900_, p.
143.]
[Footnote 548: _Astr. and Astrophysics_, vol. xiii., p. 662.]
[Footnote 549: See _infra_, p. 197.]
[Footnote 550: Abney, _Phil. Trans._, vol. clxxx., p. 119.]
[Footnote 551: _Comptes Rendus_, t. xcvii., p. 592.]
[Footnote 552: _Memoirs National Ac. of Sciences_, vol. ii., p. 102.]
[Footnote 553: _Wash. Obs._, 1867, App. ii., p. 64.]
[Footnote 554: _The Sun_, p. 357.]
[Footnote 555: _Proc. Roy. Soc._, vol. xvii., p. 289.]
[Footnote 556: _Comptes Rendus_, t. lxxiii., p. 434.]
[Footnote 557: _Wash. Obs._, 1867, App. ii., p. 195.]
[Footnote 558: Stokes, Anniversary Address, _Nature_, vol. xxxv., p.
114.]
[Footnote 559: _Comptes Rendus_, t. ci., p. 50.]
[Footnote 560: _Harvard Annals_, vol. xviii., p. 99.]
[Footnote 561: Wesley, _Phil. Trans._, vol. clxxx., p. 350.]
[Footnote 562: _Harvard Annals_, vol. xviii, p. 108.]
[Footnote 563: _Lick Report_, p. 20.]
[Footnote 564: _Ibid._, p. 14.]
[Footnote 565: _Ibid._, p. 155.]
[Footnote 566: _Pub. Astr. Soc. of the Pacific_, vol. iii., p. 158.]
[Footnote 567: Professor Holden concluded, with less qualification,
"that so-called 'polar' rays exist at all latitudes on the sun's
surface." _Lick Report_, p. 19.]
[Footnote 568: Holden, _Report on Eclipse of December, 1889_, p. 18;
Charroppin, _Pub. Astr. Soc. of the Pacific_, vol. iii., p. 26.]
[Footnote 569: Published as the Frontispiece to the _Observatory_, No.
160.]
[Footnote 570: Wesley, _Ibid._, p. 107.]
[Footnote 571: _Lick Observatory Contributions_, No. 4, p. 108.]
[Footnote 572: _Astr. and Astrophysics_, vol. xiii. p. 307.]
[Footnote 573: Lockyer, _Phil. Trans._, vol. clxxxvii., p. 592.]
[Footnote 574: He died in London, November 20, 1898.]
[Footnote 575: _Bull. Acad. St. Pétersbourg_, t. vi., p. 253.]
[Footnote 576: W. H. Wesley, _Phil. Trans._, vol. cxc, p. 204.]
[Footnote 577: _Lick Reports on Eclipse of January 1, 1889_, p. 204.]
[Footnote 578: _Astroph. Jour._, vol. xi., p. 226.]
[Footnote 579: _Observatory_, vol. xxi., p. 157.]
[Footnote 580: _The Indian Eclipse_, 1898, p. 114.]
[Footnote 581: _Science_, June 22, 1900; _Astroph. Jour._, vol. xii., p.
370.]
[Footnote 582: _Ann. der Physik_, Bd. xlviii., p. 528. See also Wood,
_Physical Review_, vol. iv., p. 191, 1896.]
[Footnote 583: _Science_, August 3, 1900.]
[Footnote 584: _Lick Observatory Bulletin_, No. 9.]
[Footnote 585: _Observatory_, vol. xxiv., pp. 321, 375.]
[Footnote 586: _Lick Report on Eclipse of December 22, 1889_, p. 47;
_Month. Not._, vol. l., p. 372.]
[Footnote 587: _Lick Obs. Bull._, No. 9.]
[Footnote 588: _Bull. de l'Acad. St. Pétersbourg_, t. iv., p. 289.]
[Footnote 589: _The Solar Corona discussed by Spherical Harmonics_,
Smithsonian Institution, 1889.]
[Footnote 590: Bakerian Lecture, _Proc. Roy. Soc._, vol. xxxix.]
[Footnote 591: _Astr. and Astrophysics_, vol. xi., p. 483.]
[Footnote 592: _Ibid._, vol. xii., p. 804.]
[Footnote 593: _Am. Journ. of Science_, vol. xi., p. 253, 1901.]
[Footnote 594: See Huggins, _Proc. Roy. Soc._, vol. xxxix., p. 108;
Young, _North Am. Review_, February, 1885, p. 179.]
[Footnote 595: Professor W. A. Norton, of Yale College, appears to have
been the earliest formal advocate of the Expulsion Theory of the solar
surroundings, in the second (1845) and later editions of his _Treatise
on Astronomy_.]
CHAPTER IV
_SOLAR SPECTROSCOPY_
The new way struck out by Janssen and Lockyer was at once and eagerly
followed. In every part of Europe, as well as in North America,
observers devoted themselves to the daily study of the chromosphere and
prominences. Foremost among these were Lockyer in England, Zöllner at
Leipzig, Spörer at Anclam, Young at Hanover, New Hampshire, Secchi and
Respighi at Rome. There were many others, but these names stood out
conspicuously.
The first point to be cleared up was that of chemical composition.
Leisurely measurements verified the presence above the sun's surface of
hydrogen in prodigious volumes, but showed that sodium had nothing to do
with the orange-yellow ray identified with it in the haste of the
eclipse. From its vicinity to the D-pair (than which it is slightly more
refrangible), the prominence-line was, however, designated D_3, and the
unknown substance emitting it was named by Lockyer "helium." Its
terrestrial discovery ensued after twenty-six years. In March, 1895,
Professor Ramsay obtained from the rare mineral clevite a volatile gas,
the spectrum of which was found to include the yellow prominence-ray.
Helium was actually at hand, and available for examination. The
identification cleared up many obscurities in chromospheric chemistry.
Several bright lines, persistently seen at the edge of the sun, and
early suspected by Young[596] to emanate from the same source as D_3,
were now derived from helium in the laboratory; and all the complex
emissions of that exotic substance ranged themselves into six sets or
series, the members of which are mutually connected by numerical
relations of a definite and simple kind. Helium is of rather more than
twice the density of hydrogen, and has no chemical affinities. In almost
evanescent quantities it lurks in the earth's crust, and is diffused
through the earth's atmosphere.
The importance of the part played in the prominence-spectrum by the
violet line of calcium was noticed by Professor Young in 1872, but since
H and K lie near the limit of the visible spectrum, photography was
needed for a thorough investigation of their appearances. Aided by its
resources, Professor George E. Hale, then at the beginning of his
career, detected in 1889 their unfailing and conspicuous presence.[597]
The substance emitting them not only constitutes a fundamental
ingredient of the chromosphere, but rises, in the fantastic jets thence
issuing, to greater heights than hydrogen itself. The isolation of H and
K in solar prominences from any other of the lines usually distinctive
of calcium was experimentally proved by Sir William and Lady Huggins in
1897 to be due to the extreme tenuity of the emitting vapour.[598]
Hydrogen, helium, and calcium form, then, the chief and unvarying
materials of the solar sierra and its peaks; but a number of metallic
elements make their appearance spasmodically under the influence of
disturbances in the layers beneath. In September, 1871, Young[599] drew
up at Dartmouth College a list of 103 lines significant of injections
into the chromosphere of iron, titanium, chromium, magnesium, and many
other substances. During two months' observation in the pure air of
Mount Sherman (8,335 feet high) in the summer of 1872, these tell-tale
lines mounted up to 273;[600] and he believes their number might still
be doubled by steady watching. Indeed, both Young and Lockyer have more
than once seen the whole field of the spectroscope momentarily inundated
with bright rays, as if the "reversing layer" had been suddenly thrust
upwards into the chromosphere, and as quickly allowed to drop back
again. The opinion would thus appear to be well-grounded that the two
form one continuous region, of which the lower parts are habitually
occupied by the heaviest vapours, but where orderly arrangement is
continually overturned by violent eruptive disturbances.
The study of the _forms_ of prominences practically began with Huggins's
observation of one through an "open slit" February 13, 1869.[601] At
first it had been thought possible to examine them only in
sections--that is, by admitting mere narrow strips or "lines" of their
various kinds of light; while the actual shape of the objects emitting
those lines had been arrived at by such imperfect devices as that of
giving to the slit of the spectroscope a vibratory moment rapid enough
to enable the eye to retain the impression of one part while others were
successively presented to it. It was an immense gain to find that their
rays had strength to bear so much of dilution with ordinary light as was
involved in opening the spectroscopic shutter wide enough to exhibit the
tree-like, or horn-like, or flame-shaped bodies rising over the sun's
rim in their undivided proportions. Several diversely-coloured images of
them are formed in the spectroscope; each may be seen under a crimson, a
yellow, a green, and a deep blue aspect. The crimson, however (built up
out of the C-line of hydrogen), is the most intense, and is commonly
used for purposes of observation and illustration.
Friedrich Zöllner was, by a few days, beforehand with Huggins in
describing the open-slit method, but was somewhat less prompt in
applying it. His first survey of a complete prominence, pictured in, and
not simply intersected by, the slit of his spectroscope, was obtained
July 1, 1869.[602] Shortly afterwards the plan was successfully adopted
by the whole band of investigators.
A difference in kind was very soon perceived to separate these objects
into two well-marked classes. Its natural and obvious character was
shown by its having struck several observers independently. The
distinction of "cloud-prominences" from "flame-prominences" was
announced by Lockyer, April 27; by Zöllner, June 2; and by Respighi,
December 4, 1870.
The first description are tranquil and relatively permanent, sometimes
enduring without striking change for many days. Certain of the included
species mimic terrestrial cloud-scenery--now appearing like fleecy
cirrus transpenetrated with the red glow of sunset--now like prodigious
masses of cumulo-stratus hanging heavily above the horizon. The solar
clouds, however, have the peculiarity of possessing _stems_. Slender
columns can ordinarily be seen to connect the surface of the
chromosphere with its outlying portions. Hence the fantastic likeness to
forest scenery presented by the long ranges of fiery trunks and foliage
occasionally seeming to fringe the sun's limb. But while this mode of
structure suggests an actual outpouring of incandescent material,
certain facts require a different interpretation. At a distance, and
quite apart from the chromosphere, prominences have been perceived, both
by Secchi and Young, to _form_, just as clouds form in a clear sky,
condensation being replaced by ignition. Filaments were then thrown out
downward towards the chromosphere, and finally the usual appearance of a
"stemmed prominence" was assumed. Still more remarkable was an
observation made by Trouvelot at Harvard College Observatory, June 26,
1874.[603] A gigantic comma-shaped prominence, 82,000 miles high,
vanished from before his eyes by a withdrawal of light as sudden as the
passage of a flash of lightning. The same observer has frequently
witnessed a gradual illumination or gradual extinction of such objects,
testifying to changes in the thermal or electrical condition of matter
already _in situ_.
The first photograph of a prominence, as shown by the spectroscope in
daylight, was taken by Professor Young in 1870.[604] But neither his
method, nor that described by Dr. Braun in 1872,[605] had any practical
success. This was reserved to reward the efforts towards the same end of
Professor Hale. Begun at Harvard College in 1889,[606] they were
prosecuted soon afterwards at the Kenwood Observatory, Chicago. The
great difficulty was to extricate the coloured image of the gaseous
structure, spectroscopically visible at the sun's limb, from the
encompassing glare, a very little of which goes a long way in _fogging_
sensitive plates. To counteract its mischievous effects, a second
slit,[607] besides the usual narrow one in front of the collimator, was
placed on guard, as it were, behind the dispersing apparatus, so as to
shut out from the sensitised surface all light save that of the required
quality. The sun's image being then allowed to drift across the outer
slit, while the plate holder was kept moving at the same rate, the
successive sectional impressions thus rapidly obtained finally "built
up" a complete picture of the prominence. Another expedient was soon
afterwards contrived.[608] The H and K rays of calcium are always, as we
have seen, bright in the spectrum of prominences. They are besides fine
and sharp, while the corresponding absorption-lines in the ordinary
solar spectrum are wide and diffuse. Hence, prominences formed by the
spectroscope out of these particular qualities of violet light, can be
photographed entire and at once, for the simple reason that they are
projected upon a naturally darkened background. Atmospheric glare is
abolished by local absorption. This beautiful method was first realised
by Professor Hale in June, 1891.
A "spectroheliograph," consisting of a spectroscopic and a photographic
apparatus of special type, attached to the eye-end of an equatoreal
twelve inches in aperture, was erected at Kenwood in March, 1891; and
with its aid, Professor Hale entered upon original researches of high
promise for the advancement of solar physics. Noteworthy above all is
his achievement of photographing both prominences and faculæ on the very
face of the sun. The latter had, until then, been very imperfectly
observed. They were only visible, in fact, when relieved by their
brilliancy against the dusky edge of the solar disc. Their convenient
emission of calcium light, however, makes it possible to photograph them
in all positions, and emphasises their close relationship to
prominences. The simultaneous picturing, moreover, of the entire
chromospheric ring, with whatever trees or fountains of fire chance to
be at the moment issuing from it, has been accomplished by a very simple
device. The disc of the sun itself having been screened with a circular
metallic diaphragm, it is only necessary to cause the slit to traverse
the virtually eclipsed luminary, in order to get an impression of the
whole round of its fringing appendages. And the record can be extended
to the disc by removing the screen, and carrying the slit back at a
quicker rate, when an "image of the sun's surface, with the faculæ and
spots, is formed on the plate exactly within the image of the
chromosphere formed during the first exposure. The whole operation,"
Professor Hale continues, "is completed in less than a minute, and the
resulting photographs give the first true pictures of the sun, showing
all of the various phenomena at its surface."[609] Most of these novel
researches were, by a remarkable coincidence, pursued independently and
contemporaneously by M. Deslandres, of the Paris Observatory.[610]
The ultra-violet prominence spectrum was photographed for the first time
from an uneclipsed sun, in June, 1891, at Chicago. Besides H and K, four
members of the Huggins-series of hydrogen-lines imprinted themselves on
the plate.[611] Meanwhile M. Deslandres was enabled, by fitting quartz
lenses to his spectroscope, and substituting a reflecting for a
refracting telescope, to get rid of the obstructive action of glass upon
the shorter light-waves, and thus to widen the scope of his inquiry into
the peculiarities of those derived from prominences.[612] As the result,
not only all the nine white-star lines were photographed from a
brilliant sun-flame, but five additional ones were found to continue the
series upward. The wave-lengths of these last had, moreover, been
calculated beforehand with singular exactness, from a simple formula
known as "Balmer's Law."[613] The new lines, accordingly, filled places
in a manner already prepared for them, and were thus unmistakably
associated with the hydrogen-spectrum. This is now known to be
represented in prominences by twenty-seven lines,[614] forming a kind of
harmonic progression, only four of which are visibly darkened in the
Fraunhofer spectrum of the sun.
PLATE I.
[Illustration: Photographs of the Solar Chromosphere and Prominences.
Taken with the Spectroheliograph of the Kenwood Observatory, Chicago, by
Professor George E. Hale.]
The chemistry of "cloud-prominences" is simple. Hydrogen, helium, and
calcium are their chief constituents. "Flame-prominences," on the other
hand, show, in addition, the characteristic rays of a number of metals,
among which iron, titanium, barium, strontium, sodium, and magnesium are
conspicuous. They are intensely brilliant; sharply defined in their
varying forms of jets, spikes, fountains, waterspouts; of rapid
formation and speedy dissolution, seldom attaining to the vast
dimensions of the more tranquil kind. Eruptive or explosive by origin,
they occur in close connection with spots; whether causally, the
materials ejected as "flames" cooling and settling down as dark,
depressed patches of increased absorption;[615] or consequentially, as a
reactive effect of falls of solidified substances from great heights in
the solar atmosphere.[616] The two classes of phenomena, at any rate,
stand in a most intimate relation; they obey the same law of
periodicity, and are confined to the same portions of the sun's surface,
while quiescent prominences may be found right up to the poles and close
to the equator.
The general distribution of prominences, including both genera, follows
that of faculæ much more closely than that of spots. From Father
Secchi's and Professor Respighi's observations, 1869-71, were derived
the first clear ideas on the subject, which have been supplemented and
modified by the later researches of Professors Tacchini and Riccò at
Rome and Palermo. The results are somewhat complicated, but may be
stated broadly as follows. The district of greatest prominence-frequency
covers and overlaps by several degrees that of the greatest
spot-frequency. That is to say, it extends to about 40° north and south
of the equator.[617] There is a visible tendency to a second pair of
maxima nearer the poles. The poles themselves, as well as the equator,
are regions of minimum occurrence. Distribution in time is governed by
the spot-cycle, but the maximum lasts longer for prominences than for
spots.
The structure of the chromosphere was investigated in 1869 and
subsequent years by Professor Respighi, director of the Capitoline
Observatory, as well as by Spörer, and Brédikhine of the Moscow
Observatory. They found this supposed solar envelope to be of the same
eruptive nature as the vast protrusions from it, and to be made up of a
congeries of minute flames[618] set close together like blades of grass.
"The appearance," Professor Young writes,[619] "which probably indicates
a fact, is as if countless jets of heated gas were issuing through vents
and spiracles over the whole surface, thus clothing it with flame which
heaves and tosses like the blaze of a conflagration."
The summits of these filaments of fire are commonly inclined, as if by a
wind sweeping over them, when the sun's activity is near its height, but
erect during his phase of tranquillity. Spörer, in 1871, inferred the
influence of permanent polar currents,[620] but Tacchini showed in 1876
that the deflections upon which this inference was based ceased to be
visible as the spot-minimum drew near.[621]
Another peculiarity of the chromosphere, denoting the remoteness of its
character from that of a true atmosphere,[622] is the irregularity of
its distribution over the sun's surface. There are no signs of its
bulging out at the equator, as the laws of fluid equilibrium in a
rotating mass would require; but there are some that the fluctuations in
its depth are connected with the phases of solar agitation. At times of
minimum it seems to accumulate and concentrate its activity at the
poles; while maxima probably bring a more equable general distribution,
with local depressions at the base of great prominences and above spots.
A low-lying stratum of carbon-vapour was, in 1897, detected in the
chromosphere by Professor Hale with a grating-spectroscope attached to
the 40-inch Yerkes refractor.[623] The eclipse-photographs of 1893
disclosed to Hartley's examination the presence there of gallium;[624]
and those taken by Evershed in 1898 were found by Jewell[625] to be
crowded with ultra-violet lines of the equally rare metal scandium. The
general rule had been laid down by Sir Norman Lockyer that the metallic
radiations from the chromosphere are those "enhanced" in the electric
spark.[626] Hence, the comparative study of conditions prevalent in the
arc and the spark has acquired great importance in solar physics.
The reality of the appearance of violent disturbance presented by the
"flaming" kind of prominence can be tested in a very remarkable manner.
Christian Doppler,[627] professor of mathematics at Prague, enounced in
1842 the theorm that the colour of a luminous body, like the pitch of a
sonorous body, must be changed by movements of approach or recession.
The reason is this. Both colour and pitch are physiological effects,
depending, not upon absolute wave-length, but upon the number of waves
entering the eye or ear in a given interval of time. And this number, it
is easy to see, must be increased if the source of light or sound is
diminishing its distance, and diminished if it is decreasing it. In the
one case, the vibrating body _pursues_ and crowds together the waves
emanating from it; in the other, it _retreats_ from them, and so
lengthens out the space covered by an identical number. The principle
may be thus illustrated. Suppose shots to be fired at a target at fixed
intervals of time. If the marksman advances, say twenty paces between
each discharge of his rifle, it is evident that the shots will fall
faster on the target than if he stood still; if, on the contrary, he
retires by the same amount, they will strike at correspondingly longer
intervals. The result will of course be the same whether the target or
the marksman be in movement.
So far Doppler was altogether right. As regards sound, anyone can
convince himself that the effect he predicted is a real one, by
listening to the alternate shrilling and sinking of the steam-whistle
when an express train rushes through a station. But in applying this
principle to the colours of stars he went widely astray; for he omitted
from consideration the double range of invisible vibrations which
partake of, and to the eye exactly compensate, changes of refrangibility
in the visible rays. There is, then, no possibility of finding a
criterion of velocity in the hue of bodies shining, like the sun and
stars, with continuous light. The entire spectrum is slightly shifted up
or down in the scale of refrangibility; certain rays normally visible
become exalted or degraded (as the case may be) into invisibility, and
certain other rays at the opposite end undergo the converse process; but
the sum total of impressions on the retina continues the same.
We are not, however, without the means of measuring this sub-sensible
transportation of the light-gamut. Once more the wonderful Fraunhofer
lines came to the rescue. They were called by the earlier physicists
"fixed lines;" but it is just because they are _not_ fixed that, in this
instance, we find them useful. They share, and in sharing betray, the
general shift of the spectrum. This aspect of Doppler's principle was
adverted to by Fizeau in 1848,[628] and the first tangible results in
the estimation of movements of approach and recession between the earth
and the stars, were communicated by Sir William Huggins to the Royal
Society, April 23, 1868. Eighteen months later, Zöllner devised his
"reversion-spectroscope"[629] for doubling the measurable effects of
line-displacements; aided by which ingenious instrument, and following a
suggestion of its inventor, Professor H. C. Vogel succeeded at Bothkamp,
June 9, 1871,[630] in detecting effects of that nature due to the solar
rotation. This application constitutes at once the test and the triumph
of the method.[631]
The eastern edge of the sun is continually moving towards us with an
equatorial speed of about a mile and a quarter per second, the western
edge retreating at the same rate. The displacements--towards the violet
on the east, towards the red on the west--corresponding to this velocity
are very small; so small that it seems hardly credible that they should
have been laid bare to perception. They amount to but 1/150th part of
the interval between the two constituents of the D-line of sodium; and
the D-line of sodium itself can be separated into a pair only by a
powerful spectroscope. Nevertheless, Professor Young[632] was able to
show quite satisfactorily, in 1876, not only deviations in the solar
lines from their proper places indicating a velocity of rotation (1·42
miles per second) slightly in excess of that given by observations of
spots, but the exemption of terrestrial lines (those produced by
absorption in the earth's atmosphere) from the general push upwards or
downwards. Shortly afterwards, Professor Langley, then director of the
Allegheny Observatory, having devised a means of comparing with great
accuracy light from different portions of the sun's disc, found that
while the obscure rays in two juxtaposed spectra derived from the solar
poles were absolutely continuous, no sooner was the instrument rotated
through 90°, so as to bring its luminous supplies from opposite
extremities of the equator, than the same rays became perceptibly
"notched." The telluric lines, meanwhile, remained unaffected, so as to
be "virtually mapped" by the process.[633] This rapid and unfailing mode
of distinction was used by Cornu with perfect ease during his
investigation of atmospheric absorption near Loiret in August and
September, 1883.[634]
A beautiful experiment of the same kind was performed by M. Thollon, of
M. Bischoffsheim's observatory at Nice, in the summer of 1880.[635] He
confined his attention to one delicately defined group of four lines in
the orange, of which the inner pair are solar (iron) and the outer
terrestrial. At the centre of the sun the intervals separating them were
sensibly equal; but when the light was taken alternately from the right
and left limbs, a relative shift in alternate directions of the solar,
towards and from the stationary telluric rays became apparent. A
parallel observation was made at Dunecht, December 14, 1883, when it was
noticed that a strong iron-line in the yellow part of the solar spectrum
is permanently double on the sun's eastern, but single on his western
limb;[636] opposite motion-displacements bringing about this curious
effect of coincidence with, and separation from, an adjacent stationary
line of our own atmosphere's production, according as the spectrum is
derived from the retreating or advancing margin of the solar globe.
Statements of fact so precise and authoritative amount to a
demonstration that results of this kind are worthy of confidence; and
they already occupy an important place among astronomical data.
The subtle method of which they served to assure the validity was
employed in 1887-9 by M. Dunér to test and extend Carrington's and
Spörer's conclusions as to the anomalous nature of the sun's axial
movement.[637] His observations for the purpose, made with a fine
diffraction-spectroscope, just then mounted at the observatory of
Upsala, were published in 1891.[638] Their upshot was to confirm and
widen the law of retardation with increasing latitude derived from the
progressive motions of spots. Determinations made within 15° of the
pole, consequently far beyond the region of spots, gave a
rotation-period of 38-1/2, that of the equatorial belt being of 25-1/2
days. Spots near the equator indeed complete their rounds in a period
shorter by at least half a day; and proportionate differences were found
to exist elsewhere in corresponding latitudes; but Dunér's observations,
it must be remembered, apply to a distinct part of the complex solar
machine from the disturbed photospheric surface. It is amply possible
that the absorptive strata producing the Fraunhofer lines, significant,
by their varying displacements at either limb, of the inferred varying
rates of rotation, may gyrate more slowly than the spot-generating
level. Moreover, faculæ appear to move at a quicker pace than
either;[639] so that we have, for three solar formations, three
different periods of average rotation, the shortest of which belongs to
the faculæ, one of intermediate length to the spots, and the most
protracted to the reversing layer. All, however, agree in lengthening
progressively from the equator towards the poles. Professor Holden aptly
compared the sun to "a vast whirlpool where the velocities of rotation
depend not only on the situation of the rotating masses as to latitude,
but also as to depth beneath the exterior surface."[640]
Sir Norman Lockyer[641] promptly perceived the applicability of the
surprising discovery of line-shiftings through end-on motion to the
study of prominences, the discontinuous light of which affords precisely
the same means of detecting movement without seeming change of place, as
do lines of absorption in a continuous spectrum. Indeed, his
observations at the sun's edge almost compelled recourse to an
explanation made available just when the need of it began to be felt. He
saw bright lines, not merely pushed aside from their normal places by a
barely perceptible amount, but bent, torn, broken, as if by the stress
of some tremendous violence. These remarkable appearances were quite
simply interpreted as the effects of movements varying in amount and
direction in the different parts of the extensive mass of incandescent
vapours falling within a single field of view. Very commonly they are of
a cyclonic character. The opposite distortions of the same coloured rays
betray the fury of "counter-gales" rushing along at the rate of 120
miles a second; while their undisturbed sections prove the persistence
of a "heart of peace" in the midst of that unimaginable fiery whirlwind.
Velocities up to 250 _miles a second_, or 15,000 times that of an
express train at the top of its speed, were thus observed by Young
during his trip to Mount Sherman, August 2, 1872; and these were
actually doubled in an extraordinary outburst observed by Father Jules
Fényi, on June 17, 1891, at the Haynald Observatory in Hungary, as well
as by M. Trouvelot at Meudon.[642]
Motions ascertainable in this way near the limb are, of course,
horizontal as regards the sun's surface; the analogies they present
might, accordingly, be styled _meteorological_ rather than _volcanic_.
But vertical displacements on a scale no less stupendous can also be
shown to exist. Observations of the spectra of spots centrally situated
(where motions in the line of sight are vertical) disclose the progress
of violent uprushes and downrushes of ignited gases, for the most part
in the penumbral or outlying districts. They appear to be occasioned by
fitful and irregular disturbances, and have none of the systematic
quality which would be required for the elucidation of sun-spot
theories. Indeed, they almost certainly take place at a great height
above the actual openings in the photosphere.
As to vertical motions above the limb, on the other hand, we have direct
visual evidence of a truly amazing kind. The projected glowing matter
has, by the aid of the spectroscope, been watched in its ascent. On
September 7, 1871, Young examined at noon a vast hydrogen cloud 100,000
miles long, as it showed to the eye, and 54,000 high. It floated
tranquilly above the chromosphere at an elevation of some 15,000 miles,
and was connected with it by three or four upright columns, presenting
the not uncommon aspect compared by Lockyer to that of a grove of
banyans. Called away for a few minutes at 12.30, on returning at 12.55
the observer found--
"That in the meantime the whole thing had been literally blown to shreds
by some inconceivable uprush from beneath. In place of the quiet cloud I
had left, the air, if I may use the expression, was filled with flying
débris--a mass of detached, vertical, fusiform filaments, each from 10"
to 30" long by 2" or 3" wide,[643] brighter and closer together where
the pillars had formerly stood, and rapidly ascending. They rose, with a
velocity estimated at 166 miles a second, to fully 200,000 miles above
the sun's surface, then gradually faded away like a dissolving cloud,
and at 1.15 only a few filmy wisps, with some brighter streamers low
down near the photosphere, remained to mark the place."[644]
A velocity of projection of _at least_ 500 miles per second was, by
Proctor's[645] calculation, required to account for this extraordinary
display, to which the earth immediately responded by a magnetic
disturbance, and a fine aurora. It has proved by no means an isolated
occurrence. Young saw its main features repeated, October 7, 1881,[646]
on a still vaster scale; for the exploded prominence attained, this
time, an altitude of 350,000 miles--the highest yet chronicled. Lockyer,
moreover, has seen a prominence 40,000 miles high shattered in ten
minutes; while uprushes have been witnessed by Respighi, of which the
initial velocities were judged by him to be 400 or 500 miles a second.
When it is remembered that a body starting from the sun's surface at the
rate of 383 miles a second would, if it encountered no resistance,
escape for ever from his control, it is obvious that we have, in the
enormous forces of eruption or repulsion manifested in the outbursts
just described, the means of accounting for the vast diffusion of matter
in the solar neighbourhood. Nor is it possible to explain them away, as
Cornu,[647] Faye,[648] and others have sought to do, by substituting for
the rush of matter in motion, progressive illumination through electric
discharges, chemical processes,[649] or even through the mere reheating
of gases cooled by expansion.[650] All the appearances are against such
evasions of the difficulty presented by velocities stigmatised as
"fabulous" and "improbable," but which, there is the strongest reason to
believe, really exist.
On the 12th of December, 1878, Sir Norman Lockyer formally expounded
before the Royal Society his hypothesis of the compound nature of the
"chemical elements."[651] An hypothesis, it is true, over and over again
propounded from the simply terrestrial point of view. What was novel was
the supra-terrestrial evidence adduced in its support; and even this had
been, in a general and speculative way, anticipated by Professor F. W.
Clarke of Washington.[652] Lockyer had been led to his conclusion along
several converging lines of research. In a letter to M. Dumas, dated
December 3, 1873, he had sketched out the successive stages of
"celestial dissociation" which he conceived to be represented in the sun
and stars. The absence from the solar spectrum of metalloidal absorption
he explained by the separation, in the fierce solar furnace, of such
substances as oxygen, nitrogen, sulphur, and chlorine, into simpler
constituents possessing unknown spectra; while metals were at that time
still admitted to be capable of existing there in a state of integrity.
Three years later he shifted his position onward. He announced, as the
result of a comparative study of the Fraunhofer and electric-arc spectra
of calcium, that the "molecular grouping" of that metal, which at low
temperatures gives a spectrum with its chief line in the blue, is nearly
broken up in the sun into another or others with lines in the
violet.[653] This came to be regarded by him as "a truly typical
case."[654]
During four years (1875-78 inclusive) this diligent observer was engaged
in mapping a section of the more refrangible part of the solar spectrum
(wave-lengths 3,800-4,000) on a scale of magnitude such that, if
completed down to the infra-red, its length would have been about _half
a furlong_. The attendant laborious investigation, by the aid of
photography, of metallic spectra, seemed to indicate the existence of
what he called "basic lines." These held their ground persistently in
the spectra of two or more metals after all possible "impurities" had
been eliminated, and were therefore held to attest the presence of a
common substratum of matter in a simpler state of aggregation than any
with which we are ordinarily acquainted.
Later inquiries have shown, however, that between the spectral lines of
different substances there are probably no _absolute_ coincidences.
"Basic" lines are really formed of doublets or triplets merged together
by insufficient dispersion. Of Thalèn's original list of seventy rays
common to several spectra,[655] very few resisted Thollon's and Young's
powerful spectroscopes; and the process of resolution was completed by
Rowland. Thus the argument from community of lines to community of
substance has virtually collapsed. It was replaced by one founded on
certain periodical changes on the spectra of sun-spots. They emerged
from a series of observations begun at South Kensington under Sir Norman
Lockyer's direction in 1879, and continued for fifteen years.[656]
The principle of the method employed is this. The whole range of
Fraunhofer lines is visible when the light from a spot is examined with
the spectroscope; but relatively few are widened. Now these widened
lines alone constitute (presumably) the true spot-spectrum; they, and
they alone, tell what kinds of vapour are thrust down into the strange
dusky pit of the nucleus, the unaffected lines taking their accustomed
origin from the over-lying strata of the normal solar atmosphere. Here
then we have the criterion that was wanted--the means of distinguishing,
spectroscopically and chemically, between the cavity and the absorbing
layers piled up above it. By its persistent employment some marked
peculiarities have been brought out, such as the unfamiliar character of
numerous lines in spot-spectra, especially at epochs of disturbance; and
the strange _individuality_ in the behaviour of every one of these
darkened and distended rays. Each seems to act on its own account; it
comports itself as if it were the sole representative of the substance
emitting it; its appearance is unconditioned by that of any of its
terrestrial companions in the same spectrum.
The most curious fact, however, elicited by these inquiries was that of
the attendance of chemical vicissitudes upon the advance of the sun-spot
period. As the maximum approached, unknown replaced known components of
the spot-spectra in a most pronounced and unmistakable way.[657] It
seemed as if the vapours emitting lines of iron, titanium, nickel, etc.,
had ceased to exist as such, and their room been taken by others, total
strangers in terrestrial laboratories. These were held by Lockyer to be
simply the finer constituents of their predecessors, dissociation having
been effected by the higher temperature ensuing upon increased solar
activity. But Father Cortie's supplementary investigations at
Stonyhurst[658] modified, while they in the main substantiated, the
South Kensington results. They showed that the substitution of unknown
for known lines characterizes disturbed spots, at all stages of the
solar cycle, so that no systematic course of chemical change can be said
to affect the sun as a whole. They showed further[659]--from evidence
independent of that obtained by Young in 1892[660]--the remarkable
conspicuousness in spot-spectra of vanadium lines excessively faint in
the Fraunhofer spectrum. Lockyer's "unknown lines" may probably thus be
accounted for. They represent absorption, not by new, but by scarce
elements, especially, Father Cortie thinks, those with atomic weights of
about 50. The circumstance of their development in solar commotions,
largely to the exclusion of iron, is none the less curious; but it
cannot be explained by any process of dissociation.
The theory has, however, to be considered under still another aspect. It
frequently happens that the contortions or displacements due to motion
are seen to affect a single line belonging to a particular substance,
while the other lines of _that same substance_ remain imperturbable.
Now, how is this most singular fact, which seems at first sight to imply
that a body may be at rest and in motion at one and the same instant, to
be accounted for? It is accounted for, on the present hypothesis, easily
enough, by supposing that the rays thus discrepant in their testimony,
do _not_ belong to one kind of matter, but to several, combined at
ordinary temperatures to form a body in appearance "elementary." Of
these different vapours, one or more may of course be rushing rapidly
towards or from the observer, while the others remain still; and since
the line of sight across the average prominence-region penetrates, at
the sun's edge, a depth of about 300,000 miles,[661] all the
incandescent materials separately occurring along which line are
projected into a single "flame" or "cloud," it will be perceived that
there is ample room for diversities of behaviour.
The alternative mode of escape from the perplexity consists in assuming
that the vapour in motion is rendered luminous under conditions which
reduce its spectrum to a few rays, the unaffected lines being derived
from a totally distinct mass of the same substance shining with its
ordinary emissions.[662] Thus, calcium can be rendered virtually
monochromatic by attenuation, and analogous cases are not rare.
Sir Norman Lockyer only asks us to believe that effects which follow
certain causes on the earth are carried a stage further in the sun,
where the same causes must be vastly intensified. We find that the
bodies we call "compound" split asunder at fixed degrees of heat
_within_ the range of our resources. Why should we hesitate to admit
that the bodies we call "simple" do likewise at degrees of heat
_without_ the range of our resources? The term "element" simply
expresses terrestrial incapability of reduction. That, in celestial
laboratories, the means and their effect here absent should be present,
would be an inference challenging, in itself, no expression of
incredulity.
There are indeed theoretical objections to it which, though probably not
insuperable, are unquestionably grave. Our seventy chemical "elements,"
for instance, are placed by the law of specific heats on a separate
footing from their known compounds. We are not, it is true, compelled by
it to believe their atoms to be really and absolutely such--to contain,
that is, the "irreducible minimum" of material substance; but we do
certainly gather from it that they are composed on a different principle
from the salts and oxides made and unmade at pleasure by chemists. Then
the multiplication of the species of matter with which Lockyer's results
menace us, is at first sight startling. They may lead, we are told, to
eventual unification, but the prospect appears remote. Their only
obvious outcome is the disruption into several constituents of each
terrestrial "element." The components of iron alone should be counted by
the dozen. And there are other metals, such as cerium, which, giving a
still more complex spectrum, would doubtless be still more numerously
resolved. Sir Norman Lockyer interprets the observed phenomena as
indicating the successive combinations, in varying proportions, of a
very few original ingredients;[663] but no definite sign of their
existence is perceptible; "protyle" seems likely long to evade
recognition; and the only intelligible underlying principle for the
reasonings employed--that of "one line, one element"--implies a throng
beyond counting of formative material units.
Thus, added complexity is substituted for that fundamental unity of
matter which has long formed the dream of speculators. And it is
extremely remarkable that Sir William Crookes, working along totally
different lines, has been led to analogous conclusions. To take only one
example. As the outcome of extremely delicate operations of sifting and
testing carried on for years, he finds that the metal yttrium splits up
into five, if not eight constituents.[664] Evidently, old notions are
doomed, nor are any preconceived ones likely to take their place. It
would seem, on the contrary, as if their complete reconstruction were at
hand. Subversive facts are steadily accumulating; the revolutionary
ideas springing from them tend, if we interpret them aright, towards the
substitution of electrical for chemical theories of matter. Dissociation
by the brute force of heat is already nearly superseded, in the thoughts
of physicists, by the more delicate process of "ionisation." Precisely
what this implies and involves we do not know; but the symptoms of its
occurrence are probably altogether different from those gathered by Sir
Norman Lockyer from the collation of celestial spectra.
A. J. Ångström of Upsala takes rank after Kirchhoff as a subordinate
founder, so to speak, of solar spectroscopy. His great map of the
"normal" solar spectrum[665] was published in 1868, two years before he
died. Robert Thalèn was his coadjutor in its execution, and the immense
labour which it cost was amply repaid by its eminent and lasting
usefulness. For more than a score of years it held its ground as the
universal standard of reference in all spectroscopic inquiries within
the range of the _visible_ emanations. Those that are invisible by
reason of the quickness of their vibrations were mapped by Dr. Henry
Draper, of New York, in 1873, and with superior accuracy by M. Cornu in
1881. The infra-red part of the spectrum, investigated by Langley,
Abney, and Knut Ångström, reaches perhaps no definite end. The
radiations oscillating too slowly to affect the eye as light may pass by
insensible gradations into the long Hertzian waves of electricity.[666]
Professor Rowland's photographic map of the solar spectrum, published in
1886, and in a second enlarged edition in 1889, opened fresh
possibilities for its study, from far down in the red to high up in the
ultra-violet, and the accompanying scale of absolute wave-lengths[667]
has been, with trifling modifications, universally adopted. His new
table of standard solar lines was published in 1893.[668] Through his
work, indeed, knowledge of the solar spectrum so far outstripped
knowledge of terrestrial spectra, that the recognition of their common
constituents was hampered by intolerable uncertainties. Thousands of the
solar lines charted with minute precision remained unidentified for want
of a corresponding precision in the registration of metallic lines.
Rowland himself, however, undertook to provide a remedy. Aided by Lewis
E. Jewell, he redetermined, at the Johns Hopkins University, the
wave-lengths of about 16,000 solar lines,[669] photographing for
comparison with them the spectra of all the known chemical elements
except gallium, of which he could procure no specimen. The labour of
collation was well advanced when he died at the age of fifty-two, April
16, 1901. Investigations of metallic arc-spectra have also been carried
out with signal success by Hasselberg,[670] Kayser and Runge, O.
Lohse,[671] and others.
Another condition _sine quâ non_ of progress in this department is the
separation of true solar lines from those produced by absorption in our
own atmosphere. And here little remains to be done. Thollon's great
Atlas[672] was designed for this purpose of discrimination. Each of its
thirty-three maps exhibits in quadruplicate a subdivision of the solar
spectrum under varied conditions of weather and zenith-distance.
Telluric effects are thus made easily legible, and they account wholly
for 866, partly for 246, out of a total of 3,200 lines. But the death of
the artist, April 8, 1887, unfortunately interrupted the half-finished
task of the last seven years of his life. A most satisfactory record,
meanwhile, of selective atmospheric action has been supplied by the
experiments and determinations of Janssen, Cornu and Egoroff, by Dr.
Becker's drawings,[673] and Mr. McClean's photographs of the analysed
light of the sun at high, low, and medium altitudes; and the autographic
pictures obtained by Mr. George Higgs, of Liverpool, of certain
rhythmical groups in the red, emerging with surprising strength near
sunset, excite general and well-deserved admiration.[674] The main
interest, however, of all these documents resides in the information
afforded by them regarding the chemistry of the sun.
The discovery that hydrogen exists in the atmosphere of the sun was made
by Ångström in 1862. His list of solar elements published in that
year,[675] the result of an investigation separate from, though
conducted on the same principle as Kirchhoff's, included the substance
which we now know to be predominant among them. Dr. Plücker of Bonn had
identified in 1859 the Fraunhofer line F with the green ray of hydrogen,
but drew no inference from his observation. The agreement was verified
by Ångström; two further coincidences were established; and in 1866 a
fourth hydrogen line in the extreme violet (named _h_) was detected in
the solar spectrum. With Thalèn, he besides added manganese, titanium,
and cobalt to the constituents of the sun enumerated by Kirchhoff, and
raised the number of identical rays in the solar and terrestrial spectra
of iron to no less than 460.[676]
Thus, when Sir Norman Lockyer entered on that branch of inquiry in 1872,
fourteen substances were recognised as common to the earth and sun.
Early in 1878 he was able to increase the list provisionally to
thirty-three,[677] all except hydrogen metals. This rapid success was
due to his adoption of the test of _length_ in lieu of that of
_strength_ in the comparison of lines. He measured their relative
significance, in other words, rather by their persistence through a wide
range of temperature, than by their brilliancy at any one temperature.
The distinction was easily drawn. Photographs of the electric arc, in
which any given metal had been volatilised, showed some of the rays
emitted by it stretching across the axis of the light to a considerable
distance on either side, while many others clung more or less closely to
its central hottest core. The former "long lines," regarded as certainly
representative, were those primarily sought in the solar spectrum; while
the attendant "short lines," often, in point of fact, due to foreign
admixtures, were set aside as likely to be misleading.[678] The
criterion is a valuable one, and its employment has greatly helped to
quicken the progress of solar chemistry.
Carbon was the first non-metallic element discovered in the sun. Messrs.
Trowbridge and Hutchins of Harvard College concluded in 1887,[679] on
the ground of certain spectral coincidences, that this protean substance
is vaporised in the solar atmosphere at a temperature approximately that
of the voltaic arc. Partial evidence to the same effect had earlier been
alleged by Lockyer, as well as by Liveing and Dewar; and the case was
rendered tolerably complete by photographs taken by Kayser and Runge in
1889.[680] It was by Professor Rowland shown to be irresistible. Two
hundred carbon-lines were, through his comparisons, sifted out from
sunlight, and it contains others significant of the presence of
silicon--a related substance, and one as important to rock-building on
the earth, as carbon is to the maintenance of life. The general result
of Rowland's labours was the establishment among solar materials, not
only of these two out of the fourteen metalloids, or non-metallic
substances, but of thirty-three metals, including silver and tin. Gold,
mercury, bismuth, antimony, and arsenic were discarded from the
catalogue; platinum and uranium, with six other metals, remained
doubtful; while iron was recorded as crowding the spectrum with over two
thousand obscure rays.[681] Gallium-absorption was detected in it by
Hartley and Ramage in 1889.[682]
Dr. Henry Draper[683] announced, in 1877, his imagined discovery, in the
solar spectrum, of eighteen especially brilliant spaces corresponding to
oxygen-emissions. But the agreement proved, when put to the test of very
high dispersion, to be wholly illusory.[684] Nor has it yet been found
possible to identify, in analysed sunlight, any significant _bright_
beams.[685]
The book of solar chemistry must be read in characters exclusively of
absorption. Nevertheless, the whole truth is unlikely to be written
there. That a substance displays none of its distinctive beams in the
spectrum of the sun or of a star, affords scarcely a presumption against
its presence. For it may be situated below the level where absorption
occurs, or under a pressure such as to efface lines by widening and
weakening them; it may be at a temperature so high that it gives out
more light than it takes up, and yet its incandescence may be masked by
the absorption of other bodies; finally, it may just balance absorption
by emission, with the result of complete spectral neutrality. An
instructive example is that of the chromospheric element helium. Father
Secchi remarked in 1868[686] that there is no dark line in the solar
spectrum matching its light; and his observation has been fully
confirmed.[687] Helium-absorption is, however, occasionally noticed in
the penumbræ of spots.[688]
Our terrestrial vital element might then easily subsist unrecognisably
in the sun. The inner organisation of the oxygen molecule is a
considerably _plastic_ one. It is readily modified by heat, and these
modifications are reflected in its varying modes of radiating light. Dr.
Schuster enumerated in 1879[689] four distinct oxygen spectra,
corresponding to various stages of temperature, or phases of electrical
excitement; and a fifth has been added by M. Egoroff's discovery in
1883[690] that certain well-known groups of dark lines in the red end of
the solar spectrum (Fraunhofer's A and B) are due to absorption by the
cool oxygen of our air. These persist down to the lowest temperatures,
and even survive a change of state. They are produced essentially the
same by liquid, as by aërial oxygen.[691]
It seemed, however, possible to M. Janssen that these bands owned a
joint solar and terrestrial origin. Oxygen in a fit condition to produce
them might, he considered, exist in the outer atmosphere of the sun; and
he resolved to decide the point. No one could bring more skill and
experience to bear upon it than he.[692] By observations on the summit
of the Faulhorn, as well as by direct experiment, he demonstrated,
nearly thirty years ago, the leading part played by water-vapour in
generating the atmospheric spectrum; and he had recourse to similar
means for appraising the share in it assignable to oxygen. An electric
beam, transmitted from the Eiffel Tower to Meudon in the summer of 1888,
having passed through a weight of oxygen about equal to that piled above
the surface of the earth, showed the groups A and B just as they appear
in the high-sun spectrum.[693] Atmospheric action is then adequate to
produce them. But M. Janssen desired to prove, in addition, that they
diminish proportionately to its amount. His ascent of Mont Blanc[694] in
1890 was undertaken with this object. It was perfectly successful. In
the solar spectrum, examined from that eminence, oxygen-absorption was
so much enfeebled as to leave no possible doubt of its purely telluric
origin. Under another form, nevertheless, it has been detected as
indubitably solar. A triplet of dark lines low down in the red,
photographed from the sun by Higgs and McClean, was clearly identified
by Runge and Paschen in 1896[695] with the fundamental group of an
oxygen series, first seen by Piazzi Smyth in the spectrum of a
vacuum-tube in 1883.[696] The _pabulum vitæ_ of our earth is then to
some slight extent effective in arresting transmitted sunlight, and
oxygen must be classed as a solar element.
The rays of the sun, besides being stopped selectively in our
atmosphere, suffer also a marked general absorption. This tells chiefly
upon the shortest wave-lengths; the ultra-violet spectrum is in fact
closed, as if by the interposition of an opaque screen. Nor does the
screen appear very sensibly less opaque from an elevation of 10,000
feet. Dr. Simony's spectral photographs, taken on the Peak of
Teneriffe,[697] extended but slightly further up than M. Cornu's, taken
in the valley of the Loire. Could the veil be withdrawn, some
indications as to the originating temperature of the solar spectrum
might be gathered from its range, since the proportion of quick
vibrations given out by a glowing body grows with the intensity of its
incandescence. And this brings us to the subject of our next Chapter.
FOOTNOTES:
[Footnote 596: _Phil. Mag._, vol. xlii., p. 380, 1871.]
[Footnote 597: _Astr. Nach._, No. 3,053, _Amer. Jour._, vol. xlii., p.
162; Deslandres, _Comptes Rendus_, t. cxiii., p. 307.]
[Footnote 598: _Proc. Roy. Society_, vol. lxi., p. 433.]
[Footnote 599: _Phil. Mag._, vol. xlii., p. 377.]
[Footnote 600: Frost-Scheiner, _Astr. Spectroscopy_, pp. 184, 423.]
[Footnote 601: _Proc. Roy. Soc._, vol. xvii., p. 302.]
[Footnote 602: _Astr. Nach._, No. 1,769.]
[Footnote 603: _Am. Jour. of Science_, vol. xv., p. 85.]
[Footnote 604: _Journ. Franklin Institute_, vol. xl., p. 232_a_.]
[Footnote 605: _Pogg. Annalen_, Bd. cxlvi., p. 475; _Astr. Nach._, No.
3,014.]
[Footnote 606: _Astr. Nach._, Nos. 3,006, 3,037.]
[Footnote 607: This device was suggested by Janssen in 1869.]
[Footnote 608: _Astr. and Astrophysics_, vol. xi., pp. 70, 407.]
[Footnote 609: _Astr. and Astrophysics_, vol. xi., p. 604.]
[Footnote 610: _Comptes Rendus_, t. cxiii., p. 307.]
[Footnote 611: _Astr. and Astrophysics_, vol. xi., p. 50.]
[Footnote 612: _Ibid._, pp. 60, 314.]
[Footnote 613: Wiedemann's _Annalen der Physik_, Bd. xxv., p. 80.]
[Footnote 614: Evershed, _Knowledge_, vol. xxi., p. 133.]
[Footnote 615: Secchi, _Le Soleil_, t. ii., p. 294.]
[Footnote 616: Lockyer, _Chemistry of the Sun_, p. 418.]
[Footnote 617: _L'Astronomie_, August, 1884, p. 292 (Riccò); see also
Evershed, _Jour. British Astr. Ass._, vol. ii., p. 174.]
[Footnote 618: Averaging about 100 miles across and 300 high. _Le
Soleil_, t. ii., p. 35.]
[Footnote 619: _The Sun_, p. 192.]
[Footnote 620: _Astr. Nach._, No. 1,854.]
[Footnote 621: _Mem. degli Spettroscopisti Italiani_, t. v., p. 4;
Secchi, _ibid._, t. vi., p. 56.]
[Footnote 622: Its non-atmospheric character was early defined by
Proctor, _Month. Not._, vol. xxxi., p. 196.]
[Footnote 623: _Astroph. Jour._, vol. vi., p. 412.]
[Footnote 624: _Ibid._, vol. xi., p. 165.]
[Footnote 625: _Ibid._, p. 243.]
[Footnote 626: _Sun's Place in Nature_, pp. 111, 288.]
[Footnote 627: _Abh. d. Kön. Böhm Ges. d. Wiss._, Bd. ii., 1841-42, p.
467.]
[Footnote 628: In a paper read before the Société Philomathique de
Paris, December 23, 1848, and first published _in extenso_ in _Ann. de
Chim. et de Phys._, t. xix., p. 211 (1870). Hippolyte Fizeau died in
September, 1896.]
[Footnote 629: _Astr. Nach._, No. 1,772.]
[Footnote 630: _Ibid._, No. 1,864.]
[Footnote 631: A. Cornu, _Sur la Méthode Doppler-Fizeau_, p. D. 23.]
[Footnote 632: _Am. Jour. of Sc._, vol. xii., p. 321.]
[Footnote 633: _Ibid._, vol. xiv., p. 140.]
[Footnote 634: _Bull. Astronom._, February, 1884, p. 77.]
[Footnote 635: _Comptes Rendus_, t. xci., p. 368.]
[Footnote 636: _Month. Not._, vol. xliv., p. 170.]
[Footnote 637: See _ante_, p. 147.]
[Footnote 638: _Recherches sur la Rotation du Soleil_, Upsal, 1891.]
[Footnote 639: Harzer, _Astr. Nach._, No. 3,026; Stratonoff, _Ibid._,
No. 3,344.]
[Footnote 640: _Publ. Astr. Pacific Soc._, vol. ii., p. 193.]
[Footnote 641: _Proc. Roy. Society_, vols. xvii., p. 415; xviii., p.
120.]
[Footnote 642: _Comptes Rendus_, t. cxii., p. 1421; t. cxiii., p. 310.]
[Footnote 643: At the sun's distance, one second of arc represents about
450 miles.]
[Footnote 644: _Amer. Jour. of Sc._, vol. ii., p. 468, 1871.]
[Footnote 645: _Month. Not._, vol. xxxii., p. 51.]
[Footnote 646: _Nature_, vol. xxiii., p. 281.]
[Footnote 647: _Comptes Rendus_, t. lxxxvii., p. 532.]
[Footnote 648: _Ibid._, t. xcvi., p. 359.]
[Footnote 649: A. Brester, _Théorie du Soleil_, p. 66.]
[Footnote 650: Such prominences as have been seen to grow by the spread
of incandescence are of the quiescent kind, and present no deceptive
appearance of violent motion.]
[Footnote 651: _Proc. Roy. Soc._, vol. xxviii., p. 157.]
[Footnote 652: "Evolution and the Spectroscope," _Pop. Science Monthly_,
January, 1873.]
[Footnote 653: _Proc. Roy. Soc._, vol. xxiv., p. 353. These are the H
and K of prominences. H. W. Vogel discovered in 1879 a hydrogen-line
nearly coincident with H (_Monatsb. Preuss. Ak._, February, 1879, p.
118).]
[Footnote 654: _Proc. Roy. Soc._, vol. xxviii., p. 444.]
[Footnote 655: Many of these were referred by Lockyer himself, who first
sifted the matter, to traces of the metals concerned.]
[Footnote 656: _Chemistry of the Sun_, p. 312; _Proc. Roy. Society_,
vol. lvii., p. 199.]
[Footnote 657: _Lockyer's Chemistry of the Sun_, p. 324.]
[Footnote 658: _Month. Not._, vol. li., p. 76.]
[Footnote 659: _Ibid._, vol. lviii., p. 370.]
[Footnote 660: _Astr. and Astrophysics_, vol. xi., p. 615.]
[Footnote 661: Thollon's estimate (_Comptes Rendus_, t. xcvii., p. 902)
of 300,000 _kilometres_, seems considerably too low. Limiting the
"average prominence region" to a shell 54,000 miles deep (2' of arc as
seen from the earth), the visual line will, at mid-height (27,000 miles
from the sun's surface), travel through (in round numbers) 320,000 miles
of that region.]
[Footnote 662: Liveing and Dewar, _Phil. Mag._, vol. xvi. (5th ser.), p.
407.]
[Footnote 663: _Chemistry of the Sun_, p. 260.]
[Footnote 664: _Nature_, October 14, 1886.]
[Footnote 665: The normal spectrum is that depending exclusively upon
wave-length--the fundamental constant given by nature as regards light.
It is obtained by the interference of rays, in the manner first
exemplified by Fraunhofer, and affords the only unvarying standard for
measurement. In the refraction spectrum (upon which Kirchhoff's map was
founded), the relative positions of the lines vary with the material of
the prisms.]
[Footnote 666: Scheiner, _Die Spectralanalyse der Gestirne_, p. 168.]
[Footnote 667: _Phil. Mag._, vol. xxvii., p. 479.]
[Footnote 668: _Astr. and Astrophysics_, vol. xii., p. 321;
Frost-Scheiner, _Astr. Spectr._, p. 363.]
[Footnote 669: Published in _Astroph. Jour._, vols. i. to vi.]
[Footnote 670: _Astr. and Astrophysics_, vol. xi., p. 793.]
[Footnote 671: _Astroph. Jour._, vol. vi., p. 95.]
[Footnote 672: _Annales de l'Observatoire de Nice_, t. iii., 1890.]
[Footnote 673: _Trans. Royal Society of Edinburgh_, vol. xxxvi., p. 99.]
[Footnote 674: Rev. A. L. Cortie, _Astr. and Astrophysics_, vol. xi., p.
401. Specimens of his photographs were given by Ranyard in _Knowledge_,
vol. xiii., p. 212.]
[Footnote 675: _Ann. d. Phys._, Bd. cxvii., p. 296.]
[Footnote 676: _Comptes Rendus_, t. lxiii., p. 647.]
[Footnote 677: _Ibid._, t. lxxxvi., p. 317. Some half dozen of these
identifications have proved fallacious.]
[Footnote 678: _Chemistry of the Sun_, p. 143.]
[Footnote 679: _Amer. Jour. of Science_, vol. xxxiv., p. 348.]
[Footnote 680: _Berlin Abhandlungen_, 1889.]
[Footnote 681: _Amer. Jour. of Science_, vol. xli., p. 243. See
Appendix, Table II.]
[Footnote 682: _Astrophy. Jour._, vol. ix., p. 219; Fowler, _Knowledge_,
vol. xxiii., p. 11.]
[Footnote 683: _Amer. Jour, of Science_, vol. xiv., p. 89; _Nature_,
vol. xvi., p. 364; _Month. Not._, vol. xxxix., p. 440.]
[Footnote 684: _Month. Not._, vol. xxxviii., p. 473; Trowbridge and
Hutchins, _Amer. Jour. of Science_, vol. xxxiv., p. 263.]
[Footnote 685: Scheiner, _Die Spectralanalyse_, p. 180.]
[Footnote 686: _Comptes Rendus_, t. lxvii., p. 1123.]
[Footnote 687: Rev. A. L. Cortie, _Month. Not._, vol. li., p. 18.]
[Footnote 688: Young, _The Sun_, p. 135; Hale, _Astr. and Astrophysics_,
vol. xi., p. 312 Buss, _Jour. Brit. Astr. Ass._, vol. ix., p. 253.]
[Footnote 689: _Phil. Trans._, vol. clxx., p. 46.]
[Footnote 690: _Comptes Rendus_, t. xcvii., p. 555; t. ci., p. 1145.]
[Footnote 691: Liveing and Dewar, _Astr. and Astrophysics_, vol. xi., p.
705.]
[Footnote 692: _Comptes Rendus_, t. lx., p. 213; t. lxiii., p. 289.]
[Footnote 693: _Ibid._, t. cviii., p. 1035.]
[Footnote 694: _Ibid._, t. cxi., p. 431.]
[Footnote 695: _Astroph. Jour._, vols. iv., p. 317; vi., p. 426.]
[Footnote 696: _Trans. Roy. Soc. Edin._, vol. xxxii., p. 452.]
[Footnote 697: _Comptes Rendus_, t. cxi., p. 941; Huggins, _Proc. Roy.
Soc._, vol. xlvi., p. 168.]
CHAPTER V
_TEMPERATURE OF THE SUN_
Newton was the first who attempted to measure the quantity of heat
received by the earth from the sun. His object in making the experiment
was to ascertain the temperature encountered by the comet of 1680 at its
passage through perihelion. He found it, by multiplying the observed
heating effects of direct sunshine according to the familiar rule of the
"inverse squares of the distances," to be about 2,000 times that of
red-hot iron.[698]
Determinations of the sun's thermal power, made with some scientific
exactness, date, however, from 1837. A few days previous to the
beginning of that year, Herschel began observing at the Cape of Good
Hope with an "actinometer," and obtained results agreeing quite
satisfactorily with those derived by Pouillet from experiments made in
France some months later with a "pyrheliometer."[699] Pouillet found
that the vertical rays of the sun falling on each square centimetre of
the earth's surface are competent (apart from atmospheric absorption) to
raise the temperature of 1·7633 grammes of water one degree Centigrade
per minute. This number (1·7633) he called the "solar constant"; and the
unit of heat chosen is known as the "calorie." Hence it was computed
that the total amount of solar heat received during a year would suffice
to melt a layer of ice covering the entire earth to a depth of 30·89
metres, or 100 feet; while the heat emitted would melt, at the sun's
surface, a stratum 11·80 metres thick each minute. A careful series of
observations showed that nearly half the heat incident upon our
atmosphere is stopped in its passage through it.
Herschel got somewhat larger figures, though he assigned only a third as
the spoil of the air. Taking a mean between his own and Pouillet's, he
calculated that the ordinary expenditure of the sun per minute would
have power to melt a cylinder of ice 184 feet in diameter, reaching from
his surface to that of Alpha Centauri; or, putting it otherwise,
that an ice-rod 45·3 miles across, continually darted into the sun with
the velocity of light, would scarcely consume, in dissolving, the
thermal supplies now poured abroad into space.[700] It is nearly certain
that this estimate should be increased by about two-thirds in order to
bring it up to the truth.
Nothing would, at first sight, appear simpler than to pass from a
knowledge of solar emission--a strictly measurable quantity--to a
knowledge of the solar temperature; this being defined as the
temperature to which a surface thickly coated with lamp-black (that is,
of standard radiating power) should be raised to enable it to send us,
from the sun's distance, the amount of heat actually received from the
sun. Sir John Herschel showed that heat-rays at the sun's surface must
be 92,000 times as dense as when they reach the earth; but it by no
means follows that either the surface emitting, or a body absorbing
those heat-rays must be 92,000 times hotter than a body exposed here to
the full power of the sun. The reason is, that the rate of
emission--consequently the rate of absorption, which is its
correlative--increases very much faster than the temperature. In other
words, a body radiates or cools at a continually accelerated pace as it
becomes more and more intensely heated above its surroundings.
Newton, however, took it for granted that radiation and temperature
advance _pari passu_--that you have only to ascertain the quantity of
heat received from, and the distance of a remote body in order to know
how hot it is.[701] And the validity of this principle, known as
"Newton's Law" of cooling, was never questioned until De la Roche
pointed out, in 1812,[702] that it was approximately true only over a
low range of temperature; while five years later, Dulong and Petit
generalised experimental results into the rule, that while temperature
grows by arithmetical, radiation increases by geometrical
progression.[703] Adopting this formula, Pouillet derived from his
observations on solar heat a solar temperature of somewhere between
1,461° and 1,761° C. Now, the higher of these points--which is nearly
that of melting platinum--is undoubtedly surpassed at the focus of
certain burning-glasses which have been constructed of such power as
virtually to bring objects placed there within a quarter of a million of
miles of the photosphere. In the rays thus concentrated, platinum and
diamond become rapidly vaporised, notwithstanding the great loss of heat
by absorption, first in passing through the air, and again in traversing
the lens. Pouillet's maximum is then manifestly too low, since it
involves the absurdity of supposing a radiating mass capable of heating
a distant body more than it is itself heated.
Less demonstrably, but scarcely less surely, Mr. J. J. Waterston, who
attacked the problem in 1860, erred in the opposite direction. Working
up, on Newton's principle, data collected by himself in India and at
Edinburgh, he got for the "potential temperature" of the sun 12,880,000°
Fahr.,[704] equivalent to 7,156,000° C. The phrase _potential
temperature_ (for which Violle substituted, in 1876, _effective
temperature_) was designed to express the accumulation in a single
surface, postulated for the sake of simplicity, of the radiations not
improbably received from a multitude of separate solar layers
reinforcing each other; and might thus (it was explained) be
considerably higher than the _actual_ temperature of any one stratum.
At Rome, in 1861, Father Secchi repeated Waterston's experiments, and
reaffirmed his conclusion;[705] while Soret's observations, made on the
summit of Mont Blanc in 1867,[706] furnished him with materials for a
fresh and even higher estimate of ten million degrees Centigrade.[707]
Yet from the very same data, substituting Dulong and Petit's for
Newton's law, Vicaire deduced in 1872 a _provisional_ solar temperature
of 1,398°.[708] This is below that at which iron melts, and we know that
iron-vapour exists high up in the sun's atmosphere. The matter was taken
into consideration on the other side of the Atlantic by Ericsson in
1871. He attempted to re-establish the shaken credit of Newton's
principle, and arrived, by its means, at a temperature of 4,000,000°
Fahrenheit.[709] Subsequently, an "underrated computation," based upon
observation of the quantity of heat received by his "sun motor," gave
him 3,000,000°. And the result, as he insisted, followed inevitably from
the principle that the temperature produced by radiant heat is
proportional to its density, or inversely as its diffusion.[710] The
principle, however, is demonstrably unsound.
In 1876 the sun's temperature was proposed as the subject of a prize by
the Paris Academy of Sciences; but although the essay of M. Jules Violle
was crowned, the problem was declared to remain unsolved. Violle (who
adhered to Dulong and Petit's formula) arrived at an _effective_
temperature of 1,500° C., but considered that it might _actually_ reach
2,500° C., if the emissive power of the photospheric clouds fell far
short (as seemed probable) of the lamp-black standard.[711] Experiments
made in April and May, 1881, giving a somewhat higher result, he raised
this figure to 3,000° C.[712]
Appraisements so outrageously discordant as those of Waterston, Secchi,
and Ericsson on the one hand, and those of the French _savants_ on the
other, served only to show that all were based upon a vicious principle.
Professor F. Rosetti,[713] accordingly, of the Paduan University, at
last perceived the necessity for getting out of the groove of "laws"
plainly in contradiction with facts. The temperature, for instance, of
the oxy-hydrogen flame was fixed by Bunsen at 2,800° C.--an estimate
certainly not very far from the truth. But if the two systems of
measurement applied to the sun be used to determine the heat of a solid
body rendered incandescent in this flame, it comes out, by Newton's mode
of calculation, 45,000° C.; by Dulong and Petit's, 870° C.[714] Both,
then, are justly discarded, the first as convicted of exaggeration, the
second of undervaluation. The formula substituted by Rosetti in 1878 was
tested successfully up to 2,000° C.; but since, like its predecessors,
it was a purely empirical rule, guaranteed by no principle, and hence
not to be trusted out of sight, it was, like them, liable to break down
at still higher elevations. Radiation by this new prescription increases
as the _square_ of the _absolute_ temperature--that is, of the number of
degrees counted from the "absolute zero" of -273° C. Its employment gave
for the sun's radiating surface an effective temperature of 20,380° C.
(including a supposed loss of one-half in the solar atmosphere); and
setting a probable deficiency in emission (as compared with lamp-black)
against a probable mutual reinforcement of superposed strata, Professor
Rosetti considered "effective" as nearly equivalent to "actual"
temperature. A "law of cooling," proposed by M. Stefan at Vienna in
1879,[715] was shown by Boltzmann, many years later, to have a certain
theoretical validity.[716] It is that emission grows as the fourth power
of absolute temperature. Hence the temperature of the photosphere would
be proportional to the square root of the square root of its heating
effects at a distance, and appeared, by Stefan's calculations from
Violle's measures of solar radiative intensity, to be just 6,000° C.;
while M. H. Le Chatelier[717] derived 7,600° from a formula, conveying
an intricate and unaccountable relation between the temperature of an
incandescent body and the intensity of its red radiations.
From a series of experiments carefully conducted at Daramona, Ireland,
with a delicate thermal balance, of the kind invented by Boys and
designated a "radio-micrometer," Messrs. Wilson and Gray arrived in
1893, with the aid of Stefan's Law, at a photospheric temperature of
7,400° C.,[718] reduced by the first-named investigator in 1901 to
6,590°.[719] Dr. Paschen, of Hanover, on the other hand, ascribed to the
sun a temperature of 5,000° from comparisons between solar radiative
intensity and that of glowing platinum;[720] while F. W. Very showed in
1895[721] that a minimum value of 20,000° C. for the same datum resulted
from Paschen's formula connecting temperature with the position of
maximum spectral energy.
A new line of inquiry was struck out by Zöllner in 1870. Instead of
tracking the solar radiations backward with the dubious guide of
empirical formulæ, he investigated their intensity at their source. He
showed[722] that, taking prominences to be simple effects of the escape
of powerfully compressed gases, it was possible, from the known
mechanical laws of heat and gaseous constitution, to deduce minimum
values for the temperatures prevailing in the area of their development.
These came out 27,700° C. for the strata lying immediately above, and
68,400° C. for the strata lying immediately below the photosphere, the
former being regarded as the region _into_ which, and the latter as the
region _from_ which the eruptions took place. In this calculation, no
prominences exceeding 40,000 miles (1·5') in height were included. But
in 1884, G. A. Hirn of Colmar, having regard to the enormous velocities
of projection observed in the interim, fixed two million degrees
Centigrade as the lowest _internal_ temperature by which they could be
accounted for; although admitting the photospheric condensations to be
incompatible with a higher _external_ temperature than 50,000° to
100,000° C.[723]
This method of going straight to the sun itself, observing what goes on
there, and inferring conditions, has much to recommend it; but its
profitable use demands knowledge we are still very far from possessing.
We are quite ignorant, for instance, of the actual circumstances
attending the birth of the solar flames. The assumption that they are
nothing but phenomena of elasticity is a purely gratuitous one.
Spectroscopic indications, again, give hope of eventually affording a
fixed point of comparison with terrestrial heat sources; but their
interpretation is still beset with uncertainties; nor can, indeed, the
expression of transcendental temperatures in degrees of impossible
thermometers be, at the best, other than a futile attempt to convey
notions respecting a state of things altogether outside the range of our
experience.
A more tangible, as well as a less disputable proof of solar radiative
intensity than any mere estimates of temperature, was provided in some
experiments made by Professor Langley in 1878.[724] Using means of
unquestioned validity, he found the sun's disc to radiate 87 times as
much heat, and 5,300 times as much light as an equal area of metal in a
Bessemer converter after the air-blast had continued about twenty
minutes. The brilliancy of the incandescent steel, nevertheless, was so
blinding, that melted iron, flowing in a dazzling white-hot stream into
the crucible, showed "deep brown by comparison, presenting a contrast
like that of dark coffee poured into a white cup." Its temperature was
estimated (not quite securely)[725] at about 2,000° C.; and no
allowances were made, in computing relative intensities, for atmospheric
ravages on sunlight, for the extra impediments to its passage presented
by the smoke-laden air of Pittsburgh, or for the obliquity of its
incidence. Thus, a very large balance of advantage lay on the side of
the metal.
A further element of uncertainty in estimating the intrinsic strength of
the sun's rays has still to be considered. From the time that his disc
first began to be studied with the telescope, it was perceived to be
less brilliant near the edges. Lucas Valerius, of the Lyncean Academy,
seems to have been the first to note this fact, which, strangely enough,
was denied by Galileo in a letter to Prince Cesi of January 25,
1613.[726] Father Scheiner, however, fully admitted it, and devoted some
columns of his bulky tome to the attempt to find its appropriate
explanation.[727] In 1729 Bouguer measured, with much accuracy, the
amount of this darkening; and from his data, Laplace, adopting a
principle of emission now known to be erroneous, concluded that the sun
loses eleven-twelfths of his light through absorption in his own
atmosphere.[728] The real existence of this atmosphere, which is totally
distinct from the beds of ignited vapours producing the Fraunhofer
lines, is not open to doubt, although its nature is still a matter of
conjecture. The separate effects of its action on luminous, thermal, and
chemical rays were carefully studied by Father Secchi, who in 1870[729]
inferred the total absorption to be 88/100 of all radiations taken
together, and added the important observation that the light from the
limb is no longer white, but reddish-brown. Absorptive effects were thus
seen to be unequally distributed; and they could evidently be studied to
advantage only by taking the various rays of the spectrum separately,
and finding out how much each had suffered in transmission.
This was done by H. C. Vogel in 1877.[730] Using a polarising
photometer, he found that only 13 per cent. of the violet rays escape at
the edge of the solar disc, 16 of the blue and green, 25 of the yellow,
and 30 per cent. of the red. Midway between centre and limb, 88·7 of
violet light and 96·7 of red penetrate the absorbing envelope, the
abolition of which would increase the intensity of the sun's visible
spectrum above two and a half times in the most, and once and a half
times in the least refrangible parts. The nucleus of a small spot was
ascertained to be of the same luminous intensity as a portion of the
unbroken surface about two and a half minutes from the limb. These
experiments having been made during a spot-minimum when there is reason
to think that absorption is below its average strength, Vogel suggested
their repetition at a time of greater activity. They were extended to
the heat-rays by Edwin B. Frost. Detailed inquiries made at Potsdam in
1892[731] went to show that, were the sun's atmosphere removed, his
thermal power, as regards ourselves, would be increased 1·7 times. They
established, too, the practical uniformity in radiation of all parts of
his disc. A confirmatory result was obtained about the same time by
Wilson and Rambaut, who found that the unveiled sun would be once and a
half times hotter than the actual sun.[732]
Professor Langley, now of Washington, gave to measures of the kind a
refinement previously undreamt of. Reliable determinations of the
"energy" of the individual spectral rays were, for the first time,
rendered possible by his invention of the "bolometer" in 1880.[733] This
exquisitely sensitive instrument affords the means of measuring heat,
not directly, like the thermopile, but in its effects upon the
conduction of electricity. It represents, in the phrase of the inventor,
the finger laid upon the throttle-valve of a steam-engine. A minute
force becomes the modulator of a much greater force, and thus from
imperceptible becomes conspicuous. By locally raising the temperature of
an inconceivably fine strip of platinum serving as the conducting-wire
in a circuit, the flow of electricity is impeded at that point, and the
included galvanometer records a disturbance of the electrical flow.
Amounts of heat were thus detected in less than ten seconds, which,
expended during a thousand years on the melting of a kilogramme of ice,
would leave a part of the work still undone; and further improvements
rendered this marvellous instrument capable of thrilling to changes of
temperature falling short of one ten-millionth of a degree
Centigrade.[734]
The heat contained in the diffraction spectrum is, with equal
dispersions, barely one-tenth of that in the prismatic spectrum. It had,
accordingly, never previously been found possible to measure it in
detail--that is, ray by ray. But it is only from the diffraction, or
normal spectrum that any true idea can be gained as to the real
distribution of energy among the various constituents, visible and
invisible, of a sunbeam. The effect of passage through a prism is to
crowd together the red rays very much more than the blue. To this
prismatic distortion was owing the establishment of a pseudo-maximum of
heat in the infra-red, which disappeared when the natural arrangement by
wave-length was allowed free play. Langley's bolometer has shown that
the hottest part of the normal spectrum virtually coincides with its
most luminous part, both lying in the orange, close to the D-line.[735]
Thus the last shred of evidence in favour of the threefold division of
solar radiations vanished, and it became obvious that the varying
effects--thermal, luminous, or chemical--produced by them are due, not
to any distinction of quality in themselves, but to the different
properties of the substances they impinge upon. They are simply bearers
of energy, conveyed in shorter or longer vibrations; the result in each
separate case depending upon the capacity of the material particles
meeting them for taking up those shorter or longer vibrations, and
turning them variously to account in their inner economy.
A long series of experiments at Allegheny was completed in the summer of
1881 on the crest of Mount Whitney in the Sierra Nevada. Here, at an
elevation of 14,887 feet, in the driest and purest air, perhaps, in the
world, atmospheric absorptive inroads become less sensible, and the
indications of the bolometer, consequently, surer and stronger. An
enormous expansion was at once given to the invisible region in the
solar spectrum below the red. Captain Abney had got chemical effects
from undulations twelve ten-thousandths of a millimetre in length. These
were the longest recognised as, or indeed believed, on theoretical
grounds, to be capable of existing. Professor Langley now got heating
effects from rays of above twice that wave-length, his delicate thread
of platinum groping its way down nearly to thirty ten-thousandths of a
millimetre, or three "microns." The known extent of the solar spectrum
was thus at once more than doubled. Its visible portion covers a range
of about one octave; bolometric indications already in 1884 comprised
between three and four. The great importance of the newly explored
region appears from the fact that three-fourths of the entire energy of
sunlight reside in the infra-red, while scarcely more than one-hundredth
part of that amount is found in the better known ultra-violet
space.[736] These curious facts were reinforced, in 1886,[737] by
further particulars learned with the help of rock-salt lenses and
prisms, glass being impervious to very slow, as to very rapid
vibrations. Traces were thus detected of solar heat distributed into
bands of transmission alternating with bands of atmospheric absorption,
far beyond the measurable limit of 5·3 microns.
In 1894, Langley described at the Oxford Meeting of the British
Association[738] his new "bolographic" researches, in which the
sensitive plate was substituted for the eye in recording deflections of
the galvanometer responding to variations of invisible heat. Finally, in
1901,[739] he embodied in a splendid map of the infra-red spectrum 740
absorption-lines of determinate wave-lengths, ranging from 0·76 to 5·3
microns. Their chemical origin, indeed, remains almost entirely unknown,
no extensive investigations having yet been undertaken of the slower
vibrations distinctive of particular substances; but there is evidence
that seven of the nine great bands crossing the "new spectrum" (as
Langley calls it)[740] are telluric, and subject to seasonal change.
Here, then, he thought, might eventually be found a sure standing-ground
for vitally important previsions of famines, droughts, and
bonanza-crops.
Atmospheric absorption had never before been studied with such precision
as it was by Langley on Mount Whitney. Aided by simultaneous
observations from Lone Pine, at the foot of the Sierra, he was able to
calculate the intensity belonging to each ray before entering the
earth's gaseous envelope--in other words, to construct an
extra-atmospheric curve of energy in the spectrum. The result showed
that the blue end suffered far more than the red, absorption varying
inversely as wave-length. This property of stopping predominantly the
quicker vibrations is shared, as both Vogel and Langley[741] have
conclusively shown, by the solar atmosphere. The effect of this double
absorption is as if two plates of reddish glass were interposed between
us and the sun, the withdrawal of which would leave his orb, not only
three or four times more brilliant, but in colour distinctly
greenish-blue.[742]
The fact of the uncovered sun being _blue_ has an important bearing upon
the question of his temperature, to afford a somewhat more secure answer
to which was the ultimate object of Professor Langley's persevering
researches; for it is well known that as bodies grow hotter, the
proportionate representation in their spectra of the more refrangible
rays becomes greater. The lowest stage of incandescence is the familiar
one of _red_ heat. As it gains intensity, the quicker vibrations come
in, and an optical balance of sensation is established at _white_ heat.
The final term of _blue_ heat, as we now know, is attained by the
photosphere. On this ground alone, then, of the large original
preponderance of blue light, we must raise our estimate of solar heat;
and actual measurements show the same upward tendency. Until quite
lately, Pouillet's figure of 1.7 calories per minute per square
centimetre of terrestrial surface, was the received value for the "solar
constant." Forbes had, it is true, got 2.85 from observations on the
Faulhorn in 1842;[743] but they failed to obtain the confidence they
merited. Pouillet's result was not definitely superseded until Violle,
from actinometrical measures at the summit and base of Mont Blanc in
1875, computed the intensity of solar radiation at 2.54,[744] and Crova,
about the same time, at Montpellier, showed it to be above two
calories.[745] Langley went higher still. Working out the results of the
Mount Whitney expedition, he was led to conclude atmospheric absorption
to be fully twice as effective as had hitherto been supposed. Scarcely
60 per cent., in fact, of those solar radiations which strike
perpendicularly through a seemingly translucent sky, were estimated to
attain the sea-level. The rest are reflected, dispersed, or absorbed.
This discovery involved a large addition to the original supply so
mercilessly cut down in transmission, and the solar constant rose at
once to three calories. Nor did the rise stop there. M. Savélieff
deduced for it a value of 3.47 from actinometrical observations made at
Kieff in 1890;[746] and Knut Ångström, taking account of the arrestive
power of carbonic acid, inferred enormous atmospheric absorption, and a
solar constant of four calories.[747] In other words, the sun's heat
reaching the outskirts of our atmosphere is capable of doing without
cessation the work of an engine of four-horse power for each square yard
of the earth's surface. Thus, modern inquiries tend to render more and
more evident the vastness of the thermal stores contained in the great
central reservoir of our system, while bringing into fair agreement the
estimates of its probable temperature. This is in great measure due to
the acquisition of a workable formula by which to connect temperature
with radiation. Stefan's rule of a fourth-power relation, if not
actually a law of nature, is a colourable imitation of one; and its
employment has afforded a practical certainty that the sun's
temperature, so far as it is definable, neither exceeds 12,000° C., nor
falls short of 6,500° C.
FOOTNOTES:
[Footnote 698: _Principia_, p. 498 (1st ed.).]
[Footnote 699: _Comptes Rendus_, t. vii., p. 24.]
[Footnote 700: _Results of Astr. Observations_, p. 446.]
[Footnote 701: "Est enim calor solis ut radiorum densitas, hoc est,
reciproce ut quadratum distantiæ locorum a sole."--_Principia_, p. 508
(3d ed., 1726).]
[Footnote 702: _Jour. de Physique_, t. lxxv., p. 215.]
[Footnote 703: _Ann. de Chimie_, t. vii., 1817, p. 365.]
[Footnote 704: _Phil. Mag._, vol. xxiii. (4th ser.), p. 505.]
[Footnote 705: _Nuovo Cimento_, t. xvi., p. 294.]
[Footnote 706: _Comptes Rendus_, t. lxv., p. 526.]
[Footnote 707: The direct result of 5-1/3 million degrees was doubled in
allowance for absorption in the sun's own atmosphere. _Comptes Rendus_,
t. lxxiv., p. 26.]
[Footnote 708: _Ibid._, p. 31.]
[Footnote 709: _Nature_, vols. iv., p. 204; v., p. 505.]
[Footnote 710: _Ibid._, vol. xxx., p. 467.]
[Footnote 711: _Ann. de Chim._, t. x. (5th ser.), p. 361.]
[Footnote 712: _Comptes Rendus_, t. xcvi., p. 254.]
[Footnote 713: _Phil. Mag._, vol. viii., p. 324, 1879.]
[Footnote 714: _Ibid._, p. 325.]
[Footnote 715: _Sitzungsberichte_, Wien, Bd. lxxix., ii., p. 391.]
[Footnote 716: _Wiedemann's Annalen_, Bd. xxii., p. 291; _Scheiner,
Strahlung und Temperatur der Sonne_, p. 27.]
[Footnote 717: _Comptes Rendus_, March 28, 1892; _Astr. and
Astrophysics_, vol. xi., p. 517.]
[Footnote 718: _Phil. Trans._, vol. clxxxv., p. 361.]
[Footnote 719: _Proc. Roy. Society_, December 12, 1901.]
[Footnote 720: Scheiner, _Temp. der Sonne_, p. 36.]
[Footnote 721: _Astroph. Jour._, vol. ii., p. 318.]
[Footnote 722: _Astr. Nach._, Nos. 1,815-16.]
[Footnote 723: _L'Astronomie_, September, 1884, p. 334.]
[Footnote 724: _Amer. Jour. of Science_, vol. i. (3rd ser.), p. 653.]
[Footnote 725: Young, _The Sun_, p. 310.]
[Footnote 726: _Op._, t. vi., p. 198.]
[Footnote 727: _Rosa Ursina_, lib. iv., p. 618.]
[Footnote 728: _Méc. Cél._, liv. x., p. 323.]
[Footnote 729: _Le Soleil_ (1st ed.), p. 136.]
[Footnote 730: _Monatsber._, Berlin, 1877, p. 104.]
[Footnote 731: _Astr. Nach._, Nos. 3,105-6; _Astr. and Astrophysics_,
vol. xi., p. 720.]
[Footnote 732: _Proc. Roy. Irish Acad._, vol. ii., No. 2, 1892.]
[Footnote 733: _Am. Jour. of Sc._, vol. xxi., p. 187.]
[Footnote 734: _Amer. Jour. of Science_, vol. v., p. 245, 1898.]
[Footnote 735: For J. W. Draper's partial anticipation of this result,
see _Ibid_. vol. iv., 1872, p. 174.]
[Footnote 736: _Phil. Mag._, vol. xiv., p. 179, 1883.]
[Footnote 737: "The Solar and the Lunar Spectrum," _Memoirs National
Acad. of Science_, vol. xxxii.; "On hitherto Unrecognised Wave-lengths,"
_Amer. Jour. of Science_, vol. xxxii., August, 1886.]
[Footnote 738: _Astroph. Jour._, vol. i., p. 162.]
[Footnote 739: _Annals of the Smithsonian Astroph. Observatory_, vol.
i.; _Comptes Rendus_, t. cxxxi., p. 734; _Astroph. Jour._, vol. iii., p.
63.]
[Footnote 740: _Phil. Mag._, July, 1901.]
[Footnote 741: _Comptes Rendus_, t. xcii., p. 701.]
[Footnote 742: _Nature_, vol. xxvi., p. 589.]
[Footnote 743: _Phil. Trans._, vol. cxxxii., p. 273.]
[Footnote 744: _Ann. de Chim._, t. x., p. 321.]
[Footnote 745: _Ibid._, t. xi., p. 505.]
[Footnote 746: _Comptes Rendus_, t. cxii., p. 1200.]
[Footnote 747: _Wied. Ann._, Bd. xxxix., p. 294; Scheiner, _Temperatur
der Sonne_, pp. 36, 38.]
CHAPTER VI
_THE SUN'S DISTANCE_
The question of the sun's distance arises naturally from the
consideration of his temperature, since the intensity of the radiations
emitted as compared with those received and measured, depends upon it.
But the knowledge of that distance has a value quite apart from its
connection with solar physics. The semi-diameter of the earth's orbit is
our standard measure for the universe. It is the great fundamental datum
of astronomy--the unit of space, any error in the estimation of which is
multiplied and repeated in a thousand different ways, both in the
planetary and sidereal systems. Hence its determination was called by
Airy "the noblest problem in astronomy." It is also one of the most
difficult. The quantities dealt with are so minute that their sure grasp
tasks all the resources of modern science. An observational inaccuracy
which would set the moon nearer to, or farther from us than she really
is by one hundred miles, would vitiate an estimate of the sun's distance
to the extent of sixteen million![748] What is needed in order to attain
knowledge of the desired exactness is no less than this: to measure an
angle about equal to that subtended by a halfpenny 2,000 feet from the
eye, within a little more than a thousandth part of its value.
The angle thus represented is what is called the "horizontal parallax"
of the sun. By this amount--the breadth of a halfpenny at 2,000 feet--he
is, to a spectator on the rotating earth, removed at rising and setting
from his meridian place in the heavens. Such, in other terms, would be
the magnitude of the terrestrial radius as viewed from the sun. If we
knew this magnitude with certainty and precision, we should also know
with certainty and precision--the dimensions of the earth being, as they
are, well ascertained--the distance of the sun. In fact, the one
quantity commonly stands for the other in works treating professedly of
astronomy. But this angle of parallax or apparent displacement cannot be
directly measured--cannot even be perceived with the finest instruments.
Not from its smallness. The parallactic shift of the nearest of the
stars as seen from opposite sides of the earth's orbit, is many times
smaller. But at the sun's limb, and close to the horizon, where the
visual angle in question opens out to its full extent, atmospheric
troubles become overwhelming, and altogether swamp the far more minute
effects of parallax.
There remain indirect methods. Astronomers are well acquainted with the
proportions which the various planetary orbits bear to each other. They
are so connected, in the manner expressed by Kepler's Third Law, that
the periods being known, it only needs to find the interval between any
two of them in order to infer at once the distances separating them all
from one another and from the sun. The plan is given; what we want to
discover is the scale upon which it is drawn; so that, if we can get a
reliable measure of the distance of a single planet from the earth, our
problem is solved.
Now some of our fellow-travellers in our unending journey round the sun,
come at times well within the scope of celestial trigonometry. The orbit
of Mars lies at one point not more than thirty-five million miles
outside that of the earth, and when the two bodies happen to arrive
together in or near the favourable spot--a conjuncture which occurs
every fifteen years--the desired opportunity is granted. Mars is then
"in opposition," or on the _opposite_ side of us from the sun, crossing
the meridian consequently at midnight.[749] It was from an opposition of
Mars, observed in 1672 by Richer at Cayenne in concert with Cassini in
Paris, that the first scientific estimate of the sun's distance was
derived. It appeared to be nearly eighty-seven millions of miles
(parallax 9·5"); while Flamsteed deduced 81,700,000 (parallax 10") from
his independent observations of the same occurrence--a difference quite
insignificant at that stage of the inquiry. But Picard's result was just
half Flamsteed's (parallax 20"; distance forty-one million miles); and
Lahire considered that we must be separated from the hearth of our
system by an interval of _at least_ 136 million miles.[750] So that
uncertainty continued to have an enormous range.
Venus, on the other hand, comes closest to the earth when she passes
between it and the sun. At such times of "inferior conjunction" she is,
however, still twenty-six million miles, or (in round numbers) 109 times
as distant as the moon. Moreover, she is so immersed in the sun's rays
that it is only when her path lies across his disc that the requisite
facilities for measurement are afforded. These "partial eclipses of the
sun by Venus" (as Encke termed them) are coupled together in pairs,[751]
of which the components are separated by eight years, recurring at
intervals alternately of 105-1/2 and 121-1/2 years. Thus, the first
calculated transit took place in December, 1631, and its companion
(observed by Horrocks) in the same month (N.S.), 1639. Then, after the
lapse of 121-1/2 years, came the June couple of 1761 and 1769; and again
after 105-1/2, the two last observed, December 8, 1874, and December 6,
1882. Throughout the twentieth century there will be no transit of
Venus; but the astronomers of the twenty-first will only have to wait
four years for the first of a June pair. The rarity of these events is
due to the fact that the orbits of the earth and Venus do not lie in the
same plane. If they did, there would be a transit each time that our
twin-planet overtakes us in her more rapid circling--that is, on an
average, every 584 days. As things are actually arranged, she passes
above or below the sun, except when she happens to be very near the line
of intersection of the two tracks.
Such an occurrence as a transit of Venus seems, at first sight, full of
promise for solving the problem of the sun's distance. For nothing would
appear easier than to determine exactly either the duration of the
passage of a small, dark orb across a large brilliant disc, or the
instant of its entry upon or exit from it. And the differences in these
times (which, owing to the comparative nearness of Venus, are quite
considerable), as observed from remote parts of the earth, can be
translated into differences of space--that is, into apparent or
parallactic displacements, whereby the distance of Venus becomes known,
and thence, by a simple sum in proportion, the distance of the sun. But
in that word "exactly" what snares and pitfalls lie hid! It is so easy
to think and to say; so indefinitely hard to realise. The astronomers of
the eighteenth century were full of hope and zeal. They confidently
expected to attain, through the double opportunity offered them, to
something like a permanent settlement of the statistics of our system.
They were grievously disappointed. The uncertainty as to the sun's
distance, which they had counted upon reducing to a few hundred thousand
miles, remained at many millions.
In 1822, however, Encke, then director of the Seeberg Observatory near
Gotha, undertook to bring order out of the confusion of discordant, and
discordantly interpreted observations. His combined result for both
transits (1761 and 1769) was published in 1824,[752] and met universal
acquiescence. The parallax of the sun thereby established was 8·5776",
corresponding to a mean distance[753] of 95-1/4 million miles. Yet this
abolition of doubt was far from being so satisfactory as it seemed.
Serenity on the point lasted exactly thirty years. It was disturbed in
1854 by Hansen's announcement[754] that the observed motions of the moon
could be drawn into accord with theory only on the terms of bringing the
sun considerably nearer to us than he was supposed to be.
Dr. Matthew Stewart, professor of mathematics in the University of
Edinburgh, had made a futile attempt in 1763 to deduce the sun's
distance from his disturbing power over our satellite.[755] Tobias Mayer
of Göttingen, however, whose short career was fruitful of suggestions,
struck out the right way to the same end; and Laplace, in the seventh
book of the _Mécanique Céleste_,[756] gave a solar parallax derived from
the lunar "parallactic inequality" substantially identical with that
issuing from Encke's subsequent discussion of the eighteenth-century
transits. Thus, two wholly independent methods--the trigonometrical,
or method by survey, and the gravitational, or method by
perturbation--seemed to corroborate each the upshot of the use of the
other until the nineteenth century was well past its meridian. It is
singular how often errors conspire to lead conviction astray.
Hansen's note of alarm in 1854 was echoed by Leverrier in 1858.[757] He
found that an apparent monthly oscillation of the sun which reflects a
real monthly movement of the earth round its common centre of gravity
with the moon, and which depends for its amount solely on the mass of
the moon and the distance of the sun, required a diminution in the
admitted value of that distance by fully four million miles. Three years
later he pointed out that certain perplexing discrepancies between the
observed and computed places both of Venus and Mars, would vanish on the
adoption of a similar measure.[758] Moreover, a favourable opposition of
Mars gave the opportunity in 1862 for fresh observations, which,
separately worked out by Stone and Winnecke, agreed with all the newer
investigations in fixing the great unit at slightly over 91 million
miles. In Newcomb's hands they gave 92-1/2 million.[759] The
accumulating evidence in favour of a large reduction in the sun's
distance was just then reinforced by an auxiliary result of a totally
different and unexpected kind.
The discovery that light does not travel instantaneously from point to
point, but takes some short time in transmission, was made by Olaus
Römer in 1675, through observing that the eclipses of Jupiter's
satellites invariably occurred later, when the earth was on the far
side, than when it was on the near side of its orbit. Half the
difference, or the time spent by a luminous vibration in crossing the
"mean radius" of the earth's orbit, is called the "light-equation"; and
the determination of its precise value has claimed the minute care
distinctive of modern astronomy. Delambre in 1792 made it 493 seconds.
Glasenapp, a Russian astronomer, raised the estimate in 1874 to 501,
Professor Harkness adopts a safe medium value of 498 seconds. Hence, if
we had any independent means of ascertaining how fast light travels, we
could tell at once how far off the sun is.
There is yet another way by which knowledge of the swiftness of light
would lead us straight to the goal. The heavenly bodies are perceived,
when carefully watched and measured, to be pushed forward out of their
true places, in the direction of the earth's motion, by a very minute
quantity. This effect (already adverted to) has been known since
Bradley's time as "aberration." It arises from a combination of the two
movements of the earth round the sun and of the light-waves through the
ether. If the earth stood still, or if light spent no time on the road
from the stars, such an effect would not exist. Its amount represents
the proportion between the velocities with which the earth and the
light-rays pursue their respective journeys. This proportion is,
roughly, one to ten thousand. So that here again, if we knew the rate
per second of luminous transmission, we should also know the rate per
second of the earth's movement, consequently the size of its orbit and
the distance of the sun.
But, until lately, instead of finding the distance of the sun from the
velocity of light, there has been no means of ascertaining the velocity
of light except through the imperfect knowledge possessed as to the
distance of the sun. The first successful terrestrial experiments on the
point date from 1849; and it is certainly no slight triumph of human
ingenuity to have taken rigorous account of the delay of a sunbeam in
flashing from one mirror to another. Fizeau led the way,[760] and he was
succeeded, after a few months, by Léon Foucault,[761] who, in 1862, had
so far perfected Wheatstone's method of revolving mirrors, as to be able
to announce with authority that light travelled slower, and that the sun
was in consequence nearer than had been supposed.[762] Thus a third line
of separate research was found to converge to the same point with the
two others.
Such a conspiracy of proof was not to be resisted, and at the
anniversary meeting of the Royal Astronomical Society in February, 1864,
the correction of the solar distance took the foremost place in the
annals of the year. Lest, however, a sudden bound of four million miles
nearer to the centre of our system should shake public faith in
astronomical accuracy, it was explained that the change in the solar
parallax corresponding to that huge leap, amounted to no more than the
breadth of a human hair 125 feet from the eye![763] The Nautical Almanac
gave from 1870 the altered value of 8.95", for which Newcomb's result of
8.85", adopted in 1869 in the Berlin Ephemeris, was substituted some ten
years later. In astronomical literature the change was initiated by Sir
Edmund Beckett in the first edition (1865) of his _Astronomy without
Mathematics_.
If any doubt remained as to the misleading character of Encke's
deduction, so long implicitly trusted in, it was removed by Powalky's
and Stone's rediscussions, in 1864 and 1868 respectively, of the transit
observations of 1769. Using improved determinations of the longitude of
the various stations, and a selective judgment in dealing with their
materials, which, however indispensable, did not escape adverse
criticism, they brought out results confirmatory of the no longer
disputed necessity for largely increasing the solar parallax, and
proportionately diminishing the solar distance. Once more in 1890, and
this time with better success, the eighteenth-century transits were
investigated by Professor Newcomb.[764] Turning to account the
experience gained in the interim regarding the optical phenomena
accompanying such events, he elicited from the mass of somewhat
discordant observations at his command, a parallax (8·79") in close
agreement with the value given by sundry modes of recent research.
Conclusions on the subject, however, were still regarded as purely
provisional. A transit of Venus was fast approaching, and to its
arbitrament, as to that of a court of final appeal, the pending question
was to be referred. It is true that the verdict in the same case by the
same tribunal a century earlier had proved of so indecisive a character
as to form only a starting-point for fresh litigation; but that century
had not passed in vain, and it was confidently anticipated that
observational difficulties, then equally unexpected and insuperable,
would yield to the elaborate care and skill of forewarned modern
preparation.
The conditions of the transit of December 8, 1874, were sketched out by
Sir George Airy, then Astronomer-Royal, in 1857,[765] and formed the
subject of eager discussion in this and other countries down to the very
eve of the occurrence. In these Mr. Proctor took a leading part; and it
was due to his urgent representations that provision was made for the
employment of the method identified with the name of Halley,[766] which
had been too hastily assumed inapplicable to the first of each
transit-pair. It depends upon the difference in the length of time taken
by the planet to cross the sun's disc, as seen from various points of
the terrestrial surface, and requires, accordingly, the visibility of
both entrance and exit at the same station. Since these were, in 1874,
separated by about three and a half hours, and the interval may be much
longer, the choice of posts for the successful use of the "method of
durations" is a matter of some difficulty.
The system described by Delisle in 1760, on the other hand, involves
merely noting the instant of ingress or egress (according to situation)
from opposite extremities of a terrestrial diameter; the disparity in
time giving a measure of the planet's apparent displacement, hence of
its actual rate of travel in miles per minute, from which its distances
severally from earth and sun are immediately deducible. Its chief
attendant difficulty is the necessity for accurately fixing the
longitudes of the points of observation. But this was much more sensibly
felt a century ago than it is now, the improved facility and certainty
of modern determinations tending to give the Delislean plan a decided
superiority over its rival.
These two traditional methods were supplemented in 1874 by the camera
and the heliometer. From photography, above all, much was expected.
Observations made by its means would have the advantages of
impartiality, multitude, and permanence. Peculiarities of vision and
bias of judgment would be eliminated; the slow progress of the
phenomenon would permit an indefinite number of pictures to be taken,
their epochs fixed to a fraction of a second; while subsequent leisurely
comparison and measurement could hardly fail, it was thought, to educe
approximate truth from the mass of accumulated evidence. The use of the
heliometer (much relied on by German observers) was so far similar to
that of the camera that the object aimed at by both was the
determination of the relative positions of the _centres_ of the sun and
Venus viewed, at the same absolute instant, from opposite sides of the
globe. So that the principle of the two older methods was to ascertain
the exact times of meeting between the solar and planetary limbs; that
of the two modern to determine the position of the dark body already
thrown into complete relief by its shining background. The former are
"methods by contact," the latter "methods by projection."
Every country which had a reputation to keep or to gain for scientific
zeal was forward to co-operate in the great cosmopolitan enterprise of
the transit. France and Germany each sent out six expeditions;
twenty-six stations were in Russian, twelve in English, eight in
American, three in Italian, one in Dutch occupation. In all, at a cost
of nearly a quarter of a million, some fourscore distinct posts of
observation were provided; among them such inhospitable, and all but
inaccessible rocks in the bleak Southern Ocean, as St. Paul's and
Campbell Islands, swept by hurricanes, and fitted only for the
habitation of seabirds, where the daring votaries of science, in the
wise prevision of a long leaguer by the elements, were supplied with
stores for many months, or even a whole year. Siberia and the Sandwich
Islands were thickly beset with observers; parties of three
nationalities encamped within the mists of Kerguelen Island,
expressively termed the "Land of Desolation," in the sanguine, though
not wholly frustrated hope of a glimpse of the sun at the right moment.
M. Janssen narrowly escaped destruction from a typhoon in the China seas
on his way to Nagasaki; Lord Lindsay (now Earl of Crawford and
Balcarres) equipped, at his private expense, an expedition to Mauritius,
which was in itself an epitome of modern resource and ingenuity.
During several years, the practical methods best suited to insure
success for the impending enterprise formed a subject of European
debate. Official commissions were appointed to receive and decide upon
evidence; and experiments were in progress for the purpose of defining
the actual circumstances of contacts, the precise determination of which
constituted the only tried, though by no means an assuredly safe road to
the end in view. In England, America, France, and Germany, artificial
transits were mounted, and the members of the various expeditions were
carefully trained to unanimity in estimating the phases of junction and
separation between a moving dark circular body and a broad illuminated
disc. In the previous century, a formidable and prevalent phenomenon,
which acquired notoriety as the "Black Drop" or "Black Ligament," had
swamped all pretensions to rigid accuracy. It may be described as
substituting adhesion for contact, the limbs of the sun and planet,
instead of meeting and parting with the desirable clean definiteness,
_clinging_ together as if made of some glutinous material, and
prolonging their connection by means of a dark band or dark threads
stretched between them. Some astronomers ascribed this baffling
appearance entirely to instrumental imperfections; others to atmospheric
agitation; others again to the optical encroachment of light upon
darkness known as "irradiation." It is probable that all these causes
conspired, in various measure, to produce it; and it is certain that its
_conspicuous_ appearance may, by suitable precautions, be obviated.
The organisation of the British forces reflected the utmost credit on
the energy and ability of Lieutenant-Colonel Tupman, who was responsible
for the whole. No useful measure was neglected. Each observer went out
ticketed with his "personal equation," his senses drilled into a species
of martial discipline, his powers absorbed, so far as possible, in the
action of a cosmopolitan observing machine. Instrumental uniformity and
uniformity of method were obtainable, and were attained; but diversity
of judgment unhappily survived the best-directed efforts for its
extirpation.
The eventful day had no sooner passed than telegrams began to pour in,
announcing an outcome of considerable, though not unqualified success.
The weather had proved generally favourable; the manifold arrangements
had worked well; contacts had been plentifully observed; photographs in
lavish abundance had been secured; a store of materials, in short, had
been laid up, of which it would take years to work out the full results
by calculation. Gradually, nevertheless, it came to be known that the
hope of a definitive issue must be abandoned. Unanimity was found to be
as remote as ever. The dreaded "black ligament" gave, indeed, less
trouble than was expected; but another appearance supervened which took
most observers by surprise. This was the illumination due to the
atmosphere of Venus. Astronomers, it is true, were not ignorant that the
planet had, on previous occasions, been seen girdled with a lucid ring;
but its power to mar observations by the distorting effect of refraction
had scarcely been reckoned with. It proved, however, to be very great.
Such was the difficulty of determining the critical instant of internal
contact, that (in Colonel Tupman's words) "observers side by side, with
adequate optical means, differed as much as twenty or thirty seconds in
the times they recorded for phenomena which they have described in
almost identical language."[767]
Such uncertainties in the data admitted of a corresponding variety in
the results. From the British observations of ingress and egress Sir
George Airy[768] derived, in 1877, a solar parallax of 8·76" (corrected
to 8·754"), indicating a mean distance of 93,375,000 miles. Mr. Stone
obtained a value of ninety-two millions (parallax 8·88"), and held any
parallax less than 8·84" or more than 8·93" to be "absolutely negatived"
by the documents available.[769] Yet, from the same, Colonel Tupman
deduced 8·81",[770] implying a distance 700,000 miles greater than Stone
had obtained. The best French observations of contacts gave a parallax
of about 8·88"; French micrometric measures the obviously exaggerated
one of 9·05".[771]
Photography, as practised by most of the European parties, was a total
failure. Utterly discrepant values of the microscopic displacements
designed to serve as sounding lines for the solar system, issued from
attempts to measure even the most promising pictures. "You might as well
try to measure the zodiacal light," it was remarked to Sir George Airy.
Those taken on the American plan of using telescopes of so great focal
length as to afford, without further enlargement, an image of the
requisite size, gave notably better results. From an elaborate
comparison of those dating from Vladivostock, Nagasaki, and Pekin, with
others from Kerguelen and Chatham Islands, Professor D. P. Todd, of
Amherst College, deduced a solar distance of about ninety-two million
miles (parallax 8·883" ±0·034"),[772] and the value was much favoured by
concurrent evidence.
On the whole, estimates of the great spatial unit cannot be said to have
gained any security from the combined effort of 1874. A few months
before the transit, Mr. Proctor considered that the uncertainty then
amounted to 1,448,000 miles;[773] five years after the transit,
Professor Harkness judged it to be still 1,575,950 miles;[774] yet it
had been hoped that it would have been brought down to 100,000. As
regards the end for which it had been undertaken, the grand campaign had
come to nothing. Nevertheless, no sign of discouragement was apparent.
There was a change of view, but no relaxation of purpose. The problem,
it was seen, could be solved by no single heroic effort, but by the
patient approximation of gradual improvements. Astronomers, accordingly,
looked round for fresh means or more refined expedients for applying
those already known. A new phase of exertion was entered upon.
On September 5, 1877, Mars came into opposition near the part of his
orbit which lies nearest to that of the earth, and Dr. Gill (now Sir
David) took advantage of the circumstance to appeal once more to him for
a decision on the _quæstio vexata_ of the sun's distance. He chose, as
the scene of his labours, the Island of Ascension, and for their plan a
method recommended by Airy in 1857,[775] but never before fairly tried.
This is known as the "diurnal method of parallaxes." Its principle
consists in substituting successive morning and evening observations
from the same spot, for simultaneous observations from remote spots, the
rotation of the earth supplying the necessary difference in the points
of view. Its great advantage is that of unity in performance. A single
mind, looking through the same pair of eyes, reinforced with the same
optical appliances, is employed throughout, and the errors inseparable
from the combination of data collected under different conditions are
avoided. There are many cases in which one man can do the work of two
better than two men can do the work of one. The result of Gill's skilful
determinations (made with Lord Lindsay's heliometer) was a solar
parallax of 8·78", corresponding to a distance of 93,080,000 miles.[776]
The bestowal of the Royal Astronomical Society's gold medal stamped the
merit of this distinguished service.
But there are other subjects for this kind of inquiry besides Mars and
Venus. Professor Galle of Breslau suggested in 1872[777] that some of
the minor planets might be got to repay astronomers for much
disinterested toil spent in unravelling their motions, by lending aid to
their efforts towards a correct celestial survey. Ten or twelve come
near enough, and are bright enough for the purpose; in fact, the absence
of sensible magnitude is one of their chief recommendations, since a
point of light offers far greater facilities for exact measurement than
a disc. The first attempt to work this new vein was made at the
opposition of Phocæa in 1872; and from observations of Flora in the
following year at twelve observatories in the northern and southern
hemispheres, Galle deduced a solar parallax of 8·87".[778] At Mauritius
in 1874, Lord Lindsay and Sir David Gill applied the "diurnal method" to
Juno, then conveniently situated for the purpose; and the continued use
of similar occasions affords an unexceptionable means for improving
knowledge of the sun's distance. They frequently recur; they need no
elaborate preparation; a single astronomer armed with a heliometer can
do all the requisite work. Dr. Gill, however, organized a more complex
plan of operations upon Iris in 1888, and upon Victoria and Sappho in
1889. A novel method was adopted. Its object was to secure simultaneous
observations made from opposite sides of the globe just when the planet
lay in the plane passing through the centre of the earth and the two
observers, the same pair of reference-stars being used on each occasion.
The displacements caused by parallax were thus in a sense doubled, since
the star to which the planet seemed approximated in the northern
hemisphere, showed as if slightly removed from it in the southern, and
_vice versâ_. As the planet pursued its course, fresh star-couples came
into play, during the weeks that the favourable period lasted. In these
determinations, only heliometers were employed. Dr. Elkin, of Yale
college, co-operated throughout, and the heliometers of Dresden,
Göttingen, Bamberg, and Leipzig, shared in the work, while Dr. Auwers of
Berlin was Sir David Gill's personal coadjutor at the Cape. Voluminous
data were collected; meridian observations of the stars of reference for
Victoria occupied twenty-one establishments during four months; the
direct work of triangulation kept four heliometers in almost exclusive
use for the best part of a year; and the ensuing toilsome computations,
carried out during three years at the Cape Observatory, filled two bulky
tomes[779] with their details. Gill's final result, published in 1897,
was a parallax of 8·802", equivalent to a solar distance of 92,874,000;
and it was qualified by a probable error so small that the value might
well have been accepted as definitive but for an unlooked-for discovery.
The minor planet Eros, detected August 14, 1898, was found to pursue a
course rendering it an almost ideal intermediary in solar
parallax-determinations. Once in thirty years, it comes within fifteen
million miles of the earth; and although the next of these choice epochs
must be awaited for some decades, an opposition too favourable to be
neglected occurred in 1900. At an International Conference, accordingly,
held at Paris in July of that year, a plan of photographic operations
was concerted between the representatives of no less than 58
observatories.[780] Its primary object was to secure a large stock of
negatives showing the planet with the comparison-stars along the route
traversed by it from October, 1900, to March, 1901,[781] and this at
least was successfully attained. Their measurement will in due time
educe the apparent displacements of the moving object as viewed
simultaneously from remote parts of the earth; and the upshot should be
a solar parallax adequate in accuracy to the exigent demands of the
twentieth century.
The second of the nineteenth-century pair of Venus-transits was looked
forward to with much abated enthusiasm. Russia refused her active
co-operation in observing it, on the ground that oppositions of the
minor planets were trigonometrically more useful, and financially far
less costly; and her example was followed by Austria; while Italian
astronomers limited their sphere of action to their own peninsula.
Nevertheless, it was generally held that a phenomenon which the world
could not again witness until it was four generations older should, at
the price of any effort, not be allowed to pass in neglect.
The persuasion of its importance justified the summoning of an
International Conference at Paris in 1881, from which, however, America,
preferring independent action, held aloof. It was decided to give
Delisle's method another trial; and the ambiguities attending and
marring its use were sought to be obviated by careful regulations for
insuring agreement in the estimation of the critical moments of ingress
and egress.[782] But in fact (as M. Puiseux had shown[783]), contacts
between the limbs of the sun and planet, so far from possessing the
geometrical simplicity attributed to them, are really made up of a
prolonged succession of various and varying phases, impossible either to
predict or identify with anything like rigid exactitude. Sir Robert Ball
compared the task of determining the precise instant of their meeting or
parting, to that of telling the hour with accuracy on a watch without a
minute hand; and the comparison is admittedly inadequate. For not only
is the apparent movement of Venus across the sun extremely slow, being
but the excess of her real motion over that of the earth; but three
distinct atmospheres--the solar, terrestrial, and Cytherean--combine to
deform outlines and mask the geometrical relations which it is desired
to connect with a strict count of time.
The result was very much what had been expected. The arrangements were
excellent, and were only in a few cases disconcerted by bad weather. The
British parties, under the experienced guidance of Mr. Stone, the late
Radcliffe observer, took up positions scattered over the globe, from
Queensland to Bermuda; the Americans collected a whole library of
photographs; the Germans and Belgians trusted to the heliometer; the
French used the camera as an adjunct to the method of contacts. Yet
little or no approach was made to solving the problem. Thus, from 606
measures of Venus on the sun, taken with a new kind of heliometer at
Santiago in Chili, M. Houzeau, of the Brussels Observatory, derived a
solar parallax of 8.907", and a distance of 91,727,000 miles.[784] But
the "probable errors" of this determination amounted to 0.084" either
way: it was subject to a "more or less" of 900,000, or to a total
uncertainty of 1,800,000 miles. The "probable error" of the English
result, published in 1887, was less formidable,[785] yet the details of
the discussion showed that no great confidence could be placed in it.
The sun's distance came out 92,560,000 miles; while 92,360,000 was given
by Professor Harkness's investigation of 1,475 American
photographs.[786] Finally, Dr. Auwers deduced from the German
heliometric measures the unsatisfactorily small value of 92,000,000
miles.[787] The transit of 1882 had not, then, brought about the desired
unanimity.
The state and progress of knowledge on this important topic were summed
up by Faye and Harkness in 1881.[788] The methods employed in its
investigation fall (as we have seen) into three separate classes--the
trigonometrical, the gravitational, and the "phototachymetrical"--an
ungainly adjective used to describe the method by the velocity of light.
Each has its special difficulties and sources of error; each has
counter-balancing advantages. The only trustworthy result from celestial
surveys, was at that time furnished by Gill's observations of Mars in
1877. But the method by lunar and planetary disturbances is unlike all
the others in having time on its side. It is this which Leverrier
declared with emphasis must inevitably prevail, because its accuracy is
continually growing.[789] The scarcely perceptible errors which still
impede its application are of such a nature as to accumulate year by
year; eventually, then, they will challenge, and must receive, a more
and more perfect correction. The light-velocity method, however,
claimed, and for some years justified, M. Faye's preference.
By a beautiful series of experiments on Foucault's principle, Michelson
fixed in 1879 the rate of luminous transmission at 299,930 (corrected
later to 299,910) kilometres a second.[790] This determination was held
by Professor Todd to be entitled to four times as much confidence as any
previous one; and if the solar parallax of 8·758" deduced from it by
Professor Harkness errs somewhat by defect, it is doubtless because
Glasenapp's "light-equation," with which it was combined, errs slightly
by excess. But all earlier efforts of the kind were thrown into the
shade by Professor Newcomb's arduous operations at Washington in
1880-1882.[791] The scale upon which they were conducted was in itself
impressive. Foucault's entire apparatus in 1862 had been enclosed in a
single room; Newcomb's revolving and fixed mirrors, between which the
rays of light were to run their timed course, were set up on opposite
shores of the Potomac, at a distance of nearly four kilometres. This
advantage was turned to the utmost account by ingenuity and skill in
contrivance and execution; and the deduced velocity of 299,860
kilometres = 186,328 miles a second, had an estimated error (30
kilometres) only one-tenth that ascribed by Cornu to his own result in
1874.
Just as these experiments were concluded in 1882, M. Magnus Nyrén, of
St. Petersburg, published an elaborate investigation of the small
annular displacements of the stars due to the successive transmission of
light, involving an increase of Struve's "constant of aberration" from
20·445" to 20·492". And from the new value, combined with Newcomb's
light-velocity, was derived a valuable approximation to the sun's
distance, concluded at 92,905,021 miles (parallax = 8·794"). Yet it is
not quite certain that Nyrén's correction was an improvement. A
differential method of determining the amount of aberration, struck out
by M. Loewy of Paris,[792] avoids most of the objections to the absolute
method previously in vogue; and the upshot of its application in 1891
was to show that Struve's constant might better be retained than
altered, Loewy's of 20·447" varying from it only to an insignificant
extent. Professor Hall had, moreover, deduced nearly the same value
(20·454") from the Washington observations since 1862, of Alpha
Lyræ (Vega); whence, in conjunction with Newcomb's rate of light
transmission, he arrived at a solar parallax of 8·81".[793] Inverting
the process, Sir David Gill in 1897 derived the constant from the
parallax. If the earth's orbit have a mean radius, as found by him, of
92,874,000 miles, then, he calculated, the aberration of
light--Newcomb's measures of its velocity being supposed exact--amounts
to 20.467". This figure can need very slight correction.
Professor Harkness surveyed in 1891,[794] from an eclectic point of
view, the general situation as regarded the sun's parallax. Convinced
that no single method deserved an exclusive preference, he reached a
plausible result through the combination, on the principle of least
squares--that is, by the mathematical rules of probability--of all the
various quantities upon which the great datum depends. It thus summed up
and harmonised the whole of the multifarious evidence bearing upon the
point, and, as modified in 1894,[795] falls very satisfactorily into
line with the Cape determination. We may, then, at least provisionally,
accept 92,870,000 miles as the length of our measuring-rod for space.
Nor do we hazard much in fixing 100,000 miles as the outside limit of
its future correction.
FOOTNOTES:
[Footnote 748: Airy, _Month. Not._, vol. xvii., p. 210.]
[Footnote 749: Mars comes into opposition once in about 780 days; but
owing to the eccentricity of both orbits, his distance from the earth at
those epochs varies from thirty-five to sixty-two million miles.]
[Footnote 750: J. D. Cassini, _Hist. Abrégée de la Parallaxe du Soleil_,
p. 122, 1772.]
[Footnote 751: The present period of coupled eccentric transits will, in
the course of ages, be succeeded by a period of single, nearly central
transits. The alignments by which transits are produced, of the earth,
Venus, and the sun, close to the place of intersection of the two
planetary orbits, now occur, the first a little in front of, the second,
after eight years less two and a half days, a little behind the node.
But when the first of these two meetings takes place very near the node,
giving a nearly central transit, the second falls too far from it, and
the planet escapes projection on the sun. The reason of the liability to
an eight-yearly recurrence is that eight revolutions of the earth are
accomplished in only a very little more time than thirteen revolutions
of Venus.]
[Footnote 752: _Die Entfernung der Sonne: Fortsetzung_, p. 108. Encke
slightly corrected his results of 1824 in _Berlin Abh._, 1835, p. 295.]
[Footnote 753: Owing to the ellipticity of its orbit, the earth is
nearer to the sun in January than in June by 3,100,000 miles. The
quantity to be determined, or "mean distance," is that lying midway
between these extremes--is, in other words, half the major axis of the
ellipse in which the earth travels.]
[Footnote 754: _Month. Not._, vol. xv., p. 9.]
[Footnote 755: _The Distance of the Sun from the Earth determined by the
Theory of Gravity_, Edinburgh, 1763.]
[Footnote 756: _Opera_, t. iii., p. 326.]
[Footnote 757: _Comptes Rendus_, t. xlvi., p. 882. The parallax 8·95"
derived by Leverrier from the above-described inequality in the earth's
motion, was corrected by Stone to 8·91". _Month. Not._, vol. xxviii., p.
25.]
[Footnote 758: _Month. Not._, vol. xxxv., p. 156.]
[Footnote 759: _Wash. Obs._, 1865, App. ii., p. 28.]
[Footnote 760: _Comptes Rendus_, t. xxix., p. 90.]
[Footnote 761: _Ibid._, t. xxx., p. 551.]
[Footnote 762: _Ibid._, t. lv., p. 501. The previously admitted velocity
was 308 million metres per second; Foucault reduced it to 298 million.
Combined with Struve's "constant of aberration" this gave 8.86" for the
solar parallax, which exactly agreed with Cornu's result from a
repetition of Fizeau's experiments in 1872. _Comptes Rendus_, t. lxxvi.,
p. 338.]
[Footnote 763: _Month. Not._, vol. xxiv., p. 103.]
[Footnote 764: _Astr. Papers of the American Ephemeris_, vol. ii., p.
263.]
[Footnote 765: _Month. Not._, vol. xvii., p. 208.]
[Footnote 766: Because closely similar to that proposed by him in _Phil.
Trans._ for 1716.]
[Footnote 767: _Month. Not._, vol. xxxviii., p. 447.]
[Footnote 768: _Ibid._, p. 11.]
[Footnote 769: _Ibid._, p. 294.]
[Footnote 770: _Ibid._, p. 334.]
[Footnote 771: _Comptes Rendus_, t. xcii., p. 812.]
[Footnote 772: _Observatory_, vol. v., p. 205.]
[Footnote 773: _Transits of Venus_, p. 89 (1st ed.).]
[Footnote 774: _Am. Jour. of Sc._, vol. xx., p. 393.]
[Footnote 775: _Month. Not._, vol. xvii., p. 219.]
[Footnote 776: _Mem. Roy. Astr. Soc._, vol. xlvi., p. 163.]
[Footnote 777: _Astr. Nach._, No. 1,897.]
[Footnote 778: Hilfiker, _Bern Mittheilungen_, 1878, p. 109.]
[Footnote 779: _Annals of the Cape Observatory_, vols. vi., vii.]
[Footnote 780: _Rapport sur l'État de l'Observatoire de Paris pour
l'Année 1900_, p. 7.]
[Footnote 781: _Observatory_, vol. xxiii., p. 311; Newcomb, _Astr.
Jour._, No. 480.]
[Footnote 782: _Comptes Rendus_, t. xciii., p. 569.]
[Footnote 783: _Ibid._, t. xcii., p. 481.]
[Footnote 784: _Bull. de l'Acad._, t. vi., p. 842.]
[Footnote 785: _Month. Not._, vol. xlviii., p. 201.]
[Footnote 786: _Astr. Jour._, No. 182.]
[Footnote 787: _Astr. Nach._, No. 3,066.]
[Footnote 788: _Comptes Rendus_, t. xcii., p. 375; _Am. Jour. of Sc._,
vol. xxii., p. 375.]
[Footnote 789: _Month. Not._, vol. xxxv., p. 401.]
[Footnote 790: _Am. Jour. of Sc._, vol. xviii., p. 393.]
[Footnote 791: _Nature_, vol. xxxiv., p. 170; _Astron. Papers of the
American Ephemeris_, vol. ii., p. 113.]
[Footnote 792: _Comptes Rendus_, t. cxii., p. 549.]
[Footnote 793: _Astr. Journ._, Nos. 169, 170]
[Footnote 794: _The Solar Parallax and its Related Constants_,
Washington, 1891.]
[Footnote 795: _Astr. and Astrophysics_, vol. xiii., p. 626.]
CHAPTER VII
_PLANETS AND SATELLITES_
Johann Hieronymus Schröter was the Herschel of Germany. He did not, it
is true, possess the more brilliant gifts of his rival. Herschel's
piercing discernment, comprehensive intelligence, and inventive
splendour were wanting to him. He was, nevertheless, the founder of
descriptive astronomy in Germany, as Herschel was in England.
Born at Erfurt in 1745, he prosecuted legal studies at Göttingen, and
there imbibed from Kästner a life-long devotion to science. From the
law, however, he got the means of living, and, what was to the full as
precious to him, the means of observing. Entering the sphere of
Hanoverian officialism in 1788, he settled a few years later at
Lilienthal, near Bremen, as "Oberamtmann," or chief magistrate. Here he
built a small observatory, enriched in 1785 with a seven-foot reflector
by Herschel, then one of the most powerful instruments to be found
anywhere out of England. It was soon surpassed, through his exertions,
by the first-fruits of native industry in that branch. Schrader of Kiel
transferred his workshops to Lilienthal in 1792, and constructed there,
under the superintendence and at the cost of the astronomical
Oberamtmann, a thirteen-foot reflector, declared by Lalande to be the
finest telescope in existence, and one twenty-seven feet in focal
length, probably as inferior to its predecessor in real efficiency as it
was superior in size.
Thus, with instruments of gradually increasing power, Schröter studied
during thirty-four years the topography of the moon and planets. The
field was then almost untrodden; he had but few and casual predecessors,
and has since had no equal in the sustained and concentrated patience of
his hourly watchings. Both their prolixity and their enthusiasm are
faithfully reflected in his various treatises. Yet the one may be
pardoned for the sake of the other, especially when it is remembered
that he struck out a substantially new line, and that one of the main
lines of future advance. Moreover, his infectious zeal communicated
itself; he set the example of observing when there was scarcely an
observer in Germany; and under his roof Harding and Bessel received
their training as practical astronomers.
But he was reserved to see evil days. Early in 1813 the French under
Vandamme occupied Bremen. On the night of April 20, the Vale of Lilies
was, by their wanton destructiveness, laid waste with fire; the
Government offices were destroyed, and with them the chief part of
Schröter's property, including the whole stock of his books and
writings. There was worse behind. A few days later, his observatory,
which had escaped the conflagration, was broken into, pillaged, and
ruined. His life was wrecked with it. He survived the catastrophe three
years without the means to repair, or the power to forget it, and
gradually sank from disappointment into decay, terminated by death,
August 29, 1816. He had, indeed, done all the work he was capable of;
and though not of the first quality, it was far from contemptible. He
laid the foundation of the _comparative_ study of the moon's surface,
and the descriptive particulars of the planets laboriously collected by
him constituted a store of more or less reliable information hardly
added to during the ensuing half century. They rested, it is true, under
some shadow of doubt; but the most recent observations have tended on
several points to rehabilitate the discredited authority of the
Lilienthal astronomer. We may now briefly resume, and pursue in its
further progress, the course of his studies, taking the planets in the
order of their distances from the sun.
In April, 1792, Schröter saw reason to conclude, from the gradual
degradation of light on its partially illuminated disc, that Mercury
possesses a tolerably dense atmosphere.[796] During the transit of May
7, 1799, he was, moreover, struck with the appearance of a ring of
softened luminosity encircling the planet to an apparent height of three
seconds, or about a quarter of its own diameter.[797] Although a "mere
thought" in texture, it remained persistently visible both with the
seven-foot and the thirteen-foot reflectors, armed with powers up to
288. It had a well-marked grayish boundary, and reminded him, though
indefinitely fainter, of the penumbra of a sun-spot. A similar appendage
had been noticed by De Plantade at Montpellier, November 11, 1736, and
again in 1786 and 1789 by Prosperin and Flaugergues; but Herschel, on
November 9, 1802, saw the preceding limb of the planet projected on the
sun cut the luminous solar clouds with the most perfect sharpness.[798]
The presence, however, of a "halo" was unmistakable in 1832, when
Professor Moll, of Utrecht, described it as a "nebulous ring of a darker
tinge approaching to the violet colour."[799] Again, to Huggins and
Stone, November 5, 1868, it showed as lucid and most distinct. No change
in the colour of the glasses used, or the powers applied, could get rid
of it, and it lasted throughout the transit.[800] It was next seen by
Christie and Dunkin at Greenwich, May 6, 1878,[801] and with much
precision of detail by Trouvelot at Cambridge (U.S.).[802] Professor
Holden, on the other hand, noted at Hastings-on-Hudson the total absence
of all anomalous appearances.[803] Nor could any vestige of them be
perceived by Barnard at Lick on November 10, 1894.[804] Various effects
of irradiation and diffraction were, however, observed by Lowell and W.
H. Pickering at Flagstaff;[805] and Davidson was favoured at San
Francisco with glimpses of the historic aureola,[806] as well as of a
central whitish spot, which often accompanies it. That both are somehow
of optical production can scarcely be doubted.
Nothing can be learned from them regarding the planet's physical
condition. Airy showed that refraction in a Mercurian atmosphere could
not possibly originate the noted aureola, which must accordingly be set
down as "strictly an ocular nervous phenomenon."[807] It is the less
easy to escape from this conclusion that we find the virtually airless
moon capable of exhibiting a like appendage. Professor Stephen
Alexander, of the United States Survey, with two other observers,
perceived, during the eclipse of the sun of July 18, 1860, the advancing
lunar limb to be bordered with a bright band;[808] and photographic
effects of the same kind appear in pictures of transits of Venus and
partial solar eclipses.
The spectroscope affords little information as to the constitution of
Mercury. Its light is of course that of the sun reflected, and its
spectrum is consequently a faint echo of the Fraunhofer spectrum. Dr. H.
C. Vogel, who first examined it in April, 1871, _suspected_ traces of
the action of an atmosphere like ours,[809] but, it would seem, on
slight grounds. It is, however, certainly very poor in blue rays. More
definite conclusions were, in 1874,[810] derived by Zöllner from
photometric observations of Mercurian phases. A similar study of the
waxing and waning moon had afforded him the curious discovery that
light-changes dependent upon phase vary with the nature of the
reflecting surface, following a totally different law on a smooth
homogeneous globe and on a rugged and mountainous one. Now the phases of
Mercury--so far as could be determined from only two sets of
observations--correspond with the latter kind of structure. Strictly
analogous to those of the moon, they seem to indicate an analogous mode
of surface-formation. This conclusion was fully borne out by Müller's
more extended observations at Potsdam during the years 1885-1893.[811]
Practical assurance was gained from them that the innermost planet has a
rough rind of dusky rock, absorbing all but 17 per cent. of the light
poured upon it by the fierce adjacent sun. Its "albedo," in other words,
is 0·17,[812] which is precisely that ascribed to the moon. The absence
of any appreciable Mercurian atmosphere followed almost necessarily from
these results.
On March 26, 1800, Schröter, observing with his 13-foot reflector in a
peculiarly clear sky, perceived the southern horn of Mercury's crescent
to be quite distinctly blunted.[813] Interception of sunlight by a
Mercurian mountain rather more than eleven English miles high explained
the effect to his satisfaction. By carefully timing its recurrence, he
concluded rotation on an axis in a period of 24 hours 4 minutes. The
first determination of the kind rewarded twenty years of unceasing
vigilance. It received ostensible confirmation from the successive
appearances of a dusky streak and blotch in May and June, 1801.[814]
These, however, were inferred to be no permanent markings on the body of
the planet, but atmospheric formations, the streak at times drifting
forwards (it was thought) under the fluctuating influence of Mercurian
breezes. From a rediscussion of these somewhat doubtful observations
Bessel inferred that Mercury rotates on an axis inclined 70° to the
plane of its orbit in 24 hours 53 seconds.
The rounded appearance of the southern horn seen by Schröter was more or
less doubtfully caught by Noble (1864), Burton, and Franks (1877);[815]
but was obvious to Mr. W. F. Denning at Bristol on the morning of
November 5, 1882.[816] That the southern polar regions are usually less
bright than the northern is well ascertained; but the cause of the
deficiency remains dubious. If inequalities of surface are in question,
they must be on a considerable scale; and a similar explanation might be
given of the deformations of the "terminator"--or dividing-line between
darkness and light in the planet's phases--first remarked by Schröter,
and again clearly seen by Trouvelot in 1878 and 1881.[817] The
displacement, during four days, of certain brilliant and dusky spaces on
the disc indicated to Mr. Denning in 1882 rotation in about twenty-five
hours; while the general aspect of the planet reminded him of that of
Mars.[818] But the difficulties in the way of its observation are
enormously enhanced by its constant close attendance on the sun.
In his sustained study of the features of Mercury, Schröter had no
imitator until Schiaparelli took up the task at Milan in 1882. His
observations were made in daylight. It was found that much more could be
seen, and higher magnifying powers used, high up in the sky near the
sun, than at low altitudes, through the agitated air of morning or
evening twilight. A notable discovery ensued.[819] Following the planet
hour by hour, instead of making necessarily brief inspections at
intervals of about a day, as previous observers had done, it was found
that the markings faintly visible remained sensibly fixed, hence, that
there was no rotation in a period at all comparable with that of the
earth. And after long and patient watching, the conclusion was at last
reached that Mercury turns on his axis in the same time needed to
complete a revolution in his orbit. One of his hemispheres, then, is
always averted from the sun, as one of the moon's hemispheres from the
earth, while the other never shifts from beneath his torrid rays. The
"librations," however, of Mercury are on a larger scale than those of
the moon, because he travels in a more eccentric path. The temporary
inequalities arising between his "even pacing" on an axis and his
alternately accelerated and retarded elliptical movement occasion, in
fact, an oscillation to and fro of the boundaries of light and darkness
on his globe over an arc of 47° 22', in the course of his year of 88
days. Thus the regions of perpetual day and perpetual night are
separated by two segments, amounting to one-fourth of the entire
surface, where the sun rises and sets once in 88 days. Else there is no
variation from the intense glare on one side of the globe, and the
nocturnal blackness on the other.
To Schiaparelli's scrutiny, Mercury appeared as a "spotty globe,"
enveloped in a tolerably dense atmosphere. The brownish stripes and
streaks, discerned on his rose-tinged disc, and judged to be permanent,
were made the basis of a chart. They were not indeed always equally well
seen. They disappeared regularly near the limb, and were at times veiled
even when centrally situated. Some of them had been clearly perceived by
De Ball at Bothkamp in 1882.[820]
Mr. Lowell followed Schiaparelli's example by observing Mercury in the
full glare of noon. "The best time to study him," he remarked, "is when
planetary almanacs state 'Mercury invisible.'" A remarkable series of
drawings executed, some at Flagstaff in 1896, the remainder at Mexico in
1897, supplied grounds for the following, among other, conclusions.[821]
Mercury rotates synchronously with its revolution--that is, once in 88
days--on an axis sensibly perpendicular to its orbital plane. No certain
signs of a Mercurian atmosphere are visible. The globe is seamed and
furrowed with long narrow markings, explicable as cracks in cooling. It
is, and always was, a dead world. From micrometrical measures, moreover,
the inferences were drawn that the planet's mass has a probable value
about 1/20 that of the earth, while its mean density falls considerably
short of the terrestrial standard.
The theory of Mercury's movements has always given trouble. In
Lalande's,[822] as in Mästlin's time, the planet seemed to exist for no
other purpose than to throw discredit on astronomers; and even to
Leverrier's powerful analysis it long proved recalcitrant. On the 12th
of September, 1869, however, he was able to announce before the Academy
of Sciences[823] the terms of a compromise between observation and
calculation. They involved the addition of a new member to the solar
system. The hitherto unrecognised presence of a body about the size of
Mercury itself revolving at somewhat less than half its mean distance
from the sun (or, if farther, then of less mass, and _vice versâ_),
would, it was pointed out, produce exactly the effect required, of
displacing the perihelion of the former planet 38" a century more than
could otherwise be accounted for. The planes of the two orbits, however,
should not lie far apart, as otherwise a nodal disturbance would arise
not perceived to exist. It was added that a ring of asteroids similarly
placed would answer the purpose equally well, and was more likely to
have escaped notice.
Upon the heels of this forecast followed promptly a seeming
verification. Dr. Lescarbault, a physician residing at Orgères, whose
slender opportunities had not blunted his hopes of achievement, had,
ever since 1845, when he witnessed a transit of Mercury, cherished the
idea that an unknown planet might be caught thus projected on the solar
background. Unable to observe continuously until 1858, he, on March 26,
1859, saw what he had expected--a small perfectly round object slowly
traversing the sun's disc. The fruitless expectation of reobserving the
phenomenon, however, kept him silent, and it was not until December 22,
after the news of Leverrier's prediction had reached him, that he wrote
to acquaint him with his supposed discovery.[824] The Imperial
Astronomer thereupon hurried down to Orgères, and by personal inspection
of the simple apparatus used, by searching cross-examination and local
inquiry, convinced himself of the genuine character and substantial
accuracy of the reported observation. He named the new planet "Vulcan,"
and computed elements giving it a period of revolution slightly under
twenty days.[825] But it has never since been seen. M. Liais, director
of the Brazilian Coast Survey, thought himself justified in asserting
that it never had been seen. Observing the sun for twelve minutes after
the supposed ingress recorded at Orgères, he noted those particular
regions of its surface as "très uniformes d'intensité."[826] He
subsequently, however, admitted Lescarbault's good faith, at first
rashly questioned. The planet-seeking doctor was, in truth, only one
among many victims of similar illusions.
Waning interest in the subject was revived by a fresh announcement of a
transit witnessed, it was asserted, by Weber at Peckeloh, April 4,
1876.[827] The pseudo-planet, indeed, was detected shortly afterwards on
the Greenwich photographs, and was found to have been seen by M. Ventosa
at Madrid in its true character of a sun-spot without penumbra; but
Leverrier had meantime undertaken the investigation of a list of twenty
similar dubious appearances, collected by Haase, and republished by Wolf
in 1872.[828] From these, five were picked out as referring in all
likelihood to the same body, the reality of whose existence was now
confidently asserted, and of which more or less probable transits were
fixed for March 22, 1877, and October 15, 1882.[829] But, widespread
watchfulness notwithstanding, no suspicious object came into view at
either epoch.
The next announcement of the discovery of "Vulcan" was on the occasion
of the total solar eclipse of July 29, 1878.[830] This time it was
stated to have been seen at some distance south-west of the obscured
sun, as a ruddy star with a minute planetary disc; and its simultaneous
detection by two observers--the late Professor James C. Watson,
stationed at Rawlins (Wyoming Territory), and Professor Lewis Swift at
Denver (Colorado)--was at first readily admitted. But their separate
observations could, on a closer examination, by no possibility be
brought into harmony, and, if valid, certainly referred to two distinct
objects, if not to four; each astronomer eventually claiming a pair of
planets. Nor could any one of the four be identified with Lescarbault's
and Leverrier's Vulcan, which, if a substantial body revolving round the
sun, must then have been found on the _east_ side of that luminary.[831]
The most feasible explanation of the puzzle seems to be that Watson and
Swift merely saw each the same two stars in Cancer: haste and excitement
doing the rest.[832] Nevertheless, they strenuously maintained their
opposite conviction.[833]
Intra-Mercurian planets have since been diligently searched for when the
opportunity of a total eclipse offered, especially during the long
obscuration at Caroline Island. Not only did Professor Holden "sweep" in
the solar vicinity, but Palisa and Trouvelot agreed to divide the field
of exploration, and thus make sure of whatever planetary prey there
might be within reach; yet with only negative results. Photographic
explorations during recent eclipses have been equally fruitless. Belief
in the presence of any considerable body or bodies within the orbit of
Mercury is, accordingly, at a low ebb. Yet the existence of the anomaly
in the Mercurian movements indicated by Leverrier has been made only
surer by further research.[834] Its elucidation constitutes one of the
"pending problems" of astronomy.
* * * * *
From the observation at Bologna in 1666-67 of some very faint spots,
Domenico Cassini concluded a rotation or libration of Venus--he was not
sure which--in about twenty-three hours.[835] By Bianchini in 1726 the
period was augmented to twenty-four _days_ eight hours. J. J. Cassini,
however, in 1740, showed that the data collected by both observers were
consistent with rotation in twenty-three hours twenty minutes.[836] So
the matter rested until Schröter's time. After watching nine years in
vain, he at last, February 28, 1788, perceived the ordinarily uniform
brightness of the planet's disc to be marbled with a filmy streak, which
returned periodically to the same position in about twenty-three hours
twenty-eight minutes. This approximate estimate was corrected by the
application of a more definite criterion. On December 28, 1789, the
southern horn of the crescent Venus was seen truncated, an outlying
lucid point interrupting the darkness beyond. Precisely the same
appearance recurred two years later, giving for the planet's rotation a
period of 23h. 21m.[837] To this only twenty-two seconds were added by
De Vico, as the result of over 10,000 observations made with the
Cauchoix refractor of the Collegio Romano, 1839-41.[838] The axis of
rotation was found to be much more bowed towards the orbital plane than
that of the earth, the equator making with it an angle of 53° 11'.
These conclusions inspired, it is true, much distrust, consequently
there were no received ideas on the subject to be subverted.
Nevertheless, a shock of surprise was felt at Schiaparelli's
announcement, early in 1890,[839] that Venus most probably rotates after
the fashion just previously ascribed to Mercury. A continuous series of
observations, from November, 1877, to February, 1878, with their records
in above a hundred drawings, supplied the chief part of the data upon
which he rested his conclusions. They certainly appeared exceptionally
well-grounded; and the doubts at first qualifying them were removed by a
fresh set of determinations in July, 1895.[840] Most observers had
depended, in their attempts to ascertain the rotation-period of Venus,
upon evanescent shadings, most likely of atmospheric origin, and
scarcely recognisable from day to day. Schiaparelli fixed his attention
upon round, defined, lustrously white spots, the presence of which near
the cusps of the illuminated crescent has been attested for close upon
two centuries. His steady watch over them showed the invariability of
their position with regard to the terminator; and this is as much as to
say that the regions of day and night do not shift on the surface of the
planet. In other words, she keeps the same face always turned towards
the sun. Moreover, since her orbit is nearly circular, libratory effects
are very small. They amount in fact to only just one-thirtieth of those
serving to modify the severe contrasts of climate in Mercury.
Confirmatory evidence of Schiaparelli's result for Venus is not wanting.
Thus, observations irreconcilable with a swift rate of rotation were
made at Bothkamp in 1871 by Vogel and Lohse;[841] and a drawing executed
by Professor Holden with the great Washington reflector, December 15,
1877, showed the same markings in the positions recorded at Milan to
have been occupied by them eight hours previously. Further, a series of
observations, carried out by M. Perrotin at Nice, May 15 to October 4,
1890, and from Mount Mounier in 1895-6, with the special aim of testing
the inference of synchronous rotation and revolution, proved strongly
corroborative of it.[842] A remarkable collection of drawings made by
Mr. Lowell in 1896 appeared decisive in its favour;[843] Tacchini at
Rome,[844] Mascari at Catania and Etna,[845] Cerulli at Terano,[846]
obtained in 1892-6 evidence similar in purport. On the other hand,
Niesten of Brussels found reason to revert to Vico's discarded elements
for the planet's rotation;[847] and Trouvelot,[848] Stanley
Williams,[849] Villiger,[850] and Leo Brenner,[851] so far agreed with
him as to adopt a period of approximately twenty-four hours. Finally, E.
Von Oppolzer suggested an appeal to the spectroscope;[852] and
Bélopolsky secured in 1900[853] spectrograms apparently marked by the
minute displacements corresponding to a rapid rate of axial movement.
But they were avowedly taken only as an experiment, with unsuitable
apparatus; and the desirable verification of their supposed import is
not yet forthcoming. Until it is, Schiaparelli's period of 225 days must
be allowed to hold the field.
Effects attributed to great differences of level in the surface of Venus
have struck many observers. Francesco Fontana at Naples in 1643 noticed
irregularities along the inner edge of the crescent.[854] Lahire in 1700
considered them--regard being had to difference of distance--to be much
more strongly marked than those visible in the moon.[855] Schröter's
assertions to the same effect, though scouted with some unnecessary
vehemence by Herschel,[856] have since been repeatedly confirmed;
amongst others by Mädler, De Vico, Langdon, who in 1873 saw the broken
line of the terminator with peculiar distinctness through a veil of
auroral cloud;[857] by Denning,[858] March 30, 1881, despite preliminary
impressions to the contrary, as well as by C. V. Zenger at Prague,
January 8, 1883. The great mountain mass, presumed to occasion the
periodical blunting of the southern horn, was precariously estimated by
the Lilienthal observer to rise to the prodigious height of nearly
twenty-seven miles, or just five times the elevation of Mount Everest!
Yet the phenomenon persists, whatever may be thought of the explanation.
Moreover, the speck of light beyond, interpreted as the visible sign of
a detached peak rising high enough above the encircling shadow to catch
the first and last rays of the sun, was frequently discerned by Baron
Van Ertborn in 1876;[859] while an object near the northern horn of the
crescent, strongly resembling a lunar ring-mountain, was delineated both
by De Vico in 1841 and by Denning forty years later.
We are almost equally sure that Venus, as that the earth is encompassed
with an atmosphere. Yet, notwithstanding luminous appearances plainly
due to refraction during the transits both of 1761 and 1769, Schröter,
in 1792, took the initiative in coming to a definite conclusion on the
subject.[860] It was founded, first, on the rapid diminution of
brilliancy towards the terminator, attributed to atmospheric absorption;
next, on the extension beyond a semicircle of the horns of the crescent;
lastly, on the presence of a bluish gleam illuminating the early hours
of the Cytherean night with what was taken to be genuine twilight. Even
Herschel admitted that sunlight, by the same effect through which the
heavenly bodies show _visibly above_ our horizons while still
_geometrically below_ them, appeared to be bent round the shoulder of
the globe of Venus. Ample confirmation of the fact has since been
afforded. At Dorpat in May, 1849, the planet being within 3° 26' of
inferior conjunction, Mädler found the arms of waning light upon the
disc to embrace no less than 240° of its extent;[861] and in December,
1842, Mr. Guthrie, of Bervie, N.B., actually observed, under similar
conditions, the whole circumference to be lit up with a faint nebulous
glow.[862] The same curious phenomenon was intermittently seen by Mr.
Leeson Prince at Uckfield in September, 1861;[863] but with more
satisfactory distinctness by Mr. C. S. Lyman of Yale College,[864]
before and after the conjunction of December 11, 1866, and during nearly
five hours previous to the transit of 1874, when the yellowish ring of
refracted light showed at one point an approach to interruption,
possibly through the intervention of a bank of clouds. Again, on
December 2, 1898, Venus being 1° 45' from the sun's centre, Mr. H. N.
Russell, of the Halsted Observatory, descried the coalescence of the
cusps, and founded on the observation a valuable discussion of such
effects.[865] Taking account of certain features in the case left
unnoticed by Neison[866] and Proctor,[867] he inferred from them the
presence of a Cytherean atmosphere considerably less refractive than our
own, although possibly, in its lower strata, encumbered with dust or
haze.
Similar appearances are conspicuous during transits. But while the
Mercurian halo is characteristically seen on the sun, the "silver
thread" round the limb of Venus commonly shows on the part _off_ the
sun. There are, however, instances of each description in both cases.
Mr. Grant, in collecting the records of physical phenomena accompanying
the transits of 1761 and 1769, remarks that no one person saw both kinds
of annulus, and argues a dissimilarity in their respective modes of
production.[868] Such a dissimilarity probably exists, in the sense that
the inner section of the ring is illusory, the outer, a genuine result
of the bending of light in a gaseous envelope; but the distinction of
separate visibility has not been borne out by recent experience. Several
of the Australian observers during the transit of 1874 witnessed the
complete phenomenon. Mr. J. Macdonnell, at Eden, saw a "shadowy nebulous
ring" surround the whole disc when ingress was two-thirds accomplished;
Mr. Tornaghi, at Goulburn, perceived a halo, entire and unmistakable, at
half egress.[869] Similar observations were made at Sydney,[870] and
were renewed in 1882 by Lescarbault at Orgères, by Metzger in Java, and
by Barnard at Vanderbilt University.[871]
Spectroscopic indications of aqueous vapour as present in the atmosphere
of Venus, were obtained in 1874 and 1882, by Tacchini and Riccò in
Italy, and by Young in New Jersey.[872] Janssen, however, who made a
special study of the point subsequently to the transit of 1882, found
them much less certain than he had anticipated;[873] and Vogel, by
repeated examinations, 1871-73, could detect only the very slightest
variations from the pattern of the solar spectrum. Some additions there
indeed seem to be in the thickening of a few water and oxygen-lines; but
so nearly evanescent as to induce the persuasion that most of the light
we receive from Venus has traversed only the tenuous upper portion of
its atmosphere.[874] It is reflected, at any rate, with comparatively
slight diminution. On the 26th and 27th of September, 1878, a close
conjunction gave Mr. James Nasmyth the rare opportunity of watching
Venus and Mercury for several hours side by side in the field of his
reflector; when the former appeared to him like clean silver, the latter
as dull as lead or zinc.[875] Yet the light _incident_ upon Mercury is,
on an average, three and a half times as strong as the light reaching
Venus. Thus, the reflective power of Venus must be singularly strong.
And we find, accordingly, from a combination of Zöllner's with Müller's
results, that its albedo is but little inferior to that of new-fallen
snow; in other words, it gives back 77 per cent. of the luminous rays
impinging upon it.
This extraordinary brilliancy would be intelligible were it permissible
to suppose that we see nothing of the planet but a dense canopy of
clouds. But the hypothesis is discountenanced by the Flagstaff
observations, and is irreconcilable with the visibility of mountainous
elevations, and permanent surface-markings. To Mr. Lowell these were so
distinct and unchanging as to furnish data for a chart of the Cytherean
globe, and the peculiar arrangement of divergent shading exhibited in it
cannot off-hand be set down as unreal, in view of Perrotin's earlier
discernment of analogous linear traces. Gruithuisen's "snow-caps,"[876]
however--it is safe to say--do not exist as such; although shining
regions near the poles form a well-attested trait of the strange
Cytherean landscape.
The "secondary," or "ashen light," of Venus was first noticed by
Riccioli in 1643; it was seen by Derham about 1715, by Kirch in 1721, by
Schröter and Harding in 1806;[877] and the reality of the appearance has
since been authenticated by numerous and trustworthy observations. It is
precisely similar to that of the "old moon in the new moon's arms"; and
Zenger, who witnessed it with unusual distinctness, January 8,
1883,[878] supposes it due to the same cause--namely, to the faint gleam
of reflected earth-light from the night-side of the planet. When we
remember, however, that "full earth-light" on Venus, at its nearest, has
little more than 1/12000 its intensity on the moon, we see at once that
the explanation is inadequate. Nor can Professor Safarik's,[879] by
phosphorescence of the warm and teeming oceans with which Zöllner[880]
regarded the globe of Venus as mainly covered, be seriously entertained.
Vogel's suggestion is more plausible. He and O. Lohse, at Bothkamp,
November 3 to 11, 1871, saw the dark hemisphere _partially_ illuminated
by secondary light, extending 30° from the terminator, and thought the
effect might be produced by a very extensive twilight.[881] Others have
had recourse to the analogy of our auroræ, and J. Lamp suggested that
the grayish gleam, visible to him at Bothkamp, October 21 and 26,
1887,[882] might be an accompaniment of electrical processes connected
with the planet's meteorology. Whatever the origin of the phenomenon, it
may serve, on a night-enwrapt hemisphere, to dissipate some of the thick
darkness otherwise encroached upon only by "the pale light of stars."
Venus was once supposed to possess a satellite. But belief in its
existence has died out. No one, indeed, has caught even a deceptive
glimpse of such an object during the last 125 years. Yet it was
repeatedly and, one might have thought, well observed in the seventeenth
and eighteenth centuries. Fontana "discovered" it in 1645; Cassini--an
adept in the art of seeing--recognised it in 1672, and again in 1686;
Short watched it for a full hour in 1740 with varied instrumental means;
Tobias Mayer in 1759, Montaigne in 1761; several astronomers at
Copenhagen in March, 1764, noted what they considered its unmistakable
presence; as did Horrebow in 1768. But M. Paul Stroobant,[883] who in
1887 submitted all the available data on the subject to a searching
examination, identified Horrebow's satellite with Theta Libræ, a
fifth-magnitude star; and a few other apparitions were, by his industry,
similarly explained away. Nevertheless, several withstood all efforts to
account for them, and together form a most curious case of illusion. For
it is quite certain that Venus has no such conspicuous attendant.
* * * * *
The third planet encountered in travelling outward from the sun is the
abode of man. He has in consequence opportunities for studying its
physical habitudes altogether different from the baffling glimpse
afforded to him of the other members of the solar family.
Regarding the earth, then, a mass of knowledge so varied and
comprehensive has been accumulated as to form a science--or rather
several sciences--apart. But underneath all lie astronomical relations,
the recognition and investigation of which constitute one of the most
significant intellectual events of the present century.
It is indeed far from easy to draw a line of logical distinction between
items of knowledge which have their proper place here, and those which
should be left to the historian of geology. There are some, however, of
which the cosmical connections are so close that it is impossible to
overlook them. Among these is the ascertainment of the solidity of the
globe. At first sight it seems difficult to conceive what the apparent
positions of the stars can have to do with subterranean conditions; yet
it was from star measurements alone that Hopkins, in 1839, concluded the
earth to be solid to a depth of at least 800 or 1,000 miles.[884] His
argument was, that if it were a mere shell filled with liquid,
precession and nutation would be much larger than they are observed to
be. For the shell alone would follow the pull of the sun and moon on its
equatorial girdle, leaving the liquid behind; and being thus so much the
lighter, would move the more readily. There is, it is true, grave reason
to doubt whether this reasoning corresponds with the actual facts of the
case;[885] but the conclusion to which it led has been otherwise
affirmed and extended.
Indications of an identical purport have been derived from another kind
of external disturbance, affecting our globe through the same agencies.
Lord Kelvin (then Sir William Thomson) pointed out in 1862[886] that
tidal influences are brought to bear on land as well as on water,
although obedience to them is perceptible only in the mobile element.
Some bodily distortion of the earth's figure _must_, however, take
place, unless we suppose it of absolute or "preternatural" rigidity, and
the amount of such distortion can be determined from its effect in
diminishing oceanic tides below their calculated value. For if the earth
were perfectly plastic to the stresses of solar and lunar gravity,
tides--in the ordinary sense--would not exist. Continents and oceans
would swell and subside together. It is to the _difference_ in the
behaviour of solid and liquid terrestrial constituents that the ebb and
flow of the waters are due.
Six years later, the distinguished Glasgow professor suggested that this
criterion might, by the aid of a prolonged series of exact tidal
observations, be practically applied to test the interior condition of
our planet.[887] In 1882, accordingly, suitable data extending over
thirty-three years having at length become available, Mr. G. H. Darwin
performed the laborious task of their analysis, with the general result
that the "effective rigidity" of the earth's mass must be _at least_ as
great as that of steel.[888]
Ratification from an unexpected quarter has lately been brought to this
conclusion. The question of a possible mobility in the earth's axis of
rotation has often been mooted. Now at last it has received an
affirmative reply. Dr. Küstner detected, in his observations of 1884-85,
effects apparently springing from a minute variation in the latitude of
Berlin. The matter having been brought before the International Geodetic
Association in 1888, special observations were set on foot at Berlin,
Potsdam, Prague, and Strasbourg, the upshot of which was to bring
plainly to view synchronous, and seemingly periodic fluctuations of
latitude to the extent of half a second of arc. The reality of these was
verified by an expedition to Honolulu in 1891-92, the variations there
corresponding inversely to those simultaneously determined in
Europe.[889] Their character was completely defined by Mr. S. C.
Chandler's discussion in October, 1891.[890] He showed that they could
be explained by supposing the pole of the earth to describe a circle
with a radius of thirty feet in a period of fourteen months.
Confirmation of this hypothesis was found by Dr. B. A. Gould in the
Cordoba observations,[891] and it was provided with a physical basis
through the able co-operation of Professor Newcomb.[892] The earth,
owing to its ellipsoidal shape, should, apart from disturbance, rotate
upon its "axis of figure," or shortest diameter; since thus alone can
the centrifugal forces generated by its spinning balance each other.
Temporary causes, however, such as heavy falls of snow or rain limited
to one continental area, the shifting of ice-masses, even the movements
of winds, may render the globe slightly lop-sided, and thus oblige it to
forsake its normal axis, and rotate on one somewhat divergent from it.
This "instantaneous axis" (for it is incessantly changing) must, by
mathematical theory, revolve round the axis of figure in a period of 306
days. Provided, that is to say, the earth were a perfectly rigid body.
But it is far from being so; it yields sensibly to every strain put upon
it; and this yielding tends to protract the time of circulation of the
displaced pole. The length of its period, then, serves as a kind of
measure of the plasticity of the globe; which, according to Newcomb's
and S. S. Hough's independent calculations,[893] seems to be a little
less than that of steel. In an earth compacted of steel, the
instantaneous axis would revolve in 441 days; in the actual earth, the
process is accomplished in 428 days. By this new path, accordingly,
astronomers have been led to an identical estimate of the consistence of
our globe with that derived from tidal investigations.
Variations of latitude are intrinsically complex. To produce them, an
incalculable interplay of causes must be at work, each with its proper
period and law of action.[894] All the elements of the phenomenon are
then in a perpetual state of flux,[895] and absorb for their continual
redetermination, the arduous and combined labours of many astronomers.
Nor is this trouble superfluous. Minute in extent though they be, the
shiftings of the pole menace the very foundations of exact celestial
science; their neglect would leave the entire fabric insecure. Just at
the beginning of the present century they reached a predicted minimum,
but are expected again to augment their range after the year 1902. The
interesting suggestion has been made by Mr. J. Halm that such
fluctuations are, in some obscure way, affected by changes in solar
activity, and conform like them to an eleven-year cycle.[896]
In a paper read before the Geological Society, December 15, 1830,[897]
Sir John Herschel threw out the idea that the perplexing changes of
climate revealed by the geological record might be explained through
certain slow fluctuations in the eccentricity of the earth's orbit,
produced by the disturbing action of the other planets. Shortly
afterwards, however, he abandoned the position as untenable;[898] and it
was left to the late Dr. James Croll, in 1864[899] and subsequent years,
to reoccupy and fortify it. Within restricted limits (as Lagrange and,
more certainly and definitely, Leverrier proved), the path pursued by
our planet round the sun alternately contracts, in the course of ages,
into a moderate ellipse, and expands almost to a circle, the major axis,
and consequently the mean distance, remaining invariable. Even at
present, when the eccentricity approaches a minimum, the sun is nearer
to us in January than in July by above three million miles, and some
850,000 years ago this difference was more than four times as great. Dr.
Croll brought together[900] a mass of evidence to support the view,
that, at epochs of considerable eccentricity, the hemisphere of which
the winter, occurring at aphelion, was both intensified and prolonged,
must have undergone extensive glaciation; while the opposite hemisphere,
with a short, mild winter, and long, cool summer, enjoyed an approach to
perennial spring. These conditions were exactly reversed at the end of
10,500 years, through the shifting of the perihelion combined with the
precession of the equinoxes, the frozen hemisphere blooming into a
luxuriant garden as its seasons came round to occur at the opposite
sites of the terrestrial orbit, and the vernal hemisphere subsiding
simultaneously into ice-bound rigour.[901] Thus a plausible explanation
was offered of the anomalous alternations of glacial and semi-tropical
periods, attested, on incontrovertible geological evidence, as having
succeeded each other in times past over what are now temperate regions.
They succeeded each other, it is true, with much less frequency and
regularity than the theory demanded; but the discrepancy was overlooked
or smoothed away. The most recent glacial epoch was placed by Dr. Croll
about 200,000 years ago, when the eccentricity of the earth's orbit was
3·4 times as great as it is now. At present a faint representation of
such a state of things is afforded by the southern hemisphere. One
condition of glaciation in the coincidence of winter with the maximum of
remoteness from the sun, is present; the other--a high eccentricity--is
deficient. Yet the ring of ice-bound territory hemming in the southern
pole is well known to be far more extensive than the corresponding
region in the north.
The verification of this ingenious hypothesis depends upon a variety of
intricate meteorological conditions, some of which have been adversely
interpreted by competent authorities.[902] What is still more serious,
its acceptance seems precluded by time-relations of a simple kind. Dr.
Wright[903] has established with some approach to certainty that glacial
conditions ceased in Canada and the United States about ten or twelve
thousand years ago. The erosive action of the Falls of Niagara qualifies
them to serve as a clepsydra, or water-clock on a grand scale; and their
chronological indications have been amply corroborated elsewhere and
otherwise on the same continent. The astronomical Ice Age, however,
should have been enormously more antique. No reconciliation of the facts
with the theory appears possible.
The first attempt at an experimental estimate of the "mean density" of
the earth was Maskelyne's observation in 1774 of the deflection of a
plumb-line through the attraction of Schehallien. The conclusion thence
derived, that our globe weighs 4-1/2 times as much as an equal bulk of
water,[904] was not very exact. It was considerably improved upon by
Cavendish, who, in 1798, brought into use the "torsion-balance"
constructed for the same purpose by John Michell. The resulting estimate
of 5·48 was raised to 5·66 by Francis Baily's elaborate repetition of
the process in 1838-42. From experiments on the subject made in 1872-73
by Cornu and Baille the slightly inferior value of 5·56 was derived; and
it was further shown that the data collected by Baily, when corrected
for a systematic error, gave practically the same result (5·55).[905] M.
Wilsing's of 5·58, obtained at Potsdam in 1889,[906] nearly agreed with
it; while Professor Poynting, by means of a common balance, arrived at a
terrestrial mean density of 5·49.[907] Professor Boys next entered the
field with an exquisite apparatus, in which a quartz fibre performed the
functions of a torsion-rod; and the figure 5·53 determined by him, and
exactly confirmed by Dr. Braun's research at Mariaschein, Bohemia, in
1896,[908] may be called the standard value of the required datum.
Newton's guess at the average weight of the earth as five or six times
that of water has thus been curiously verified.
Operations for determining the figure of the earth were carried out during
the last century on an unprecedented scale. The Russo-Scandinavian arc,
of which the measurement was completed under the direction of the elder
Struve in 1855, reached from Hammerfest to Ismailia on the Danube,
a length of 25° 20'. But little inferior to it was the Indian arc, begun
by Lambton in the first years of the century, continued by Everest,
revised and extended by Walker. Both were surpassed in compass by the
Anglo-French arc, which embraced 28°; and considerable segments of
meridians near the Atlantic and Pacific shores of North America were
measured under the auspices of the United States Coast Survey. But these
operations shrink into insignificance by comparison with Sir David
Gill's grandiose scheme for uniting two hemispheres by a continuous
network of triangulation. The history of geodesy in South Africa began
with Lacaille's measurements in 1752. They were repeated and enlarged in
scope by Sir Thomas Maclear in 1841-48; and his determinations prepared
the way for a complete survey of Cape Colony and Natal, executed during
the ten years 1883-92 by Colonel Morris, R.E., under the direction of
Sir David Gill.[909] Bechuanaland and Rhodesia were subsequently
included in the work; and the Royal Astronomer obtained, in 1900, the
support of the International Geodetic Association for its extension to
the mouth of the Nile. Nor was this the limit of his design. By carrying
the survey along the Levantine coast, connection can be established with
Struve's system, and the magnificent amplitude of 105° will be given to
the conjoined African and European arcs. Meantime, the French have
undertaken the remeasurement of Bouguer's Peruvian arc, and a
corresponding Russo-Swedish[910] enterprise is progressing in
Spitzbergen; so that abundant materials will ere long be provided for
fresh investigations of the shape and size of our planet. The smallness
of the outstanding uncertainty can be judged of by comparing J. B.
Listing's[911] with General Clarke's[912] results, published in the same
year (1878). Listing stated the dimensions of the terrestrial spheroid
as follows: Equatorial radius = 3,960 miles; polar radius = 3,947 miles;
ellipticity = 1/288·5. Clarke's corresponding figures were: 3,963 and
3,950 miles, giving an ellipticity of 1/293·5. The value of the latter
fraction at present generally adopted is 1/292; that is to say, the
thickness of the protuberant equatorial ring is held to be 1/292 of the
equatorial radius. From astronomical considerations, it is true, Newcomb
estimated the ratio at 1/308;[913] but for obtaining this particular
datum, geodetical methods are unquestionably to be preferred.
* * * * *
The moon possesses for us a unique interest. She in all probability
shared the origin of the earth; she perhaps prefigures its decay. She is
at present its minister and companion. Her existence, so far as we can
see, serves no other purpose than to illuminate the darkness of
terrestrial nights, and to measure, by swiftly-recurring and conspicuous
changes of aspect, the long span of terrestrial time. Inquiries
stimulated by visible dependence, and aided by relatively close
vicinity, have resulted in a wonderfully minute acquaintance with the
features of the single lunar hemisphere open to our inspection.
Selenography, in the modern sense, is little more than a hundred years
old. It originated with the publication in 1791 of Schröter's
_Selenotopographische Fragmente_.[914] Not but that the lunar surface
had already been diligently studied, chiefly by Hevelius, Cassini,
Riccioli, and Tobias Mayer; the idea, however, of investigating the
moon's physical condition, and detecting symptoms of the activity there
of natural forces through minute topographical inquiry, first obtained
effect at Lilienthal. Schröter's delineations, accordingly, imperfect
though they were, afforded a starting-point for a _comparative_ study of
the superficial features of our satellite.
The first of the curious objects which he named "rills" was noted by him
in 1787. Before 1801 he had found eleven; Lohrmann added 75; Mädler 55;
Schmidt published in 1866 a catalogue of 425, of which 278 had been
detected by himself;[915] and he eventually brought the number up to
nearly 1,000. They are, then, a very persistent lunar feature, though
wholly without terrestrial analogue. There is no difference of opinion
as to their nature. They are quite obviously clefts in a rocky surface,
100 to 500 yards deep, usually a couple of miles across, and pursuing
straight, curved, or branching tracks up to 150 miles in length. As
regards their origin, the most probable view is that they are fissures
produced in cooling; but Neison inclines to consider them rather as
dried watercourses.[916]
On February 24, 1792, Schröter perceived what he took to be distinct
traces of a lunar twilight, and continued to observe them during nine
consecutive years.[917] They indicated, he thought, the presence of a
shallow atmosphere, about 29 times more tenuous than our own. Bessel, on
the other hand, considered that the only way of "saving" a lunar
atmosphere was to deny it any refractive power, the sharpness and
suddenness of star-occultations negativing the possibility of gaseous
surroundings of greater density (admitting an extreme supposition) than
1/500 that of terrestrial air.[918] Newcomb places the maximum at 1/400.
Sir John Herschel concluded "the non-existence of any atmosphere at the
moon's edge having 1/1980 part of the density of the earth's
atmosphere."[919]
This decision was fully borne out by Sir William Huggins's spectroscopic
observation of the disappearance behind the moon's limb of the small
star Eta Piscium, January 4, 1865.[920] Not the slightest sign of
selective absorption or unequal refraction was discernible. The entire
spectrum went out at once, as if a slide had suddenly dropped over it.
The spectroscope has uniformly told the same tale; for M. Thollon's
observation during the total solar eclipse at Sohag of a supposed
thickening at the moon's rim, of certain dark lines in the solar
spectrum, is now acknowledged to have been illusory. Moonlight, analysed
with the prism, is found to be pure reflected sunlight, diminished in
_quantity_, owing to the low reflective capability of the lunar surface,
to less than one-fifth its incident intensity, but wholly unmodified in
_quality_.
Nevertheless, the diameter of the moon appeared from the Greenwich
observations discussed by Airy in 1865[921] to be 4" smaller than when
directly measured; and the effect would be explicable by refraction in a
lunar atmosphere 2,000 times thinner than our own at the sea-level. But
the difference was probably illusory. It resulted in part, if not
wholly, from the visual enlargement by irradiation of the bright disc of
the moon. Professor Comstock, employing the 16-inch Clark equatoreal of
the Washburn Observatory, found in 1897 the refractive displacements of
occulted stars so trifling as to preclude the existence of a permanent
lunar atmosphere of much more than 1/5000 the density of the terrestrial
envelope.[922] The possibility, however, was admitted that, on the
illuminated side of the moon, temporary exhalations of aqueous vapour
might arise from ice-strata evaporated by sun-heat. Meantime, some
renewed evidence of actual crepuscular gleams on the moon had been
gathered by MM. Paul and Prosper Henry of the Paris Observatory, as well
as by Mr. W. H. Pickering, in the pure air of Arequipa, at an altitude
of 8,000 feet above the sea.[923] An occultation of Jupiter, too,
observed by him August 12, 1892,[924] was attended with a slight
flattening of the planet's disc through the effect, it was supposed, of
lunar refraction--but of refraction in an atmosphere possessing, at the
most, 1/4000 the density at the sea-level of terrestrial air, and
capable of holding in equilibrium no more than 1/250 of an inch of
mercury. Yet this small barometric value corresponds, Mr. Pickering
remarks, "to a pressure of hundreds of tons per square mile of the lunar
surface." The compression downward of gaseous strata on the moon should,
in any case, proceed very gradually, owing to the slight power of lunar
gravity,[925] and they might hence play an important part in the economy
of our satellite while evading spectroscopic and other tests. Thus--as
Mr. Ranyard remarked[926]--the cliffs and pinnacles of the moon bear
witness, by their unworn condition, to the efficiency of atmospheric
protection against meteoric bombardment; and Mr. Pickering shows that it
could be afforded by such a tenuous envelope as that postulated by him.
The first to emulate Schröter's selenographical zeal was Wilhelm
Gotthelf Lohrmann, a land-surveyor of Dresden, who, in 1824, published
four out of twenty-five sections of the first scientifically executed
lunar chart, on a scale of 37-1/2 inches to a lunar diameter. His sight,
however, began to fail three years later, and he died in 1840, leaving
materials from which the work was completed and published in 1878 by Dr.
Julius Schmidt, late director of the Athens Observatory. Much had been
done in the interim. Beer and Mädler began at Berlin in 1830 their great
trigonometrical survey of the lunar surface, as yet neither revised nor
superseded. A map, issued in four parts, 1834-36, on nearly the same
scale as Lohrmann's, but more detailed and authoritative, embodied the
results. It was succeeded, in 1837, by a descriptive volume bearing the
imposing title, _Der Mond; oder allgemeine vergleichende Selenographie_.
This summation of knowledge in that branch, though in truth leaving many
questions open, had an air of finality which tended to discourage
further inquiry.[927] It gave form to a reaction against the sanguine
views entertained by Hevelius, Schröter, Herschel and Gruithuisen as to
the possibilities of agreeable residence on the moon, and relegated the
"Selenites," one of whose cities Schröter thought he had discovered, and
of whose festal processions Gruithuisen had not despaired of becoming a
spectator, to the shadowy land of the Ivory Gate. All examples of change
in lunar formations were, moreover, dismissed as illusory. The light
contained in the work was, in short, a "dry light," not stimulating to
the imagination. "A mixture of a lie," Bacon shrewdly remarks, "doth
ever add pleasure." For many years, accordingly, Schmidt had the field
of selenography almost to himself.
Reviving interest in the subject was at once excited and displayed by
the appointment, in 1864, of a Lunar Committee of the British
Association. The indirect were of greater value than the direct fruits
of its labours. An English school of selenography rose into importance.
Popularity was gained for the subject by the diffusion of works
conspicuous for ingenuity and research. Nasmyth's and Carpenter's
beautifully illustrated volume (1874) was succeeded, after two years, by
a still more weighty contribution to lunar science in Mr. Neison's
well-known book, accompanied by a map, based on the survey of Beer and
Mädler, but adding some 500 measures of positions, besides the
representation of several thousand new objects. With Schmidt's _Charte
der Gebirge der Mondes_, Germany once more took the lead. This splendid
delineation, built upon Lohrmann's foundation, embraced the detail
contained in upwards of 3,000 original drawings, representing the labour
of thirty-four years. No less than 32,856 craters are represented in it,
on a scale of seventy-five inches to a diameter. An additional help to
lunar inquiries was provided at the same time in this country by the
establishment, through the initiative of the late Mr. W. R. Birt, of the
Selenographical Society.
But the strongest incentive to diligence in studying the rugged features
of our celestial helpmate has been the idea of probable or actual
variation in them. A change always seems to the inquisitive intellect of
man like a breach in the defences of Nature's secrets, through which it
may hope to make its way to the citadel. What is desirable easily
becomes credible; and thus statements and rumours of lunar convulsions
have successively, during the last hundred years, obtained credence, and
successively, on closer investigation, been rejected. The subject is one
as to which illusion is peculiarly easy. Our view of the moon's surface
is a bird's-eye view. Its conformation reveals itself indirectly through
irregularities in the distribution of light and darkness. The forms of
its elevations and depressions can be inferred only from the shapes of
the black, unmitigated shadows cast by them. But these shapes are in a
state of perpetual and bewildering fluctuation, partly through changes
in the angle of illumination, partly through changes in our point of
view, caused by what are called the moon's "librations."[928] The result
is, that no single observation can be _exactly_ repeated by the same
observer, since identical conditions recur only after the lapse of a
great number of years.
Local peculiarities of surface, besides, are liable to produce
perplexing effects. The reflection of earth-light at a particular angle
from certain bright summits completely, though temporarily, deceived
Herschel into the belief that he had witnessed, in 1783 and 1787,
volcanic outbursts on the dark side of the moon. The persistent
recurrence, indeed, of similar appearances under circumstances less
amenable to explanation inclined Webb to the view that effusions of
native light actually occur.[929] More cogent proofs must, however, be
adduced before a fact so intrinsically improbable can be admitted as
true.
But from the publication of Beer and Mädler's work until 1866, the
received opinion was that no genuine sign of activity had ever been
seen, or was likely to be seen, on our satellite; that her face was a
stereotyped page, a fixed and irrevisable record of the past. A profound
sensation, accordingly, was produced by Schmidt's announcement, in
October, 1866, that the crater "Linné," in the Mare Serenitatis, had
disappeared,[930] effaced, as it was supposed, by an igneous outflow.
The case seemed undeniable, and is still dubious. Linné had been known
to Lohrmann and Mädler, 1822-32, as a deep crater, five or six miles in
diameter, the third largest in the dusky plain known as the "Mare
Serenitatis"; and Schmidt had observed and drawn it, 1840-43, under a
practically identical aspect. Now it appears under high light as a
whitish spot, in the centre of which, as the rays begin to fall
obliquely, a pit, scarcely two miles across, emerges into view.[931] The
crateral character of this comparatively minute depression was detected
by Father Secchi, February 11, 1867.
This is not all. Schröter's description of Linné, as seen by him
November 5, 1788, tallies quite closely with modern observation;[932]
while its inconspicuousness in 1797 is shown by its omission from
Russell's lunar globe and maps.[933] We are thus driven to adopt one of
two suppositions: either Lohrmann, Mädler, and Schmidt were entirely
mistaken in the size and importance of Linné, or a real change in its
outward semblance supervened during the first half of the century, and
has since passed away, perhaps again to recur. The latter hypothesis
seems the more probable: and its probability is strengthened by much
evidence of actual obscuration or variation of tint in other parts of
the lunar surface, more especially on the floor of the great "walled
plain" named "Plato."[934] From a re-examination with a 13-inch
refractor at Arequipa in 1891-92, of this region, and of the Mare
Serenitatis, Mr. W. H. Pickering inclines to the belief that lunar
volcanic action, once apparently so potent, is not yet wholly
extinct.[935]
An instance of an opposite kind of change was alleged by Dr. Hermann J.
Klein of Cologne in March, 1878.[936] In Linné the obliteration of an
old crater had been assumed; in "Hyginus N.," the formation of a new
crater was asserted. Yet, quite possibly, the same cause may have
produced the effects thought to be apparent in both. It is, however, far
from certain that any real change has affected the neighbourhood of
Hyginus. The novelty of Klein's observation of May 19, 1877, may have
consisted simply in the detection of a hitherto unrecognised feature.
The region is one of complex formation, consequently of more than
ordinary liability to deceptive variations in aspect under rapid and
entangled fluctuations of light and shade.[937] Moreover, it seems to be
certain, from Messrs. Pratt and Capron's attentive study, that "Hyginus
N." is no true crater, but a shallow, saucer-like depression, difficult
of clear discernment.[938] Under suitable illumination, nevertheless, it
contains, and is marked by, an ample shadow.[939]
In both these controverted instances of change, lunar photography was
invoked as a witness; but, notwithstanding the great advances made in
the art by De la Rue in this country, by Draper, and, above all, by
Rutherford in America, without decisive results. Investigations of the
kind began to assume a new aspect in 1890, when Professor Holden
organised them at the Lick Observatory.[940] Autographic moon-pictures
were no longer taken casually, but on system; and Dr. Weinek's elaborate
study, and skilful reproductions of them at Prague,[941] gave them
universal value. They were designed to provide materials for an atlas on
the scale of Beer and Mädler's, of which some beautiful specimen-plates
have been issued. At Paris, in 1894, with the aid of a large "equatoreal
coudé," a work of similar character was set on foot by MM. Loewy and
Puiseux. Its progress has been marked by the successive publication of
five instalments of a splendid atlas, on a scale of about eight feet to
the lunar diameter, accompanied by theoretical dissertations, designed
to establish a science of "selenology." The moon's formations are thus
not only delineated under every variety of light-incidence, but their
meaning is sought to be elicited, and their history and mutual relations
interpreted.[942] Henceforth, at any rate, the lunar volcanoes can
scarcely, without notice taken, breathe hard in their age-long sleep.
Melloni was the first to get undeniable heating effects from moonlight.
His experiments, made on Mount Vesuvius early in 1846,[943] were
repeated with like result by Zantedeschi at Venice four years later. A
rough measure of the intensity of those effects was arrived at by Piazzi
Smyth at Guajara, on the Peak of Teneriffe, in 1856. At a distance of
fifteen feet from the thermomultiplier, a Price's candle was found to
radiate just twice as much heat as the full moon.[944] Then, after
thirteen years, in 1869-72, an exact and extensive series of
observations on the subject were made by the present Earl of Rosse. The
lunar radiations, from the first to the last quarter, displayed, when
concentrated with the Parsonstown three-foot mirror, appreciable thermal
energy, increasing with the phase, and largely due to "dark heat,"
distinguished from the quicker-vibrating sort by inability to traverse a
plate of glass. This was supposed to indicate an actual heating of the
surface, during the long lunar day of 300 hours, to about 500° F.[945]
(corrected later to 197°),[946] the moon thus acting as a direct
radiator no less than as a reflector of heat. But the conclusion was
very imperfectly borne out by Dr. Boeddicker's observations with the
same instrument and apparatus during the total lunar eclipse of October
4, 1884.[947] This initial opportunity of measuring the heat phases of
an eclipsed moon was used with the remarkable result of showing that the
heat disappeared almost completely, though not quite simultaneously,
with the light. Confirmatory evidence of the extraordinary promptitude
with which our satellite parts with heat already to some extent
appropriated, was afforded by Professor Langley's bolometric
observations at Allegheny of the partial eclipse of September 23,
1885.[948] Yet it is certain that the moon sends us a perceptible
quantity of heat _on its own account_, besides simply throwing back
solar radiations. For in February, 1885, Professor Langley succeeded,
after many fruitless attempts, in getting measures of a "lunar
heat-spectrum." The incredible delicacy of the operation may be judged
of from the statement that the sum-total of the thermal energy dispersed
by his rock-salt prisms was insufficient to raise a thermometer fully
exposed to it one-thousandth of a degree Centigrade! The singular fact
was, however, elicited that this almost evanescent spectrum is made up
of two superposed spectra, one due to reflection, the other, with a
maximum far down in the infra-red, to radiation.[949] The corresponding
temperature of the moon's sunlit surface Professor Langley considers to
be about that of freezing water.[950] Repeated experiments having failed
to get any thermal effects from the dark part of the moon, it was
inferred that our satellite "has no internal heat sensible at the
surface"; so that the radiations from the lunar soil giving the low
maximum in the heat-spectrum, "must be due purely to solar heat which
has been absorbed and almost immediately re-radiated." Professor
Langley's explorations of the terra incognita of immensely long
wave-lengths where lie the unseen heat-emissions from the earth into
space, led him to the discovery that these, contrary to the received
opinion, are in good part transmissible by our atmosphere, although they
are completely intercepted by glass. Another important result of the
Allegheny work was the abolition of the anomalous notion of the
"temperature of space," fixed by Pouillet at -140° C. For space in
itself can have no temperature, and stellar radiation is a negligible
quantity. Thus, it is safe to assume "that a perfect thermometer
suspended in space at the distance of the earth or moon from the sun,
but shielded from its rays, would sensibly indicate the absolute
zero,"[951] ordinarily placed at -273° C.
A "Prize Essay on the Distribution of the Moon's Heat" (The Hague),
1891, by Mr. Frank W. Very, who had taken an active part in Professor
Langley's long-sustained inquiry, embodies the fruits of its
continuation. They show the lunar disc to be tolerably uniform in
thermal power. The brighter parts are also indeed hotter, but not much.
The traces perceived of a slight retention of heat by the substances
forming the lunar surface, agreed well with the Parsonstown observations
of the total eclipse of the moon, January 28, 1888.[952] For they
brought out an unmistakable divergence between the heat and light
phases. A curious decrease of heat previous to the first touch of the
earth's shadow upon the lunar globe remains unexplained, unless it be
admissible to suppose the terrestrial atmosphere capable of absorbing
heat at an elevation of 190 miles. The probable range of temperature on
the moon was discussed by Professor Very in 1898.[953] He concluded it
to be very wide. Hotter than boiling water under the sun's vertical
rays, the arid surface of our dependent globe must, he found, cool in
the 14-day lunar night to about the temperature of liquid air.
Although that fundamental part of astronomy known as "celestial
mechanics" lies outside the scope of this work, and we therefore pass
over in silence the immense labours of Plana, Damoiseau, Hansen,
Delaunay, G. W. Hill, and Airy in reconciling the observed and
calculated motions of the moon, there is one slight but significant
discrepancy which is of such importance to the physical history of the
solar system, that some brief mention must be made of it.
Halley discovered in 1693, by examining the records of ancient eclipses,
that the moon was going faster then than 2,000 years previously--so much
faster, as to have got ahead of the place in the sky she would otherwise
have occupied, by about two of her own diameters. It was one of
Laplace's highest triumphs to have found an explanation of this puzzling
fact. He showed, in 1787, that it was due to a very slow change in the
ovalness of the earth's orbit, tending, during the present age of the
world, to render it more nearly circular. The pull of the sun upon the
moon is thereby lessened; the counter-pull of the earth gets the upper
hand; and our satellite, drawn nearer to us by something less than an
inch each year,[954] proportionately quickens her pace. Many thousands
of years hence the process will be reversed; the terrestrial orbit will
close in at the sides, the lunar orbit will open out under the growing
stress of solar gravity, and our celestial chronometer will lose instead
of gaining time.
This is all quite true as Laplace put it; but it is not enough. Adams,
the virtual discoverer of Neptune, found with surprise in 1853 that the
received account of the matter was "essentially incomplete," and
explained, when the requisite correction was introduced, only half the
observed acceleration.[955] What was to be done with the remaining half?
Here Delaunay, the eminent French mathematical astronomer, unhappily
drowned at Cherbourg in 1872 by the capsizing of a pleasure-boat, came
to the rescue.[956]
It is obvious to anyone who considers the subject a little attentively,
that the tides must act to some extent as a friction-brake upon the
rotating earth. In other words, they must bring about an almost
infinitely slow lengthening of the day. For the two masses of water
piled up by lunar influence on the hither and farther sides of our
globe, strive, as it were, to detach themselves from the unity of the
terrestrial spheroid, and to follow the movements of the moon. The moon,
accordingly, holds them _against_ the whirling earth, which revolves
like a shaft in a fixed collar, slowly losing motion and gaining heat,
eventually dissipated through space.[957] This must go on (so far as we
can see) until the periods of the earth's rotation and of the moon's
revolution coincide. Nay, the process will be continued--should our
oceans survive so long--by the feebler tide-raising power of the sun,
ceasing only when day and night cease to alternate, when one side of our
planet is plunged in perpetual darkness and the other seared by
unchanging light.
Here, then, we have the secret of the moon's turning always the same
face towards the earth. It is that in primeval times, when the moon was
liquid or plastic, an earth-raised tidal wave rapidly and forcibly
reduced her rotation to its present exact agreement with her period of
revolution. This was divined by Kant[958] nearly a century before the
necessity for such a mode of action presented itself to any other
thinker. In a weekly paper published at Königsberg in 1754, the modern
doctrine of "tidal friction" was clearly outlined by him, both as
regards its effects actually in progress on the rotation of the earth,
and as regards its effects already consummated on the rotation of the
moon--the whole forming a preliminary attempt at what he called a
"natural history" of the heavens. His sagacious suggestion, however,
remained entirely unnoticed until revived--it would seem
independently--by Julius Robert Mayer in 1848;[959] while similar, and
probably original, conclusions were reached by William Ferrel of
Allensville, Kentucky, in 1858.[960]
Delaunay was not then the inventor or discoverer of tidal friction; he
merely displayed it as an effective cause of change. He showed reason
for believing that its action in checking the earth's rotation, far from
being, as Ferrel had supposed, completely neutralised by the contraction
of the globe through cooling, was a fact to be reckoned with in
computing the movements, as well as in speculating on the history, of
the heavenly bodies. The outstanding acceleration of the moon was thus
at once explained. It was explained as apparent only--the reflection of
a real lengthening, by one second in 100,000 years, of the day. But on
this point the last word has not yet been spoken.
Professor Newcomb undertook in 1870 the onerous task of investigating
the errors of Hansen's Lunar Tables as compared with observations prior
to 1750. The results, published in 1878,[961] proved somewhat
perplexing. They tend, in general, to reduce the amount of acceleration
left unaccounted for by Laplace's gravitational theory, and
proportionately to diminish the importance of the part played by tidal
friction. But, in order to bring about this diminution, and at the same
time conciliate Alexandrian and Arabian observations, it is necessary to
reject _as total_ the ancient solar eclipses known as those of Thales
and Larissa. This may be a necessary, but it must be admitted to be a
hazardous expedient. Its upshot was to indicate a possibility that the
observed and calculated values of the moon's acceleration might after
all prove to be identical; and the small outstanding discrepancy was
still further diminished by Tisserand's investigation, differently
conducted, of the same Arabian eclipses discussed by Newcomb.[962] The
necessity of having recourse to a lengthening day is then less pressing
than it seemed some time ago; and the effect, if perceptible in the
moon's motion, should, M. Tisserand remarked, be proportionately so in
the motions of all the other heavenly bodies. The presence of the
apparent general acceleration that should ensue can be tested with most
promise of success, according to the same authority, by delicate
comparisons of past and future transits of Mercury.
Newcomb further showed that small residual irregularities are still
found in the movements of our satellite, inexplicable either by any
known gravitational influence, or by any _uniform_ value that could be
assigned to secular acceleration.[963] If set down to the account of
imperfections in the "time-keeping" of the earth, it could only be on
the arbitrary supposition of fluctuations in its rate of going
themselves needing explanation. This, it is true, might be found in very
slight changes of figure,[964] not altogether unlikely to occur. But
into this cloudy and speculative region astronomers for the present
decline to penetrate. They prefer, if possible, to deal only with
calculable causes, and thus to preserve for their "most perfect of
sciences" its special prerogative of assured prediction.
FOOTNOTES:
[Footnote 796: _Neueste Beyträge zur Erweiterung der Sternkunde_, Bd.
iii., p. 14 (1800).]
[Footnote 797: _Ibid._, p. 24.]
[Footnote 798: _Phil. Trans._, vol. xciii., p. 215.]
[Footnote 799: _Mem. Roy. Astr. Soc._, vol. vi., p. 116.]
[Footnote 800: _Month. Not._, vol. xix., pp. 11, 25.]
[Footnote 801: _Ibid._, vol. xxxviii., p. 398.]
[Footnote 802: _Am. Jour. of Sc._, vol. xvi., p. 124.]
[Footnote 803: _Wash. Obs._ for 1876, Part ii., p. 34.]
[Footnote 804: _Pop. Astr._, vol. ii., p. 168; _Astr. Jour._, No. 335.]
[Footnote 805: _Astr. and Astrophysics_, vol. xiii., p. 866.]
[Footnote 806: _Ibid._, p. 867.]
[Footnote 807: _Month. Not._, vol. xxiv., p. 18.]
[Footnote 808: _Ibid._, vol. xxiii., p. 234 (Challis).]
[Footnote 809: _Untersuchungen über die Spectra der Planeten_, p. 9.]
[Footnote 810: _Sirius_, vol. vii., p. 131.]
[Footnote 811: _Potsdam Publ._, No. 30; _Astr. Nach._, No. 3,171; Frost,
_Astr. and Astrophysics_, vol. xii., p. 619.]
[Footnote 812: Zöllner and Winnecke made it=O·13, _Astr. Nach._, No.
2,245.]
[Footnote 813: _Neueste Beyträge_, Bd. iii., p. 50.]
[Footnote 814: _Astr. Jahrbuch_, 1804, pp. 97-102.]
[Footnote 815: Webb, _Celestial Objects_, p. 46 (4th ed.).]
[Footnote 816: _L'Astronomie_, t. ii., p. 141.]
[Footnote 817: _Observations sur les Planètes Vénus et Mercure_, p. 87.]
[Footnote 818: _Observatory_, vol. vi., p. 40.]
[Footnote 819: _Atti dell' Accad. dei Lincei_, t. v. ii., p. 283, 1889;
_Astr. Nach._, No. 2,944.]
[Footnote 820: _Astr. Nach._ No. 2,479.]
[Footnote 821: _Memoirs Amer. Acad._, vol. xii., No. 4, p. 464.]
[Footnote 822: _Hist. de l'Astr._, p. 682.]
[Footnote 823: _Comptes Rendus_, t. xlix., p. 379.]
[Footnote 824: _Comptes Rendus_, t. l., p. 40.]
[Footnote 825: _Ibid._, p. 46.]
[Footnote 826: _Astr. Nach._, Nos. 1,248 and 1,281.]
[Footnote 827: _Comptes Rendus_, t. lxxxiii., pp. 510, 561.]
[Footnote 828: _Handbuch der Mathematik_, Bd. ii., p. 327.]
[Footnote 829: _Comptes Rendus_, t. lxxxiii., p. 721.]
[Footnote 830: _Nature_, vol. xviii., pp. 461, 495, 539.]
[Footnote 831: Oppolzer, _Astr. Nach._, No. 2,239.]
[Footnote 832: _Ibid._, Nos. 2,253-4 (C. H. F. Peters).]
[Footnote 833: _Ibid._, Nos. 2,263 and 2,277. See also Tisserand in
_Ann. Bur. des Long._, 1882, p. 729.]
[Footnote 834: See J. Bauschinger's _Untersuchungen_ (1884), summarised
in _Bull. Astr._, t. i., p. 506, and _Astr. Nach._, No. 2,594. Newcomb
finds the anomalous motion of the perihelion to be even larger (43"
instead of 38") than Leverrier made it. _Month. Not._, February, 1884,
p. 187. Harzer's attempt to account for it in _Astr. Nach._, No. 3,030,
is more ingenious than successful.]
[Footnote 835: _Jour. des Sçavans_, December, 1667, p. 122.]
[Footnote 836: _Élémens d'Astr._, p. 525. Cf. Chandler, _Pop. Astr._,
February, 1897, p. 393.]
[Footnote 837: _Beobachtungen über die sehr beträchtlichen Gebirge und
Rotation der Venus_, 1792, p. 35. Schröter's final result in 1811 was
23h. 21m. 7·977s. _Monat. Corr._, Bd. xxv., p. 367.]
[Footnote 838: _Astr. Nach._, No. 404.]
[Footnote 839: _Rendiconti del R. Istituto Lombardo_, t. xxiii., serie
ii.]
[Footnote 840: _Astr. Nach._, No. 3,304.]
[Footnote 841: _Bothkamp Beobachtungen_, Heft ii., p. 120.]
[Footnote 842: _Comptes Rendus_, t. cxi., p. 542; t. cxxii., p. 395.]
[Footnote 843: _Month. Not._, vol. lvii., p. 402; _Astr. Nach._, No.
3,406.]
[Footnote 844: _Mem. Spettroscopisti Italiani_, t. xxv., p. 93;
_Nature_, vol. liii., p. 306.]
[Footnote 845: _Astr. Nach._, No. 3,329.]
[Footnote 846: _Ibid._]
[Footnote 847: _Bull. de l'Acad. de Belgique_, t. xxi., p. 452, 1891.]
[Footnote 848: _Observations sur les Planètes Vénus et Mercure_, 1892.]
[Footnote 849: _Astr. Nach._, No. 3,300.]
[Footnote 850: _Ibid._, No. 3,332.]
[Footnote 851: _Ibid._, No. 3,314.]
[Footnote 852: _Ibid._, No. 3,170.]
[Footnote 853: _Ibid._, No. 3,641. The velocity of a point on the
equator of Venus, if Brenner's period of 23h. 57m. were exact, would be
0·28 miles per second; but the displacements due to this rate would be
doubled by reflection.]
[Footnote 854: _Novæ Observationes_, p. 92.]
[Footnote 855: _Mém. de l'Ac._, 1700, p. 296.]
[Footnote 856: _Phil. Trans._, vol. lxxxiii., p. 201.]
[Footnote 857: Webb, _Cel. Objects_, p. 58.]
[Footnote 858: _Month. Not._, vol. xlii., p. 111.]
[Footnote 859: _Bull. Ac. de Bruxelles_, t. xliii., p. 22.]
[Footnote 860: _Phil. Trans._, vol. lxxxii., p. 309;
_Aphroditographische Fragmente_, p. 85 (1796).]
[Footnote 861: _Astr. Nach._, No. 679.]
[Footnote 862: _Month. Not._, vol. xiv., p. 169.]
[Footnote 863: _Ibid._, vol. xxiv., p. 25.]
[Footnote 864: _Am. Jour. of Sc._, vol. xliii., p. 129 (2d ser.); vol.
ix., p. 47 (3d ser.).]
[Footnote 865: _Astroph. Jour._, vol. ix., p. 284.]
[Footnote 866: _Month. Not._, vol. xxxvi., p. 347.]
[Footnote 867: _Old and New Astronomy_, p. 448.]
[Footnote 868: _Hist. Phys. Astr._, p. 431.]
[Footnote 869: _Mem. Roy. Astr. Soc._, vol. xlvii., pp. 77, 84.]
[Footnote 870: _Astr. Reg._, vol. xiii., p. 132.]
[Footnote 871: _L'Astronomie_, t. ii., p. 27; _Astr. Nach._, No. 2,021;
_Am. Jour. of Sc._, vol. xxv., p. 430.]
[Footnote 872: _Mem. Spettr. Ital._, Dicembre, 1882; _Am. Jour. of Sc._,
vol. xxv., p. 328.]
[Footnote 873: _Comptes Rendus_, t. cxvi., p. 288.]
[Footnote 874: Vogel, _Spectra der Planeten_, p. 15.]
[Footnote 875: _Nature_, vol. xix., p. 23.]
[Footnote 876: _Nova Acta Acad. Naturæ Curiosorum_, Bd. x., 239.]
[Footnote 877: _Astr. Jahrbuch_, 1809, p. 164.]
[Footnote 878: _Month Not._, vol. xliii., p. 331.]
[Footnote 879: _Report Brit. Ass._, 1873, p. 407. The paper contains a
valuable record of observations of the phenomenon.]
[Footnote 880: _Photom. Untersuchungen_, p. 301.]
[Footnote 881: _Bothkamp Beobachtungen_, Heft ii., p. 126.]
[Footnote 882: _Astr. Nach._, No. 2,818.]
[Footnote 883: _Mémoires de l'Acad. de Bruxelles_, t. xlix., No. 5, 4to;
_Astr. Nach._, No. 2,809; _f._ Schorr, _Der Venusmond_, 1875.]
[Footnote 884: _Phil. Trans._, 1839, 1841, 1842.]
[Footnote 885: Delaunay objected (_Comptes Rendus_, t. lxvii., p. 65)
that the viscosity of the contained liquid (of which Hopkins took no
account) would, where the movements were so excessively slow as those of
the earth's axis, almost certainly cause it to behave like a solid. Lord
Kelvin, however (_Report Brit. Ass._, 1876, ii., p. 1), considered
Hopkins's argument valid as regards the comparatively quick solar
semi-annual and lunar fortnightly nutations.]
[Footnote 886: _Phil. Trans._, cliii., p. 573.]
[Footnote 887: _Report Brit. Ass._, 1868, p. 494.]
[Footnote 888: _Ibid._, 1882, p. 474.]
[Footnote 889: Albrecht, _Astr. Nach._, No. 3,131.]
[Footnote 890: _Astr. Jour._, Nos. 248, 249.]
[Footnote 891: _Ibid._, No. 258.]
[Footnote 892: _Month. Not._, vol. lii., p. 336.]
[Footnote 893: _Astr. Nach._, No. 3,097; _Phil. Trans._, vol. clxxxvi.,
A., p. 469; _Proc. Roy. Soc._, vol. lix.]
[Footnote 894: See Chandler's searching investigations, _Astr. Jour._,
Nos. 329, 344, 351, 392, 402, 406, 412, 446, 489, 490, 494, 495.]
[Footnote 895: Rees, _Pop. Astr._, No. 74, 1900.]
[Footnote 896: _Nature_, vol. lxi., p. 447; see also A. V. Bäcklund,
_Astr. Nach._, No. 3,787.]
[Footnote 897: _Trans. Geol. Soc._, vol. iii. (2d ser.), p. 293.]
[Footnote 898: See his _Treatise on Astronomy_, p. 199 (1833).]
[Footnote 899: _Phil. Mag._, vol. xxviii. (4th ser.), p. 121.]
[Footnote 900: _Climate and Time_, 1875; _Discussions on Climate and
Cosmology_, 1885.]
[Footnote 901: See for a popular account of the theory, Sir R. Ball's
_The Cause of an Ice Age_, 1892.]
[Footnote 902: See A. Woeikof, _Phil. Mag._, vol. xxi., p. 223.]
[Footnote 903: _The Ice Age in North America_, London, 1890.]
[Footnote 904: _Phil. Trans._, vol. lxviii., p. 783.]
[Footnote 905: _Comptes Rendus_, t. lxxvi., p. 954.]
[Footnote 906: _Potsdam Publ._, Nos. 22, 23.]
[Footnote 907: _Phil. Trans._, vol. clxxxii., p. 565; _Adams Prize Essay
for 1893._]
[Footnote 908: _Denkschriften Akad. der Wiss. Wien_, Bd. lxiv.; quoted
by Poynting. _Nature_, vol. lxii., p. 404.]
[Footnote 909: _Report on the Geodetic Survey of S. Africa_, 1894.]
[Footnote 910: _Nature_, vol. lxii., p. 622; Hollis, _Observatory_, vol.
xxiii., p. 337; Poincaré, _Comptes Rendus_, July 23, 1900.]
[Footnote 911: _Astr. Nach._, No. 2,228.]
[Footnote 912: Young's _Gen. Astr._, p. 601.]
[Footnote 913: _Astr. Constants_, p. 195.]
[Footnote 914: The second volume was published at Göttingen in 1802.]
[Footnote 915: _Ueber Rillen auf dem Monde_, p. 13. _Cf. The Moon_, by
T. Gwyn Elger, p. 20. W. H. Pickering, _Harvard Annals_, vol. xxxii., p.
249.]
[Footnote 916: _The Moon_, p. 73.]
[Footnote 917: _Selen. Fragm._, Th. ii., p. 399.]
[Footnote 918: _Astr. Nach._, No. 263 (1834); _Pop. Vorl._, pp. 615-620
(1838).]
[Footnote 919: _Outlines of Astr._, par. 431.]
[Footnote 920: _Month. Not._, vol. xxv., p. 61.]
[Footnote 921: _Month. Not._, vol. xxv., p. 264.]
[Footnote 922: _Astroph. Jour._, vol. vi., p. 422.]
[Footnote 923: _Harvard Annals_, vol. xxxii., p. 81.]
[Footnote 924: _Astr. and Astrophysics_, vol. xi., p. 778.]
[Footnote 925: Neison, _The Moon_, p. 25.]
[Footnote 926: _Knowledge_, vol. xvii., p. 85.]
[Footnote 927: Neison, _The Moon_, p. 104.]
[Footnote 928: The combination of a uniform rotational with an unequal
orbital movement causes a slight swaying of the moon's globe, now east,
now west, by which we are able to see round the edges of the averted
hemisphere. There is also a "parallactic" libration, depending on the
earth's rotation; and a species of nodding movement--the "libration in
latitude"--is produced by the inclination of the moon's axis to her
orbit, and by her changes of position with regard to the terrestrial
equator. Altogether, about 2/11 of the _invisible_ side come into view.]
[Footnote 929: _Cel. Objects_, p. 58 (4th ed.).]
[Footnote 930: _Astr. Nach._, No. 1,631.]
[Footnote 931: Cf. Leo Brenner, _Naturwiss. Wochenschrift_, January 13,
1895; _Jour. Brit. Astr. Ass._, vol. v., pp. 29, 222.]
[Footnote 932: Respighi, _Les Mondes_, t. xiv., p. 294; Huggins, _Month.
Not._, vol. xxvii., p. 298.]
[Footnote 933: Birt, _Ibid._, p. 95.]
[Footnote 934: _Report Brit. Ass._, 1872, p. 245.]
[Footnote 935: _Observatory_, vol. xv., p. 250.]
[Footnote 936: _Astr. Reg._, vol. xvi., p. 265; _Astr. Nach._, No.
2,275.]
[Footnote 937: Lindsay and Copeland, _Month. Not._, vol. xxxix., p.
195.]
[Footnote 938: _Observatory_, vols. ii., p. 296; iv., p. 373. N. E.
Green (_Astr. Reg._, vol. xvii., p. 144) concluded the object a mere
"spot of colour," dark under oblique light.]
[Footnote 939: Webb, _Cel. Objects_, p. 101.]
[Footnote 940: _Publ. Lick Observatory_, vol. iii., p. 7.]
[Footnote 941: _Ibid._, p. 21; Mee, _Knowledge_, vol. xviii., p. 135.]
[Footnote 942: _Comptes Rendus_, t. cxxii., p. 967; _Bull. Astr._,
August, 1899; _Ann. Bureau des Long._, 1898; _Nature_, vols. lii., p.
439; lvi., p. 280; lix., p. 304; lx., p. 491; _Astroph. Jour._ vol. vi.,
p. 51.]
[Footnote 943: _Comptes Rendus_, t. xxii., p. 541.]
[Footnote 944: _Phil. Trans._, vol. cxlviii., p. 502.]
[Footnote 945: _Proc. Roy. Soc._, vol. xvii., p. 443.]
[Footnote 946: _Phil. Trans._, vol. clxiii., p. 623.]
[Footnote 947: _Trans. R. Dublin Soc._, vol. iii., p. 321.]
[Footnote 948: _Science_, vol. vii., p. 9.]
[Footnote 949: _Amer. Jour. of Science_, vol. xxxviii., p. 428.]
[Footnote 950: "The Temperature of the Moon," _Memoirs National Acad. of
Sciences_, vol. iv., p. 193, 1889.]
[Footnote 951: _Temperature of the Moon_, p. iii.; see also App. ii., p.
206.]
[Footnote 952: _Trans. R. Dublin Soc._, vol. iv., p. 481, 1891; Rosse,
_Proc. Roy. Institution_, May 31, 1895.]
[Footnote 953: _Astroph. Jour._, vol. viii., pp. 199, 265.]
[Footnote 954: Airy, _Observatory_, vol. iii., p. 420.]
[Footnote 955: _Phil. Trans._, vol. cxliii., p. 397; _Proc. Roy. Soc._,
vol. vi., p. 321.]
[Footnote 956: _Comptes Rendus_, t. lxi., p. 1023.]
[Footnote 957: Professor Darwin calculated that the heat generated by
tidal friction in the course of lengthening the earth's period of
rotation from 23 to 24 hours, equalled 23 million times the amount of
its present annual loss by cooling. _Nature_, vol. xxxiv., p. 422.]
[Footnote 958: _Sämmtl. Werke_ (ed. 1839), Th. vi., pp. 5-12. See also
C. J. Monro's useful indications in _Nature_, vol. vii., p. 241.]
[Footnote 959: _Dynamik des Himmels_, p. 40.]
[Footnote 960: Gould's _Astr. Jour._, vol. iii., p. 138.]
[Footnote 961: _Wash. Obs._ for 1875, vol. xxii., App. ii.]
[Footnote 962: _Comptes Rendus_, t. cxiii., p. 669; _Annuaire_, Paris,
1892.]
[Footnote 963: Newcomb, _Pop. Astr._ (4th ed.), p. 101.]
[Footnote 964: Sir W. Thomson, _Report Brit. Ass._, 1876, p. 12.]
CHAPTER VIII
_PLANETS AND SATELLITES_--(_continued_)
"The analogy between Mars and the earth is perhaps by far the greatest
in the whole solar system." So Herschel wrote in 1783,[965] and so we
may safely say to-day, after six score further years of scrutiny. The
circumstance lends a particular interest to inquiries into the physical
habitudes of our exterior planetary neighbour.
Fontana first caught glimpses, at Naples in 1636 and 1638,[966] of dusky
stains on the ruddy disc of Mars. They were next seen by Hooke and
Cassini in 1666, and this time with sufficient distinctness to serve as
indexes to the planet's rotation, determined by the latter as taking
place in a period of twenty-four hours forty minutes.[967] Increased
confidence was given to this result through Maraldi's precise
verification of it in 1719.[968] Among the spots observed by him, he
distinguished two as stable in position, though variable in size. They
were of a peculiar character, showing as bright patches round the poles,
and had already been noticed during sixty years back. A current
conjecture of their snowy nature obtained validity when Herschel
connected their fluctuations in extent with the progress of the Martian
seasons. The inference of frozen precipitations could scarcely be
resisted when once it was clearly perceived that the shining polar zones
did actually by turns diminish and grow with the alternations of summer
and winter in the corresponding hemisphere.
This, it may be said, was the opening of our acquaintance with the state
of things prevailing on the surface of Mars. It was accompanied by a
steady assertion, on Herschel's part, of permanence in the dark
markings, notwithstanding partial obscurations by clouds and vapours
floating in a "considerable but moderate atmosphere." Hence the presumed
inhabitants of the planet were inferred to "probably enjoy a situation
in many respects similar to ours."[969]
Schröter, on the other hand, went altogether wide of the truth as
regards Mars. He held that the surface visible to us is a mere shell of
drifting cloud, deriving a certain amount of apparent stability from the
influence on evaporation and condensation of subjacent but unseen
areographical features;[970] and his opinion prevailed with his
contemporaries. It was, however, rejected by Kunowsky in 1822, and
finally overthrown by Beer and Mädler's careful studies during five
consecutive oppositions, 1830-39. They identified at each the same dark
spots, frequently blurred with mists, especially when the local winter
prevailed, but fundamentally unchanged.[971] In 1862 Lockyer established
a "marvellous agreement" with Beer and Mädler's results of 1830, leaving
no doubt as to the complete fixity of the main features, amid "daily,
nay, hourly," variations of detail through transits of clouds.[972] On
seventeen nights of the same opposition, F. Kaiser of Leyden obtained
drawings in which nearly all the markings noted in 1830 at Berlin
reappeared, besides spots frequently seen respectively by Arago in 1813,
by Herschel in 1783, and one sketched by Huygens in 1672 with a
writing-pen in his diary.[973] From these data the Leyden observer
arrived at a period of rotation of 24h. 37m. 22·62s., being just one
second shorter than that deduced, exclusively from their own
observations, by Beer and Mädler. The exactness of this result was
practically confirmed by the inquiries of Professor Bakhuyzen of
Leyden.[974] Using for a middle term of comparison the disinterred
observations of Schröter, with those of Huygens at one, and of
Schiaparelli at the other end of an interval of 220 years, he was
enabled to show, with something like certainty, that the time of
rotation (24h. 37m. 22·735s.) ascribed to Mars by Mr. Proctor[975] in
reliance on a drawing executed by Hooke in 1666, was too long by _nearly
one-tenth of a second_. The minuteness of the correction indicates the
nicety of care employed. Nor employed vainly; for, owing to the
comparative antiquity of the records available in this case, an almost
infinitesimal error becomes so multiplied by frequent repetition as to
produce palpable discrepancies in the positions of the markings at
distant dates. Hence Bakhuyzen's period of 24h. 37m. 22·66s. is
undoubtedly of a precision unapproached as regards any other heavenly
body save the earth itself.
Two facts bearing on the state of things at the surface of Mars were,
then, fully acquired to science in or before the year 1862. The first
was that of the seasonal fluctuations of the polar spots; the second,
that of the general permanence of certain dark gray or greenish patches,
perceived with the telescope as standing out from the deep yellow ground
of the disc. That these varieties of tint correspond to the real
diversities of a terraqueous globe, the "ripe cornfield"[976] sections
representing land, the dusky spots and streaks, oceans and straits, has
long been the prevalent opinion. Sir J. Herschel in 1830 led the way in
ascribing the redness of the planet's light to an inherent peculiarity
of soil.[977] Previously it had been assimilated to our sunset glows
rather than to our red sandstone formations--set down, that is, to an
atmospheric stoppage of blue rays. But the extensive Martian atmosphere,
implicitly believed in on the strength of some erroneous observations by
Cassini and Römer in the seventeenth century, vanished before the sharp
occultation of a small star in Leo, witnessed by Sir James South in
1822;[978] and Dawes's observation in 1865,[979] that the ruddy tinge is
deepest near the central parts of the disc, certified its
non-atmospheric origin. The absolute whiteness of the polar snow-caps
was alleged in support of the same inference by Sir William Huggins in
1867.[980]
All recent operations tend to show that the atmosphere of Mars is much
thinner than our own. This was to have been expected _à priori_, since
the same proportionate mass of air would on his smaller globe form a
relatively sparse covering.[981] Besides, gravity there possesses less
than four-tenths its force here, so that this sparser covering would
weigh less, and be less condensed, than if it enveloped the earth.
Atmospheric pressure would accordingly be of about two and a quarter,
instead of fifteen terrestrial pounds per square inch. This corresponds
with what the telescope shows us. It is extremely doubtful whether any
features of the earth's actual surface could be distinguished by a
planetary spectator, however well provided with optical assistance.
Professor Langley's inquiries[982] led him to conclude that fully twice
as much light is absorbed by our air as had previously been
supposed--say 40 per cent. of vertical rays in a clear sky. Of the sixty
reaching the earth, less than a quarter would be reflected even from
white sandstone; and this quarter would again pay heavy toll in escaping
back to space. Thus not more than perhaps ten or twelve out of the
original hundred sent by the sun would, under the most favourable
circumstances, and from the very centre of the earth's disc, reach the
eye of a Martian or lunar observer. The light by which he views our
world is, there is little doubt, light reflected from the various strata
of our atmosphere, cloud or mist-laden or serene, as the case may be,
with an occasional snow-mountain figuring as a permanent white spot.
This consideration at once shows us how much more tenuous the Martian
air must be, since it admits of topographical delineations of the
Martian globe. The clouds, too, that form in it seem in general to be
rather of the nature of ground-mists than of heavy cumulus.[983]
Occasionally, indeed, durable and extensive strata become visible.
During the latter half of October, 1894, for instance, a region as large
as Europe remained apparently cloud-covered. Yet most recent observers
are unable to detect the traces of aqueous absorption in the Martian
spectrum noted by Huggins in 1867[984] and by Vogel in 1873.[985]
Campbell vainly looked for them,[986] visually in 1894,
spectrographically in 1896; Keeler was equally unsuccessful;[987]
Jewell[988] holds that they could, with present appliances, only be
perceived if the atmosphere of Mars were much richer in water-vapour
than that of the earth. There can be little doubt, however, that its
supply is about the minimum adequate to the needs of a _living_, and
perhaps a life-nuturing planet.
The climate of Mars seems to be unexpectedly mild. Its _theoretical_
mean temperature, taking into account both distance from the sun and
albedo, is 34° C. below freezing.[989] Yet its polar snows are both less
extensive and less permanent than those on the earth. The southern white
hood, noticed by Schiaparelli in 1877 to have survived the summer only
as a small lateral patch, melted completely in 1894. Moreover, Mr. W. H.
Pickering observed with astonishment the disappearance, in the course of
thirty-three days of June and July, 1892, of 1,600,000 square miles of
southern snow.[990] Curiously enough, the initial stage of shrinkage in
the white calotte was marked by its division into two unequal parts, as
if in obedience to the mysterious principle of duplication governing so
many Martian phenomena.[991] Changes of the hues associated respectively
with land and water accompanied in lower latitudes, and were thought to
be occasioned by floods ensuing upon this rapid antarctic thaw. It is
true that scarcity of moisture would account for the scantiness and
transitoriness of snowy deposits easily liquefied because thinly spread.
But we might expect to see the whole wintry hemisphere, at any rate,
frost-bound, since the sun radiates less than half as much heat on Mars
as on the earth. Water seems, nevertheless, to remain, as a rule,
uncongealed everywhere outside the polar regions. We are at a loss to
imagine by what beneficent arrangement the rigorous conditions naturally
to be looked for can be modified into a climate which might be found
tolerable by creatures constituted like ourselves.
Martian topography may be said to form nowadays a separate
sub-department of descriptive astronomy. The amount of detail become
legible by close scrutiny on a little disc which, once in fifteen years,
attains a maximum of about 1/5000 the area of the full moon, must excite
surprise and might provoke incredulity. Spurious discoveries, however,
have little chance of holding their own where there are so many
competitors quite as ready to dispute as to confirm.
The first really good map of Mars was constructed in 1869 by Proctor
from drawings by Dawes. Kaiser of Leyden followed in 1872 with a
representation founded upon data of his own providing in 1862-64; and
Terby, in his valuable _Aréographie_, presented to the Brussels Academy
in 1873[992] a careful discussion of all important observations from the
time of Fontana downwards, thus virtually adding to knowledge by
summarising and digesting it. The memorable opposition of September 5,
1877, marked a fresh epoch in the study of Mars. While executing a
trigonometrical survey (the first attempted) of the disc, then of the
unusual size of 25" across, G. V. Schiaparelli, director of the Milan
Observatory, detected a novel and curious feature. What had been taken
for Martian continents were found to be, in point of fact,
agglomerations of islands, separated from each other by a network of
so-called "canals" (more properly _channels_).[993] These are obviously
extensions of the "seas," originating and terminating in them, and
sharing their gray-green hue, but running sometimes to a length of three
or four thousand miles in a straight line, and preserving throughout a
nearly uniform breadth of about sixty miles. Further inquiries have
fully substantiated the discovery made at the Brera Observatory. The
"canals" of Mars are an actually existent and permanent phenomenon. An
examination of the drawings in his possession showed M. Terby that they
had been seen, though not distinctively recognised, by Dawes, Secchi,
and Holden; several were independently traced out by Burton at the
opposition of 1879; all were recovered by Schiaparelli himself in 1879
and 1881-82; and their indefinite multiplication resulted from Lovell's
observations in 1894 and 1896.
When the planet culminated at midnight, and was therefore in opposition,
December 26, 1881, its distance was greater, and its apparent diameter
less than in 1877, in the proportion of sixteen to twenty-five. Its
atmosphere was, however, more transparent, and ours of less impediment
to northern observers, the object of scrutiny standing considerably
higher in northern skies. Never before, at any rate, had the true aspect
of Mars come out so clearly as at Milan, with the 8-3/4-inch Merz
refractor of the observatory, between December, 1881, and February,
1882. The canals were all again there, but this time they were--in as
many as twenty cases--_seen in duplicate_. That is to say, a twin-canal
ran parallel to the original one at an interval of 200 to 400
miles.[994]
We are here brought face to face with an apparently insoluble enigma.
Schiaparelli regards the "germination" of his canals as a periodical
phenomenon depending on the Martian seasons. It is, assuredly, not an
illusory one, since it was plainly apparent, during the opposition of
1886, to MM. Perrotin and Thollon at Nice,[995] and to the former, using
the new 30-inch refractor of that observatory, in 1888; Mr. A. Stanley
Williams, with the help of only a 6-1/2-inch reflector, distinctly
perceived in 1890 seven of the duplicate objects noted at Milan,[996]
and the Lick observations, both of 1890 and of 1892, together with the
drawings made at Flagstaff and Mexico during the last favourable
oppositions of the nineteenth century, brought unequivocal confirmation
to the accuracy of Schiaparelli's impressions.[997] Various conjectures
have been hazarded in explanation of this bizarre appearance. The
difficulty of conceiving a physical reality corresponding to it has
suggested recourse to an optical rationale. Proctor regarded it as an
effect of diffraction;[998] Stanislas Meunier, of oblique reflection
from overlying mist-banks;[999] Flammarion considers it possible that
companion-canals might, under special circumstances, be evoked by
refraction as a kind of mirage.[1000] But none of these speculations are
really admissible, when all the facts are taken into account. The view
that the canals of Mars are vast rifts due to the cooling of the globe,
is recommended by the circumstance that they tend to follow great
circles; nevertheless, it would break down if, as Schiaparelli holds,
the fluctuations in their visibility depend upon actual obliterations
and re-emergencies. Fantastic though the theory of their artificial
origin appear, it is held by serious astronomers. Its vogue is largely
due to Mr. Lowell's ingenious advocacy. He considers the Martian globe
to be everywhere intersected by an elaborate system of irrigation-works,
rendered necessary by a perennial water-famine, relieved periodically by
the melting of the polar snows. Nor does he admit the existence of
oceans, or lakes. What have been taken for such are really tracts
covered with vegetation, the bright areas intermixed with them
representing sandy deserts. And it is noteworthy in this connection that
Professor Barnard obtained in 1894,[1001] with the great Lick refractor,
"suggestive and impressive views" disclosing details of light and shade
on the gray-green patches so intricate and minute as almost to preclude
the supposition of their aqueous nature.
The closeness of the terrestrial analogy has thus of late been much
impaired. Even if the surface of Mars be composed of land and water,
their distribution must be of a completely original type. The
interlacing everywhere of continents with arms of the sea (if that be
the correct interpretation of the visual effects) implies that their
levels scarcely differ;[1002] and Schiaparelli carries most observers
with him in holding that their outlines are not absolutely constant,
encroachments of dusky upon bright tints suggesting extensive
inundations.[1003] The late N. E. Green's observations at Madeira in
1877 indicated, on the other hand, a rugged south polar region. The
contour of the snow-cap not only appeared indented, as if by valleys and
promontories, but brilliant points were discerned outside the white
area, attributed to isolated snow-peaks.[1004] Still more elevated, if
similarly explained, must be the "ice island" first seen in a
comparatively low latitude by Dawes in January, 1865.
On August 4, 1892, Mars stood opposite to the sun at a distance of only
34,865,000 miles from the earth. In point of vicinity, then, its
situation was scarcely less favourable than in 1877. The low altitude of
the planet, however, practically neutralised this advantage for northern
observers, and public expectation, which had been raised to the highest
pitch by the announcements of sensation-mongers, was somewhat
disappointed at the "meagreness" of the news authentically received from
Mars. Valuable series of observations were, nevertheless, made at Lick
and Arequipa; and they unite in testifying to the genuine prevalence of
surface-variability, especially in certain regions of intermediate tint,
and perhaps of the "crude consistence" of "boggy Syrtes, neither sea,
nor good dry land." Professor Holden insisted on the "enormous
difficulties in the way of completely explaining the recorded phenomena
by terrestrial analogies";[1005] Mr. W. H. Pickering spoke of
"conspicuous and startling changes." They, however, merely overlaid, and
partially disguised, a general stability. Among the novelties detected
by Mr. Pickering were a number of "lakes," or "oases" (in Lowell's
phraseology), under the aspect of black dots at the junctions of two or
more canals;[1006] and he, no less than the Lick astronomers and M.
Perrotin at Nice,[1007] observed brilliant clouds projecting beyond the
terminator, or above the limb, while carried round by the planet's
rotation. They seemed to float at an altitude of at least twenty miles,
or about four times the height of terrestrial cirrus; but this was not
wonderful, considering the low power of gravity acting upon them. Great
capital was made in the journalistic interest out of these imaginary
signals from intelligent Martians, desirous of opening communications
with (to them) problematical terrestrial beings. Similar effects had,
however, been seen before by Mr. Knobel in 1873, by M. Terby in 1888,
and at the Lick Observatory in 1890; and they were discerned again with
particular distinctness by Professor Hussey at Lick, August 27,
1896.[1008]
The first photograph of Mars was taken by Gould at Cordoba in 1879.
Little real service in planetary delineation has, it is true, been so
far rendered by the art, yet one achievement must be recorded to its
credit. A set of photographs obtained by Mr. W. H. Pickering on Wilson's
Peak, California, April 9, 1890, showed the southern polar cap of Mars
as of moderate dimensions, but with a large dim adjacent area.
Twenty-four hours later, on a corresponding set, the dim area was
brilliantly white. The polar cap had become enlarged in the interim,
apparently through a wide-spreading snow-fall, by the annexation of a
territory equal to that of the United States. The season was towards the
close of winter in Mars. Never until then had the process of glacial
extension been actually (it might be said) superintended in that distant
globe.
Mars was gratuitously supplied with a pair of satellites long before he
was found actually to possess them. Kepler interpreted Galileo's anagram
of the "triple" Saturn in this sense; they were perceived by Micromégas
on his long voyage through space; and the Laputan astronomers had even
arrived at a knowledge, curiously accurate under the circumstances, of
their distances and periods. But terrestrial observers could see nothing
of them until the night of August 11, 1877. The planet was then within
one month of its second nearest approach to the earth during the last
century; and in 1845 the Washington 26-inch refractor was not in
existence.[1009] Professor Asaph Hall, accordingly, determined to turn
the conjecture to account for an exhaustive inquiry into the
surroundings of Mars. Keeping his glaring disc just outside the field of
view, a minute attendant speck of light was "glimpsed" August 11. Bad
weather, however, intervened, and it was not until the 16th that it was
ascertained to be what it appeared--a satellite. On the following
evening a second, still nearer to the primary, was discovered, which, by
the bewildering rapidity of its passages hither and thither, produced at
first the effect of quite a crowd of little moons.[1010]
Both these delicate objects have since been repeatedly observed, both in
Europe and America, even with comparatively small instruments. At the
opposition of 1884, indeed, the distance of the planet was too great to
permit of the detection of both elsewhere than at Washington. But the
Lick equatoreal showed them, July 18, 1888, when their brightness was
only 0·12 its amount at the time of their discovery; so that they can
now be followed for a considerable time before and after the least
favourable oppositions.
The names chosen for them were taken from the Iliad, where "Deimos" and
"Phobos" (Fear and Panic) are represented as the companions in battle of
Ares. In several respects, they are interesting and remarkable bodies.
As to size, they may be said to stand midway between meteorites and
satellites. From careful photometric measures executed at Harvard in
1877 and 1879, Professor Pickering concluded their diameters to be
respectively six and seven miles.[1011] This is on the assumption that
they reflect the same proportion of the light incident upon them that
their primary does. But it may very well be that they are less
reflective, in which case they would be more extensive. The albedo of
Mars is put by Müller at 0·27; his surface, in other words, returns 27
per cent. of the rays striking it. If we put the albedo of his
satellites equal to that of our moon, 0·17, their diameters will be
increased from 6 and 7 to 7-1/2 and 9 miles, Phobos, the inner one,
being the larger. Mr. Lowell, however, formed a considerably larger
estimate of their dimensions.[1012] It is interesting to note that
Deimos, according to Professor Pickering's very distinct perception,
does not share the reddish tint of Mars.
Deimos completes its nearly circular revolutions in thirty hours
eighteen minutes, at a distance from the surface of its ruling body of
12,500 miles; Phobos traverses an elliptical orbit[1013] in seven hours
thirty-nine minutes twenty-two seconds, at a distance of only 3,760
miles. This is the only known instance of a satellite circulating faster
than its primary rotates, and is a circumstance of some importance as
regards theories of planetary development. To a Martian spectator the
curious effect would ensue of a celestial object, seemingly exempt from
the general motion of the sphere, rising in the west, setting in the
east, and culminating twice, or even thrice a day; which, moreover, in
latitudes above 69° north or south, would be permanently and altogether
hidden by the intervening curvature of the globe.
* * * * *
The detection of new members of the solar system has come to be one of
the most ordinary of astronomical events. Since 1846 no single year has
passed without bringing its tribute of asteroidal discovery. In the last
of the seventies alone, a full score of miniature planets were
distinguished from the thronging stars amid which they seem to move;
1875 brought seventeen such recognitions; their number touched a minimum
of one in 1881; it rose in 1882, and again in 1886, to eleven; dropped
to six in 1889, and sprang up with the aid of photography to
twenty-seven in 1892. That high level has since, on an average, been
maintained; and on January 1, 1902, nearly 500 asteroids were recognised
as revolving between the orbits of Mars and Jupiter. Of these,
considerably more than one hundred are claimed by one investigator
alone--Dr. Max Wolf of Heidelburg; M. Charlois of Nice comes second with
102; while among the earlier observers Palisa of Vienna contributed 86,
and C. H. F. Peters of Clinton (N. Y.), whose varied and useful career
terminated July 19, 1890, 52 to the grand total. The construction by
Chacornac and his successors at Paris, and more recently by Peters at
Clinton, of ecliptical charts showing all stars down to the thirteenth
and fourteenth magnitudes respectively, rendered the picking out of moving
objects above that brightness a mere question of time and diligence. Both,
however, are vastly economised by the photographic method. Tedious
comparisons of the sky with charts are no longer needed for the
identification of unrecorded, because simulated stars. Planetary bodies
declare themselves by appearing upon the plate, not in circular, but in
linear form. Their motion converts their images into trails, long or short
according to the time of exposure. The first asteroid (No. 323) thus
detected was by Max Wolf, December 22, 1891.[1014] Eighteen others were
similarly discovered in 1892, by the same skilful operator; and ten more
through Charlois's adoption at Nice of the novel plan now in exclusive use
for picking up errant light-specks. Far more onerous than the task of their
discovery is that of keeping them in view once discovered--of tracking out
their paths, ixing their places, and calculating the disturbing effects
upon them of the mighty Jovian mass. These complex operations have come to
be centralised at Berlin under the superintendence of Professor Tietjen,
and their results are given to the public through the medium of the
_Berliner Astronomisches Jahrbuch_.
The _cui bono?_ however, began to be agitated. Was it worth while to
maintain a staff of astronomers for the sole purpose of keeping hold
over the identity of the innumerable component particles of a cosmical
ring? The prospect, indeed, of all but a select few of the asteroids
being thrown back by their contemptuous captors into the sea of space
seemed so imminent that Professor Watson provided by will against the
dereliction of the twenty-two discovered by himself. But the fortunes of
the whole family improved through the distinction obtained by one of
them. On August 14, 1898, the trail of a rapidly-moving, star-like
object of the eleventh magnitude imprinted itself on a plate exposed by
Herr Witt at the Urania Observatory, Berlin. Its originator proved to be
unique among asteroids. "Eros" is, in sober fact,
'one of those mysterious stars
Which hide themselves between the Earth and Mars,'
divined or imagined by Shelley.[1015] True, several of its congeners
invade the Martian sphere at intervals; but the proper habitat of Eros
is within that limit, although its excursions transcend it. In other
words, its mean distance from the sun is about 135, as compared with the
Martian distance of 141 million miles. Further, its orbit being so
fortunately circumstanced as to bring it once in sixty-seven years
within some 15 millions of miles of the earth, it is of extraordinary
value to celestial surveyors. The calculation of its movements was much
facilitated by detections, through a retrospective search,[1016] of many
of its linear images among the star-dots on the Harvard plates.[1017]
The little body--which can scarcely be more than twenty miles in
diameter--shows peculiarities of behaviour as well as of position. Dr.
von Oppolzer, in February, 1901,[1018] announced it to be extensively
and rapidly variable. Once in 2 hours 38 minutes it lost about
three-fourths of its light,[1019] but these fluctuations quickly
diminished in range, and in the beginning of May ceased
altogether.[1020] Evidently, then, they depend upon the situation of the
asteroid relatively to ourselves; and, so far, events lent countenance
to M. André's eclipse hypothesis, since mutual occultations of the
supposed planetary twins could only take place when the plane of their
revolutions passed through the earth, and this condition would be
transitory. Yet the recognition in Eros of an "Algol asteroid" seems on
other grounds inadmissible;[1021] nor until the phenomenon is
conspicuously renewed--as it probably will be at the opposition of
1903--can there be much hope of finding its appropriate rationale.
The crowd of orbits disclosed by asteroidal detections invites attentive
study. D'Arrest remarked in 1851,[1022] when only thirteen minor planets
were known, that supposing their paths to be represented by solid hoops,
not one of the thirteen could be lifted from its place without bringing
the others with it. The complexity of interwoven tracks thus illustrated
has grown almost in the numerical proportion of discovery. Yet no two
actually intersect, because no two lie exactly in the same plane, so
that the chances of collision are at present _nil_. There is only one
case, indeed, in which it seems to be eventually possible. M. Lespiault
has pointed out that the curves traversed by "Fidés" and "Maïa" approach
so closely that a time may arrive when the bodies in question will
either coalesce or unite to form a binary system.[1023]
The maze threaded by the 500 asteroids contrasts singularly with the
harmoniously ordered and rhythmically separated orbits of the larger
planets. Yet the seeming confusion is not without a plan.
The established rules of our system are far from being totally
disregarded by its minor members. The orbit of Pallas, with its
inclination of 34° 42', touches the limit of departure from the ecliptic
level; the average obliquity of the asteroidal paths is somewhat less
than that of the sun's equator;[1024] their mean eccentricity is below
that of the curve traced out by Mercury, and all without exception are
pursued in the planetary direction--from west to east.
The zone in which these small bodies travel is about three times as wide
as the interval separating the earth from the sun. It extends perilously
near to Jupiter, and dovetails into the sphere of Mars.
Their distribution is very unequal. They are most densely congregated
about the place where a single planet ought, by Bode's Law, to revolve;
it may indeed be said that only stragglers from the main body are found
more than fifty million miles within or without a mean distance from the
sun 2·8 times that of the earth. Significant gaps, too, occur where some
force prohibitive of their presence would seem to be at work. The
probable nature of that force was suggested by the late Professor
Kirkwood, first in 1866, when the number of known asteroids was only
eighty-eight, and again with more confidence in 1876, from the study of
a list then run up to 172.[1025] It appears that these bare spaces are
found just where a revolving body would have a period connected by a
simple relation with that of Jupiter. It would perform two or three
circuits to his one, five to his two, nine to his five, and so on.
Kirkwood's inference was that the gaps in question were cleared of
asteroids by the attractive influence of Jupiter. For disturbances
recurring time after time--owing to commensurability of periods--nearly
at the same part of the orbit, would have accumulated until the shape of
that orbit was notably changed. The body thus displaced would have come
in contact with other cosmical particles of the same family with
itself--then, it may be assumed, more evenly scattered than now--would
have coalesced with them, and permanently left its original track. In
this way the regions of maximum perturbation would gradually have become
denuded of their occupants.
We can scarcely doubt that this law of commensurability has largely
influenced the present distribution of the asteroids. But its effects
must have been produced while they were still in an unformed, perhaps a
nebular condition. In a system giving room for considerable modification
through disturbance, the recurrence of conjunctions with a dominating
mass at the same orbital point need not involve instability.[1026] On
the whole, the correspondence of facts with Kirkwood's hypothesis has
not been impaired by their more copious collection.[1027] Some chasms of
secondary importance have indeed been bridged; but the principal stand
out more conspicuously through the denser scattering of orbits near
their margins. Nor is it doubtful that the influence of Jupiter in some
way produced them. M. de Freycinet's study of the problem they
present[1028] has, however, led him to the conclusion that they existed
_ab origine_, thus testifying rather to the preventive than to the
perturbing power of the giant planet.
The existence, too, of numerous asteroidal pairs travelling in
approximately coincident tracks, must date from a remote antiquity. They
result, Professor Kirkwood[1029] believed, from the divellent action of
Jupiter upon embryo pigmy planets, just as comets moving in pursuit of
one another are a consequence of the sundering influence of the sun.
Leverrier fixed, in 1853,[1030] one-fourth of the earth's mass as the
outside limit for the combined masses of all the bodies circulating
between Mars and Jupiter; but it is far from probable that this maximum
is at all nearly approached. M. Berberich[1031] held that the moon would
more than outweigh the whole of them, a million of the lesser bodies
shining like stars of the twelfth magnitude being needed, according to
his judgment, to constitute her mass. And M. Niesten estimated that the
whole of the 216 asteroids discovered up to August, 1880, amounted in
_volume_ to only 1/4000th of our globe,[1032] and we may safely
add--since they are tolerably certain to be lighter, bulk for bulk, than
the earth--that their proportionate _mass_ is smaller still. A fairly
concordant result was published in 1895 by Mr. B. M. Roszel.[1033] He
found that the lunar globe probably contains forty times, the
terrestrial globe 3,240 times the quantity of matter parcelled out among
the first 311 minor planets. The actual size of a few of them may now be
said to be known. Professor Pickering, from determinations of
light-intensity, assigned to Vesta a diameter of 319 miles, to Pallas
167, to Juno 94, down to twelve and fourteen for the smaller members of
the group.[1034] An albedo equal to that of Mars was assumed as the
basis of the calculation. Moreover, Professor G. Müller[1035] of Potsdam
examined photometrically the phases of seven among them, of which
four--namely, Vesta, Iris, Massalia, and Amphitrite--were found to
conform precisely to the behaviour of Mars as regards light-change from
position, while Ceres, Pallas, and Irene varied after the manner of the
moon and Mercury. The first group were hence inferred to resemble Mars
in physical constitution, nature of atmosphere, and reflective capacity;
the second to be moon-like bodies.
Finally, Professor Barnard, directly measuring with the Yerkes refractor
the minute discs presented by the original quartette, obtained the
following authentic data concerning them:[1036] Diameter of Ceres, 477
miles, albedo = 0·18; diameter of Pallas, 304 miles, albedo = 0·23;
diameter of Vesta, 239 miles, albedo = 0·74; diameter of Juno, 120
miles, albedo = 0·45. Thus, the rank of premier asteroid proves to
belong to Ceres, and to have been erroneously assigned to Vesta in
consequence of its deceptive brilliancy. What kind of surface this
indicates, it is hard to say. The dazzling whiteness of snow can hardly
be attributed to bare rock; yet the dynamical theory of gases--as Dr.
Johnstone Stoney pointed out in 1867[1037]--prohibits the supposition
that bodies of insignificant gravitative power can possess aerial
envelopes. Even our moon, it is calculated, could not permanently hold
back the particles of oxygen, nitrogen, or water-gas from escaping into
infinite space; still less, a globe one thousand times smaller. Vogel's
suspicion of an air-line in the spectrum of Vesta[1038] has,
accordingly, not been confirmed.
* * * * *
Crossing the zone of asteroids on our journey outward from the sun, we
meet with a group of bodies widely different from the "inferior" or
terrestrial planets. Their gigantic size, low specific gravity, and
rapid rotation, obviously from the first threw the "superior" planets
into a class apart; and modern research has added qualities still more
significant of a dissimilar physical constitution. Jupiter, a huge globe
86,000 miles in diameter, stands pre-eminent among them. He is, however,
only _primus inter pares_; all the wider inferences regarding his
condition may be extended, with little risk of error, to his fellows;
and inferences in his case rest on surer grounds than in the case of the
others, from the advantages offered for telescopic scrutiny by his
comparative nearness.
Now the characteristic modern discovery concerning Jupiter is that he is
a body midway between the solar and terrestrial stages of cosmical
existence--a decaying sun or a developing earth, as we choose to put
it--whose vast unexpended stores of internal heat are mainly, if not
solely, efficient in producing the interior agitations betrayed by the
changing features of his visible disc. This view, impressed upon modern
readers by Mr. Proctor's popular works, was anticipated in the last
century. Buffon wrote in his _Époques de la Nature_ (1778):[1039]--"La
surface de Jupiter est, comme l'on sait, sujette à des changemens
sensibles, qui semblent indiquer que cette grosse planète est encore
dans un état d'inconstance et de bouillonnement."
Primitive incandescence, attendant, in his fantastic view, on planetary
origin by cometary impacts with the sun, combined, he concluded, with
vast bulk to bring about this result. Jupiter has not yet had time to
cool. Kant thought similarly in 1785;[1040] but the idea did not commend
itself to the astronomers of the time, and dropped out of sight until
Mr. Nasmyth arrived at it afresh in 1853.[1041] Even still, however,
terrestrial analogies held their ground. The dark belts running parallel
to the equator, first seen at Naples in 1630, continued to be
associated--as Herschel had associated them in 1781--with Jovian
trade-winds, in raising which the deficient power of the sun was
supposed to be compensated by added swiftness of rotation. But opinion
was not permitted to halt here.
In 1860 G. P. Bond of Cambridge (U.S.) derived some remarkable
indications from experiments on the light of Jupiter.[1042] They showed
that fourteen times more of the photographic rays striking it are
reflected by the planet than by our moon, and that, unlike the moon,
which sends its densest rays from the margin, Jupiter is brightest near
the centre. But the most perplexing part of his results was that Jupiter
actually seemed to give out more light than he received. Bond, however,
rightly considered his data too uncertain for the support of so bold an
assumption as that of original luminosity, and, even if the presence of
native light were proved, thought that it might emanate from auroral
clouds of the terrestrial kind. The conception of a sun-like planet was
still a remote, and seemed an extravagant one.
Only since it was adopted and enforced by Zöllner in 1865,[1043] can it
be regarded as permanently acquired to science. The rapid changes in the
cloud-belts both of Jupiter and Saturn, he remarked, attest a high
internal temperature. For we know that all atmospheric movements on the
earth are sun-heat transformed into motion. But sun-heat at the distance
of Jupiter possesses but 1/27, at that of Saturn 1/100 of its force
here. The large amount of energy, then, obviously exerted in those
remote firmaments must have some other source, to be found nowhere else
than in their own active and all-pervading fires, not yet banked in with
a thick solid crust.
The same acute investigator dwelt, in 1871,[1044] on the similarity
between the modes of rotation of the great planets and of the sun,
applying the same principles of explanation to each case. The fact of
this similarity is undoubted. Cassini[1045] and Schröter both noticed
that markings on Jupiter travelled quicker the nearer they were to his
equator; and Cassini even hinted at their possible assimilation to
sun-spots.[1046] It is now well ascertained that, as a rule (not without
exceptions), equatorial spots give a period some 5-1/2 minutes shorter
than those in latitudes of about 30°. But, as Mr. Denning has pointed
out,[1047] no single period will satisfy the observations either of
different markings at the same epoch, or of the same markings at
different epochs. Accelerations and retardations, depending upon
processes of growth or change, take place in very much the same kind of
way as in solar maculæ, inevitably suggesting similarity of origin.
The interesting query as to Jupiter's surface incandescence has been
studied since Bond's time with the aid of all the appliances furnished
to physical inquirers by modern inventiveness, yet without bringing to
it a categorical reply. Zöllner in 1865, Müller in 1893, estimated his
albedo at 0·62 and 0·75 respectively, that of fresh-fallen snow being
0·78, and of white paper 0·70.[1048] But the disc of Jupiter is by no
means purely white. The general ground is tinged with ochre; the polar
zones are leaden or fawn coloured; large spaces are at times stained or
suffused with chocolate-browns and rosy hues. It is occasionally seen
ruled from pole to pole with dusky bars, and is never wholly free from
obscure markings. The reflection, then, by it, as a whole, of about 70
per cent. of the rays impinging upon it, might well suggest some
original reinforcement.
Nevertheless, the spectroscope gives little countenance to the
supposition of any considerable permanent light-emission. The spectrum
of Jupiter, as examined by Huggins, 1862-64, and by Vogel, 1871-73,
shows the familiar Fraunhofer rays belonging to reflected sunlight. But
it also shows lines of native absorption. Some of these are identical
with those produced by the action of our own atmosphere, especially one
or more groups due to aqueous vapours; others are of unknown origin;
and it is remarkable that one among the latter--a strong band in the
red--agrees in position with a dark line in the spectra of some ruddy
stars.[1049] There is, besides, a general absorption of blue rays,
intensified--as Le Sueur observed at Melbourne in 1869[1050]--in the
dusky markings, evidently through an increase of depth in the atmospheric
strata traversed by the light proceeding from them.
All these observations, however (setting aside the stellar line as of
doubtful significance), point to a cool planetary atmosphere. One
spectrograph, it is true, taken by Dr. Henry Draper, September 27,
1879,[1051] seemed to attest the action of intrinsic light; but the
peculiarity was referred by Dr. Vogel, with convincing clearness, to a
flaw in the film.[1052] So far, then, native emissions from any part of
Jupiter's diversified surface have not been detected; and, indeed, the
blackness of the shadows cast by his satellites on his disc sufficiently
proves that he sends out virtually none but reflected light.[1053] This
conclusion, however, by no means invalidates that of his high internal
temperature.
The curious phenomena attending Jovian satellite-transits may be
explained, partly as effects of contrast, partly as due to temporary
obscurations of the small discs projected on the large disc of Jupiter.
At their first entry upon its marginal parts, which are several times
less luminous than those near the centre, they invariably show as bright
spots, then usually vanish as the background gains lustre, to reappear,
after crossing the disc, thrown into relief, as before, against the
dusky limb. But instances are not rare, more especially of the third and
fourth satellites standing out, during the entire middle part of their
course, in such inky darkness as to be mistaken for their own shadows.
The earliest witness of a "black transit" was Cassini, September 2,
1665; Römer in 1677, and Maraldi in 1707 and 1713, made similar
observations, which have been multiplied in recent years. In some cases
the process of darkening has been visibly attended by the formation, or
emergence into view, of spots on the transiting body, as noted by the
two Bonds at Harvard, March 18, 1848.[1054] The third satellite was seen
by Dawes, half dark, half bright, when crossing Jupiter's disc, August
21, 1867;[1055] one-third dark by Davidson of California, January 15,
1884, under the same circumstances;[1056] and unmistakably spotted, both
on and off the planet, by Schröter, Secchi, Dawes, and Lassell.
The first satellite sometimes looks dusky, but never absolutely black,
in travelling over the disc of Jupiter. The second appears uniformly
white--a circumstance attributed by Dr. Spitta[1057] to its high albedo.
The singularly different aspects, even during successive transits, of
the third and fourth satellites, are connected by Professor Holden[1058]
with the varied luminosity of the segments of the planetary surface they
are projected upon, and W. H. Pickering inclines to the same opinion;
but fluctuations in their own brightness[1059] may be a concurrent
cause. Herschel concluded in 1797 that, like our moon, they always turn
the same face towards their primary, and as regards the outer satellite,
Engelmann's researches in 1871, and C. E. Burton's in 1873, made this
almost certain; while both for the third and fourth Jovian moons it was
completely assured by W. H. Pickering's and A. E. Douglass's
observations at Arequipa in 1892,[1060] and at Flagstaff in
1894-95.[1061] Strangely enough, however, the interior members of the
system have preserved a relatively swift rotation, notwithstanding the
enormous checking influence upon it of Jove-raised tides.
All the satellites are stated, on good authority, to be more or less
egg-shaped. On September 8, 1890, Barnard saw the first elongated and
bisected by a bright equatorial belt, during one of its dark
transits;[1062] and his observation, repeated August 3, 1891, was
completely verified by Schaeberle and Campbell, who ascertained,
moreover, that the longer axis of the prolate body was directed towards
Jupiter's centre.[1063] The ellipticity of its companions was determined
by Pickering and Douglass; indeed, that of No. 3 had long previously
been noticed by Secchi.[1064] No. 3 also shows equatorial stripes,
perceived in 1891 by Schaeberle and Campbell,[1065] and evident later to
Pickering and Douglass;[1066] nor need we hesitate to admit as authentic
their records of similar, though less conspicuous markings on the other
satellites. A constitution analogous to that of Jupiter himself was thus
unexpectedly suggested; and Vogel's detection of lines--or traces of
lines--in their spectra, agreeing with absorption-rays derived from
their primary, lends support to the conjecture that they possess gaseous
envelopes similar to his.
The system of Jupiter, as it was discovered by Galileo, and investigated
by Laplace, appeared in its outward aspect so symmetrical, and displayed
in its inner mechanism such harmonious dynamical relations, that it
might well have been deemed complete. Nevertheless, a new member has
been added to it. Near midnight on September 9, 1892, Professor Barnard
discerned with the Lick 36-inch "a tiny speck of light," closely
following the planet.[1067] He instantly divined its nature, watched its
hurried disappearance in the adjacent glare, and made sure of the
reality of his discovery on the ensuing night. It was a delicate
business throughout, the Liliputian luminary subsiding into invisibility
before the slightest glint of Jovian light, and tarrying, only for brief
intervals, far enough from the disc to admit of its exclusion by means
of an occulting plate. The new satellite is estimated to be of the
thirteenth stellar magnitude, and, if equally reflective of light with
its next neighbour, Io (satellite No. 1), its diameter must be about one
hundred miles. It revolves at a distance of 112,500 miles from Jupiter's
centre, and of 68,000 from his bulging equatorial surface. Its period of
11h. 57m. 23s. is just two hours longer than Jupiter's period of
rotation, so that Phobos still remains a unique example of a secondary
body revolving faster than its primary rotates. Jupiter's innermost moon
conforms in its motions strictly, indeed inevitably, to the plane of his
equatorial protuberance, following, however, a sensibly elliptical path
the major axis of which is in rapid revolution.[1068] Its very
insignificance raises the suspicion that it may not prove solitary.
Possibly it belongs to a zone peopled by asteroidal satellites. More
than fifteen thousand such small bodies could be furnished out of the
materials of a single full-sized satellite spoiled in the making. But we
must be content for the present to register the fact without seeking to
penetrate the meaning of its existence. Very high and very fine
telescopic power is needed for its perception. Outside the United
States, it has been very little observed. The only instruments in this
country successfully employed for its detection are, we believe, Dr.
Common's 5-foot reflector and Mr. Newall's 25-inch refractor.
In the course of his observations on Jupiter at Brussels in 1878, M.
Niesten was struck with a rosy cloud attached to a whitish zone beneath
the dark southern equatorial band.[1069] Its size was enormous. At the
distance of Jupiter, its measured dimensions of 13" by 3" implied a real
extension in longitude of 30,000, in latitude of something short of
7,000 miles. The earliest record of its appearance seems to be by
Professor Pritchett, director of the Morrison Observatory (U.S.), who
figured and described it July 9, 1878.[1070] It was again delineated
August 9, by Tempel at Florence.[1071] In the following year it
attracted the wonder and attention of almost every possessor of a
telescope. Its colour had by that time deepened into a full brick-red,
and was set off by contrast with a white equatorial spot of unusual
brilliancy. During three ensuing years these remarkable objects
continued to offer a visible and striking illustration of the compound
nature of the planet's rotation. The red spot completed a circuit in
nine hours fifty-five minutes thirty-six seconds; the white spot in
about five and a half minutes less. Their _relative_ motion was thus no
less than 260 miles an hour, bringing them together in the same meridian
at intervals of forty-four days ten hours forty-two minutes. Neither,
however, preserved continuously the same uniform rate of travel. The
period of each had lengthened by some seconds in 1883, while sudden
displacements, associated with the recovery of lustre after recurrent
fadings, were observed in the position of the white spot,[1072]
recalling the leap forward of a reviving sun-spot. Just the opposite
effect attended the rekindling of the companion object. While
semi-extinct, in 1882-84, it lost little motion; but a fresh access of
retardation was observed by Professor Young[1073] in connection with its
brightening in 1886. This suggests very strongly that the red spot is
_fed from below_. A shining aureola of "faculæ," described by Bredichin
at Moscow, and by Lohse at Potsdam, as encircling it in September,
1879,[1074] was held to strengthen the solar analogy.
The conspicuous visibility of this astonishing object lasted three
years. When the planet returned to opposition in 1882-83, it had faded
so considerably that Riccò's uncertain glimpse of it at Palermo, May 31,
1883, was expected to be the last. It had, nevertheless, begun to
recover in December, and presented to Mr. Denning in the beginning of
1886 much the same aspect as in October, 1882.[1075] Observed by him in
an intermediate stage, February 25, 1885, when "a mere skeleton of its
former self," it bore a striking likeness to an "elliptical ring"
descried in the same latitude by Mr. Gledhill in 1869-70. This, indeed,
might be called the preliminary sketch for the famous object brought to
perfection ten years later, but which Mr. H. C. Russell of Sydney saw
and drew still unfinished in June, 1876,[1076] before it had separated
from its matrix, the dusky south tropical belt. In earlier times, too, a
marking "at once fixed and transient" had been repeatedly perceived
attached to the southernmost of the central belts. It gave Cassini in
1665 a rotation-period of nine hours fifty-six minutes,[1077] reappeared
and vanished eight times during the next forty-three years, and was last
seen by Maraldi in 1713. It was, however, very much smaller than the
recent object, and showed no unusual colour.[1078]
The assiduous observations made on the "Great Red Spot" by Mr. Denning
at Bristol and by Professor Hough at Chicago afforded grounds only for
negative conclusions as to its nature. It certainly did _not_ represent
the outpourings of a Jovian volcano; it was in no sense attached to the
Jovian soil--if the phrase have any application to that planet; it was
_not_ a mere disclosure of a glowing mass elsewhere seethed over by
rolling vapours. It was, indeed, certainly not self-luminous, a
satellite projected upon it in transit having been seen to show as
bright as upon the dusky equatorial bands. A fundamental objection to
all three hypotheses is that the rotation of the spot was variable. It
did not then ride at anchor, but floated free. Some held that its
surface was depressed below the average cloud-level, and that the cavity
was filled with vapours. Professor Wilson, on the other hand, observing
with the 16-inch equatorial of the Goodsell Observatory in Minnesota,
received a persistent impression of the object "being at a higher level
than the other markings."[1079] A crucial experiment on this point was
proposed by Mr. Stanley Williams in 1890.[1080] A dark spot moving
faster along the same parallel was timed to overtake the red spot
towards the end of July. A unique opportunity hence appeared to be at
hand of determining the relative vertical depths of the two formations,
one of which must inevitably, it was thought, pass above the other. No
forecast included a third alternative, which was nevertheless adopted by
the dark spot. It evaded the obstacle in its path by skirting round its
southern edge.[1081] Nothing, then, was gained by the conjunction,
beyond an additional proof of the singular repellent influence exerted
by the red spot over the markings in its vicinity. It has, for example,
gradually carved out a deep bay for its accommodation in the gray belt
just north of it. The effect was not at first steadily present. A
premonitory excavation was drawn by Schwabe at Dessau, September 5,
1831, and again by Trouvelot, Barnard, and Elvins in 1879; yet there was
no sign of it in the following year. Its development can be traced in
Dr. Boeddicker's beautiful delineations of Jupiter, made with the
Parsonstown 3-foot reflector, from 1881 to 1886.[1082] They record the
belt as straight in 1881, but as strongly indented from January, 1883;
and the cavity now promises to outlast the spot. So long as it survives,
however, the forces at work in the spot can have lost little of their
activity. For it must be remembered that the belt has a shorter
rotation-period than the red spot, which, accordingly (as Mr. Elvins of
Toronto has pointed out), breasts and diverts, by its interior energy, a
current of flowing matter, ever ready to fill up its natural bed, and
override the barrier of obstruction.
The famous spot was described by Keeler in 1889, as "of a pale pink
colour, slightly lighter in the middle. Its outline was a fairly true
ellipse, framed in by bright white clouds."[1083] The fading
continuously in progress from 1887 was temporarily interrupted in 1891.
The revival, indeed, was brief. Professor Barnard wrote in August, 1892:
"The great red spot is still visible, but it has just passed through a
crisis that seemingly threatened its very existence. For the past month
it has been all but impossible to catch the feeblest trace of the spot,
though the ever-persistent bay in the equatorial belt close north of it,
and which has been so intimately connected with the history of the red
spot, has been as conspicuous as ever. It is now, however, possible to
detect traces of the entire spot. An obscuring medium seems to have been
passing over it, and has now drifted somewhat preceding the spot."[1084]
The object is now always inconspicuous, and often practically invisible,
and may be said to float passively in the environing medium.[1085] Yet
there are sparks beneath the ashes. A rosy tinge faintly suffused it in
April, 1900,[1086] and its absolute end may still be remote.
The extreme complexity of the planet's surface-movements has been
strikingly evinced by Mr. Stanley Williams's detailed investigations. He
enumerated in 1896[1087] nine principal currents, all flowing parallel
to the equator, but unsymmetrically placed north and south of it, and
showing scant signs of conformity to the solar rule of retardation with
increase of latitude. The linear rate of the planet's equatorial
rotation was spectroscopically determined by Bélopolsky and Deslandres
in 1895. Both found it to fall short of the calculated speed, whence an
enlargement, by self-refraction, of the apparent disc was
inferred.[1088]
Jupiter was systematically photographed with the Lick 36-inch telescope
during the oppositions of 1890, 1891, and 1892, the image thrown on the
plates (after eightfold direct enlargement) being one inch in diameter.
Mr. Stanley Williams's measurements and discussion of the set for 1891
showed the high value of the materials thus collected, although much
more minute details can be seen than can at present be photographed. The
red spot shows as "very distinctly annular" in several of these
pictures.[1089] Recently, the planet has been portrayed by Deslandres
with the 62-foot Meudon refractor.[1090] The extreme actinic feebleness
of the equatorial bands was strikingly apparent on his plates.
In 1870, Mr. Ranyard[1091]--whose death, December 14, 1894, was a
serious loss to astronomy--acting upon an earlier suggestion of Sir
William Huggins, collected records of unusual appearances on the disc of
Jupiter, with a view to investigate the question of their recurrence at
regular intervals. He concluded that the development of the deeper
tinges of colour, and of the equatorial "port-hole" markings girdling
the globe in regular alternations of bright and dusky, agreed, so far as
could be ascertained, with epochs of sun-spot maximum. The further
inquiries of Dr. Lohse at Bothkamp in 1873[1092] went to strengthen the
coincidence, which had been anticipated _à priori_ by Zöllner in
1871.[1093] Moreover, separate and distinct evidence was alleged by Mr.
Denning in 1899 of decennial outbreaks of disturbance in north temperate
regions.[1094] It may, indeed, be taken for granted that what Hahn terms
the universal pulse of the solar system[1095] affects the vicissitudes
of Jupiter; but the law of those vicissitudes is far from being so
obviously subordinate to the rhythmical flow of central disturbance as
are certain terrestrial phenomena. The great planet, being in fact
himself a "semi-sun," may be regarded as an originator, no less than a
recipient, of agitating influences, the combined effects of which may
well appear insubordinate to any obvious law.
It is likely that Saturn is in a still earlier stage of planetary
development than Jupiter. He is the lightest for his size of all the
planets. In fact, he would float in water. And since his density is
shown, by the amount of his equatorial bulging, to increase
centrally,[1096] it follows that his superficial materials must be of a
specific gravity so low as to be inconsistent, on any probable
supposition, with the solid or liquid states. Moreover, the chief
arguments in favour of the high temperature of Jupiter, apply, with
increased force, to Saturn; so that it may be concluded, without much
risk of error, that a large proportion of his bulky globe, 73,000 miles
in diameter, is composed of heated vapours, kept in active and agitated
circulation by the process of cooling.
His unique set of appendages has, since the middle of the last century,
formed the subject of searching and fruitful inquiries, both theoretical
and telescopic. The mechanical problem of the stability of Saturn's
rings was left by Laplace in a very unsatisfactory condition.
Considering them as rotating solid bodies, he pointed out that they
could not maintain their position unless their weight were in some way
unsymmetrically distributed; but made no attempt to determine the kind
or amount of irregularity needed to secure this end. Some observations
by Herschel gave astronomers an excuse for taking for granted the
fulfilment of the condition thus vaguely postulated; and the question
remained in abeyance until once more brought prominently forward by the
discovery of the dusky ring in 1850.
The younger Bond led the way, among modern observers, in denying the
solidity of the structure. The fluctuations in its aspect were, he
asserted in 1851,[1097] inconsistent with such a hypothesis. The fine
dark lines of division, frequently detected in both bright rings, and as
frequently relapsing into imperceptibility, were due, in his opinion, to
the real nobility of their particles, and indicated a fluid formation.
Professor Benjamin Peirce of Harvard University immediately followed
with a demonstration, on abstract grounds, of their non-solidity.[1098]
Streams of some fluid denser than water were, he maintained, the
physical reality giving rise to the anomalous appearance first disclosed
by Galileo's telescope.
The mechanism of Saturn's rings, proposed as the subject of the Adams
Prize, was dealt with by James Clerk Maxwell in 1857. His investigation
forms the groundwork of all that is at present known in the matter. Its
upshot was to show that neither solid nor fluid rings could continue to
exist, and that the only possible composition of the system was by an
aggregated multitude of unconnected particles, each revolving
independently in a period corresponding to its distance from the
planet.[1099] This idea of a satellite-formation had been, remarkably
enough, several times entertained and lost sight of. It was first put
forward by Roberval in the seventeenth century, again by Jacques Cassini
in 1715, and with perfect definiteness by Wright of Durham in
1750.[1100] Little heed, however, was taken of these casual
anticipations of a truth which reappeared, a virtual novelty, as the
legitimate outcome of the most refined modern methods.
The details of telescopic observation accord, on the whole, admirably
with this hypothesis. The displacements or disappearance of secondary
dividing-lines--the singular striated appearance, first remarked by
Short in the eighteenth century, last by Perrotin and Lockyer at Nice,
March 18, 1884[1101]--show the effects of waves of disturbance
traversing a moving mass of gravitating particles;[1102] the broken and
changing line of the planet's shadow on the ring gives evidence of
variety in the planes of the orbits described by those particles. The
whole ring-system, too, appears to be somewhat elliptical.[1103]
The satellite-theory has derived unlooked-for support from photometric
inquiries. Professor Seeliger pointed out in 1888[1104] that the
unvarying brilliancy of the outer rings under all angles of
illumination, from 0° to 30°, can be explained from no other point of
view. Nor does the constitution of the obscure inner ring offer any
difficulty. For it is doubtless formed of similar small bodies to those
aggregated in the lucid members of the system, only much more thinly
strewn, and reflecting, consequently, much less light. It is not,
indeed, at first easy to see why these sparser flights should show as a
dense dark shading on the body of Saturn. Yet this is invariably the
case. The objection has been urged by Professor Hastings of Baltimore.
The brightest parts of these appendages, he remarked,[1105] are more
lustrous than the globe they encircle; but if the inner ring consists of
identical materials, possessing presumably an equal reflective capacity,
the mere fact of their scanty distribution would not cause them to show
as dark against the same globe. Professor Seeliger, however,
replied[1106] that the darkening is due to the never-ending swarms of
their separate shadows transiting the planet's disc. Sunlight is not,
indeed, wholly excluded. Many rays come and go between the open ranks of
the meteorites. For the dusky ring is transparent. The planet it
encloses shows through it, as if veiled with a strip of crape. A
beautiful illustration of its quality in this respect was derived by
Professor Barnard from an eclipse of Japetus, November 1, 1889.[1107]
The eighth moon remained steadily visible during its passage through the
shadow of the inner ring, but with a progressive loss of lustre in
approaching its bright neighbour. There was no breach of continuity. The
satellite met no gap, corresponding to that between the dusky ring and
the body of Saturn, through which it could shine with undiminished
light, but was slowly lost sight of as it plunged into deeper and deeper
gloom. The important facts were thus established, that the brilliant and
obscure rings merge into each other, and that the latter thins out
towards the Saturnian globe.
The meteoric constitution of these appendages was beautifully
demonstrated in 1895 by Professor Keeler,[1108] then director of the
Alleghany Observatory, Pittsburgh. From spectrographs taken with the
slit adjusted to coincidence with the equatorial plane of the system, he
determined the comparative radial velocities of its different parts. And
these supply a crucial test of Clerk Maxwell's theory. For if the rings
were solid, the swiftest rates of rotation should be at their outer
edges, corresponding to wider circles described in the same period;
while, if they are pulverulent, the inverse relation must hold good.
This proved to be actually the case. The motion slowed off outward, in
agreement with the diminishing speed of particles travelling freely,
each in its own orbit. Keeler's result was promptly confirmed by
Campbell,[1109] as well as by Deslandres and Bélopolsky.
A question of singular interest, and one which we cannot refrain from
putting to ourselves, is--whether we see in the rings of Saturn a
finished structure, destined to play a permanent part in the economy of
the system; or whether they represent merely a stage in the process of
development out of the chaotic state in which it is impossible to doubt
that the materials of all planets were originally merged. M. Otto Struve
attempted to give a definite answer to this important query.
A study of early and later records of observations disclosed to him, in
1851, an apparent progressive approach of the inner edge of the bright
ring to the planet. The rate of approach he estimated at about
fifty-seven English miles a year, or 11,000 miles during the 194 years
elapsed since the time of Huygens.[1110] Were it to continue, a collapse
of the system must be far advanced within three centuries. But was the
change real or illusory--a plausible, but deceptive inference from
insecure data? M. Struve resolved to put it to the test. A set of
elaborately careful micrometrical measures of the dimensions of Saturn's
rings, executed by himself at Pulkowa in the autumn of 1851, was
provided as a standard of future comparison; and he was enabled to renew
them, under closely similar circumstances, in 1882.[1111] But the
expected diminution of the space between Saturn's globe and his rings
had not taken place. A slight extension in the width of the system, both
outward and inward, was indeed, hinted at; and it is worth notice that
just such a separation of the rings was indicated by Clerk Maxwell's
theory, so that there is an _à priori_ likelihood of its being in
progress. Yet Hall's measures in 1884-87[1112] failed to supply evidence
of alteration with time; and Barnard's, executed at Lick in
1894-95,[1113] showed no sensible divergence from them. Hence, much
weight cannot be laid upon Huygens's drawings and descriptions, which
had been held to prove conclusively a partial filling up, since 1657, of
the interval between the ring and the planet.[1114]
The rings of Saturn replace, in Professor G. H. Darwin's view,[1115] an
abortive satellite, scattered by tidal action into annular form. For
they lie closer to the planet than is consistent with the integrity of a
revolving body of reasonable bulk. The limit of possible existence for
such a mass was fixed by Roche of Montpellier, in 1848,[1116] at 2·44
mean radii of its primary; while the outer edge of the ring-system is
distant 2·38 radii of Saturn from his centre. The virtual discovery of
its pulverulent condition dates, then, according to Professor Darwin,
from 1848. He conjectures that the appendage will eventually disappear,
partly through the dispersal of its constituent particles inward, and
their subsidence upon the planet's surface, partly by their dispersal
outward, to a region beyond "Roche's limit," where coalescence might
proceed unhindered by the strain of unequal attractions. One modest
satellite, revolving inside Mimas, would then be all that was left of
the singular appurtenances we now contemplate with admiration.
There seems reason to admit that Kirkwood's law of commensurability has
had some effect in bringing about the present distribution of the matter
composing them. Here the influential bodies are Saturn's moons, while
the divisions and boundaries of the rings represent the spaces where
their disturbing action conspires to eliminate revolving particles.
Kirkwood, in fact, showed, in 1867,[1117] that a body circulating in the
chasm between the bright rings known as "Cassini's division," would have
a period nearly commensurable with those of _four_ out of the eight
moons; and Meyer of Geneva subsequently calculated all such
combinations, with the result of bringing out coincidences between
regions of maximum perturbation and the limiting and dividing lines of
the system.[1118] This is in itself a strong confirmation of the view
that the rings are made up of independently revolving small bodies.
On December 7, 1876, Professor Asaph Hall discovered at Washington a
bright equatorial spot on Saturn, which he followed and measured through
above sixty rotations, each performed in ten hours fourteen minutes
twenty-four seconds.[1119] This, he was careful to add, represented the
period, not necessarily of the _planet_, but only of the individual
spot. The only previous determination of Saturn's axial movement
(setting aside some insecure estimates by Schröter) was Herschel's in
1794, giving a period of ten hours sixteen minutes. The substantial
accuracy of Hall's result was verified by Mr. Denning in 1891.[1120] In
May and June of that year, ten vague bright markings near the equator
were watched by Mr. Stanley Williams, who derived from them a rotation
period only two seconds shorter than that determined at Washington.
Nevertheless, similarly placed spots gave in 1892 and 1893 notably
quicker rates;[1121] so that the task of timing the general drift of the
Saturnian surface by the displacements of such objects is hampered, to
an indefinite extent, by their individual proper motions.
Saturn's outermost satellite, Japetus, is markedly variable--so variable
that it sends us, when brightest, just 4-1/2 times as much light as when
faintest. Moreover, its fluctuations depend upon its orbital position in
such a way as to make it a conspicuous telescopic object when west, a
scarcely discernible one when east of the planet. Herschel's
inference[1122] of a partially obscured globe turning always the same
face towards its primary seems the only admissible one, and is confirmed
by Pickering's measurements of the varying intensity of its light. He
remarked further that the dusky and brilliant hemispheres must be so
posited as to divide the disc, viewed from Saturn, into nearly equal
parts; so that this Saturnian moon, even when "full," appears very
imperfectly illuminated over one-half of its surface.[1123]
Zöllner estimated the albedo of Saturn at 0·51, Müller at 0·88, a value
impossibly high, considering that the spectrum includes no vestige of
original emissions. Closely similar to that of Jupiter, it shows the
distinctive dark line in the red (wave-length 618), which we may call
the "red-star line"; and Janssen, from the summit of Etna in 1867[1124]
found traces in it of aqueous absorption. The light from the ring
appears to be pure reflected sunshine unmodified by original atmospheric
action.[1125]
Uranus, when favourably situated, can easily be seen with the naked eye
as a star between the fifth and sixth magnitudes. There is indeed, some
reason to suppose that he had been detected as a wandering orb by savage
"watchers of the skies" in the Pacific long before he swam into
Herschel's ken. Nevertheless, inquiries into his physical habitudes are
still in an early stage. They are exceedingly difficult of execution,
even with the best and largest modern telescopes; and their results
remain clouded with uncertainty.
It will be remembered that Uranus presents the unusual spectacle of a
system of satellites travelling nearly at right angles to the plane of
the ecliptic. The existence of this anomaly gives a special interest to
investigations of his axial movement, which might be presumed, from the
analogy of the other planets, to be executed in the same tilted plane.
Yet this is far from being certainly the case.
Mr. Buffham in 1870-72 caught traces of bright markings on the Uranian
disc, doubtfully suggesting a rotation in about twelve hours in a plane
_not_ coincident with that in which his satellites circulate.[1126]
Dusky bands resembling those of Jupiter, but very faint, were barely
perceptible to Professor Young at Princeton in 1883. Yet, though almost
necessarily inferred to be equatorial, they made a considerable angle
with the trend of the satellites' orbits.[1127] More distinctly by the
brothers Henry, with the aid of their fine refractor, two gray parallel
rulings, separated by a brilliant zone, were discerned every clear night
at Paris from January to June, 1884.[1128] What were taken to be the
polar regions appeared comparatively dusky. The direction of the
equatorial rulings (for so we may safely call them) made an angle of 40°
with the satellites' line of travel. Similar observations were made at
Nice by MM. Perrotin and Thollon, March to June, 1884, a lucid spot near
the equator, in addition, indicating rotation in a period of about ten
hours.[1129] The discrepancy was, however, considerably reduced by
Perrotin's study of the planet in 1889 with the new 30-inch
equatoreal.[1130] The dark bands, thus viewed to better advantage than
in 1884, appeared to deviate no more than 10° from the satellites'
orbit-plane. No definitive results, on the other hand, were derived by
Professors Holden, Schaeberle, and Keeler from their observations of
Uranus in 1889-90 with the potent instrument on Mount Hamilton.
Shadings, it is true, were almost always, though faintly, seen; but they
appeared under an anomalous, possibly an illusory aspect. They
consisted, not of parallel, but of forked bands.[1131]
Measurements of the little sea-green disc which represents to us the
massive bulk of Uranus, by Young, Schiaparelli,[1132] Safarik, H. C.
Wilson[1133] and Perrotin, prove it to be quite distinctly _bulged_. The
compression at once caught Barnard's trained eye in 1894,[1134] when he
undertook at Lick a micrometrical investigation of the system; and he
was surprised to perceive that the major axis of the elliptical surface
made an angle of about 28° with the line of travel pursued by the
satellites. Nothing more can be learned on this curious subject for some
years, since the pole of the planet is just now turned nearly towards
the earth; but Barnard's conclusion is unlikely to be seriously
modified. He fixed the mean diameter of Uranus at 34,900 miles. But this
estimate was materially reduced through Dr. See's elimination of
irradiative effects by means of daylight measures, executed at
Washington in 1901.[1135]
The visual spectrum of this planet was first examined by Father Secchi
in 1869, and later, with more advantages for accuracy, by Huggins,
Vogel,[1136] and Keeler.[1137] It is a very remarkable one. In lieu of
the reflected Fraunhofer lines, imperceptible perhaps through feebleness
of light, six broad bands of original absorption appear, one
corresponding to the blue-green ray of hydrogen (F), another to the
"red-star line" of Jupiter and Saturn, the rest as yet unidentified. The
hydrogen band seems much too strong and diffuse to be the mere echo of a
solar line, and might accordingly be held to imply the presence of free
hydrogen in the Uranian atmosphere. This, however, would be difficult of
reconcilement with Keeler's identification of an absorption-group in the
yellow with a telluric waterband.
Notwithstanding its high albedo--0·62, according to Zöllner--proof is
wanting that any of the light of Uranus is inherent. Mr. Albert Taylor
announced, indeed, in 1889, his detection, with Common's giant
reflector, of bright flutings in its spectrum;[1138] but Professor
Keeler's examination proved them to be merely contrast effects.[1139]
Sir William and Lady Huggins, moreover, obtained about the same time a
photograph purely solar in character. The spectrum it represented was
crossed by numerous Fraunhofer lines, and by no others. It was, then,
presumably composed entirely of reflected light.
* * * * *
Judging from the indications of an almost evanescent spectrum, Neptune,
as regards physical condition, is the twin of Uranus, as Saturn of
Jupiter. Of the circumstances of his rotation we are as good as
completely ignorant. Mr. Maxwell Hall, indeed, noticed at Jamaica, in
November and December, 1883, certain rhythmical fluctuations of
brightness, suggesting revolution on an axis in slightly less than eight
hours;[1140] but Professor Pickering reduces the supposed variability to
an amount altogether too small for certain perception, and Dr. G. Müller
denies its existence _in toto_. It is true their observations were not
precisely contemporaneous with those of Mr. Hall[1141] who believes the
partial obscurations recorded by himself to have been of a passing kind,
and to have suddenly ceased after a fortnight of prevalence. Their less
conspicuous renewal was visible to him in November, 1884, confirming a
rotation period of 7·92 hours.
It was ascertained at first by indirect means that the orbit of
Neptune's satellite is inclined about 20° to his equator. Mr.
Marth[1142] having drawn attention to the rapid shifting of its plane of
motion, M. Tisserand and Professor Newcomb[1143] independently published
the conclusion that such shifting necessarily results from Neptune's
ellipsoidal shape. The movement is of the kind exemplified--although
with inverted relations--in the precession of the equinoxes. The pole of
the satellite, owing to the pull of Neptune's equatorial protuberance,
describes a circle round the pole of his equator in a retrograde
direction, and in a period of over five hundred years. The amount of
compression indicated for the primary body is, at the outside, 1/85;
whence it can be inferred that Neptune possesses a lower rotatory
velocity than the other giant planets. Direct verification of the trend
theoretically inferred for the satellite's movement was obtained by Dr.
See in 1899. The Washington 26-inch refractor disclosed to him, under
exceptionally favourable conditions, a set of equatorial belts on the
disc of Neptune, and they took just the direction prescribed by theory.
Their objective reality cannot be doubted, although Barnard was unable,
either with the Lick or the Yerkes telescope,[1144] to detect any
definite markings on this planet. Its diameter was found by him to be
32,900 miles.
The possibility that Neptune may not be the most remote body circling
round the sun has been contemplated ever since he has been known to
exist. Within the last few years the position at a given epoch of a
planet far beyond his orbital verge has been approximately fixed by two
separate investigators.
Professor George Forbes of Edinburgh adopted in 1880 a novel plan of
search for unknown members of the solar system, the first idea of which
was thrown out by M. Flammarion in November, 1879.[1145] It depends upon
the movements of comets. It is well known that those of moderately short
periods are, for a reason already explained, connected with the larger
planets in such a way that the cometary aphelia fall near some planetary
orbit. Jupiter claims a large retinue of such partial dependents,
Neptune owns six, and there are two considerable groups, the farthest
distances of which from the sun lie respectively near 100 and 300 times
that of the earth. At each of these vast intervals, one involving a
period of 1,000, the other of 5,000 years, Professor Forbes maintains
that an unseen planet circulates. He even computed elements for the
nearer of the two, and fixed its place on the celestial sphere;[1146]
but the photographic searches made for it by Dr. Roberts at Crowborough
and by Mr. Wilson at Daramona proved unavailing. Undeterred by
Deichmüller's discouraging opinion that cometary orbits extending beyond
the recognised bounds of the solar system are too imperfectly known to
serve as the basis of trustworthy conclusions,[1147] the Edinburgh
Professor returned to the attack in 1901.[1148] He now sought to prove
that the lost comet of 1556 actually returned in 1844, but with elements
so transformed by ultra-Neptunian perturbations as to have escaped
immediate identification. If so, the "wanted" planet has just entered
the sign Libra, and, being larger than Jupiter, should be possible to
find.
Almost simultaneously with Forbes, Professor Todd set about groping for
the same object by the help of a totally different set of indications.
Adams's approved method commended itself to him; but the hypothetical
divagations of Neptune having scarcely yet had time to develop, he was
thrown back upon the "residual errors" of Uranus. They gave him a
virtually identical situation for the new planet with that derived from
the clustering of cometary aphelia.[1149] Yet its assigned distance was
little more than half that of the nearer of Professor Forbes's remote
pair, and it completed a revolution in 375 instead of 1,000 years. The
agreement in them between the positions determined, on separate grounds,
for the ultra-Neptunian traveller was merely an odd coincidence; nor can
we be certain, until it is seen, that we have really got into touch with
it.
FOOTNOTES:
[Footnote 965: _Phil. Trans._, vol. lxxiv., p. 260.]
[Footnote 966: _Novæ Observationes_, p. 105.]
[Footnote 967: _Phil. Trans._, vol. i., p. 243.]
[Footnote 968: _Mém. de l'Ac._, 1720, p. 146.]
[Footnote 969: _Phil. Trans._, vol. lxxiv., p. 273.]
[Footnote 970: A large work, entitled _Areographische Fragmente_, in
which Schröter embodied the results of his labours on Mars, 1785-1803,
narrowly escaped the conflagration of 1813, and was published at Leyden
in 1881.]
[Footnote 971: _Beiträge_, p. 124.]
[Footnote 972: _Mem. R. A. Soc._, vol. xxxii., p. 183.]
[Footnote 973: _Astr. Nach._, No. 1,468.]
[Footnote 974: _Observatory_, vol. viii., p. 437.]
[Footnote 975: _Month. Not._, vols. xxviii., p. 37; xxix., p. 232;
xxxiii., p. 552.]
[Footnote 976: Flammarion, _L'Astronomie_, t. i., p. 266.]
[Footnote 977: Smyth, _Cel. Cycle_, vol. i., p. 148 (1st ed.).]
[Footnote 978: _Phil. Trans._, vol. cxxi., p. 417.]
[Footnote 979: _Month. Not._, vol. xxv., p. 227.]
[Footnote 980: _Phil. Mag._, vol. xxxiv., p. 75.]
[Footnote 981: Proctor, _Quart. Jour. of Science_, vol. x., p. 185;
Maunder, _Sunday Mag._, January, February, March, 1882; Campbell, _Publ.
Astr. Pac. Soc._, vol. vi., p. 273.]
[Footnote 982: _Am. Jour. of Sc._, vol. xxviii., p. 163.]
[Footnote 983: Burton, _Trans. Roy. Dublin Soc._, vol. i., 1880, p.
169.]
[Footnote 984: _Month. Not._, vol. xxvii., p. 179; _Astroph. Journ._,
vol. i., p. 193.]
[Footnote 985: _Untersuchungen über die Spectra der Planeten_, p. 20;
_Astroph. Journ._, vol. i., p. 203.]
[Footnote 986: _Publ. Astr. Pac. Soc._, vols. vi., p. 228; ix., p. 109;
_Astr. and Astroph._, vol. xiii., p. 752; _Astroph. Jour._, vol. ii., p.
28.]
[Footnote 987: _Ibid._, vol. v., p. 328.]
[Footnote 988: _Ibid._, vols. i., p. 311; iii., p. 254.]
[Footnote 989: C. Christiansen, _Beiblätter_, 1886, p. 532.]
[Footnote 990: _Astr. and Astrophysics_, vol. xi., p. 671.]
[Footnote 991: Flammarion, _La Planète Mars_, p. 574.]
[Footnote 992: _Mémoires Couronnés_, t. xxxix.]
[Footnote 993: Lockyer, _Nature_, vol. xlvi., p. 447.]
[Footnote 994: _Mem. Spettr. Italiani_, t. xi., p. 28.]
[Footnote 995: _Bull. Astr._, t. iii., p. 324.]
[Footnote 996: _Journ. Brit. Astr. Ass._, vol. i., p. 88.]
[Footnote 997: _Publ. Pac. Astr. Soc._, vol. ii., p. 299; Percival
Lowell, _Mars_, 1896; _Annals of the Lowell Observatory_, vol. ii.,
1900.]
[Footnote 998: _Old and New Astr._, p. 545.]
[Footnote 999: _L'Astronomie_, t. xi., p. 445.]
[Footnote 1000: _La Planète Mars_, p. 588.]
[Footnote 1001: _Month. Notices_, vol. lvi., p. 166.]
[Footnote 1002: _L'Astronomie_, t. viii.]
[Footnote 1003: _Astr. Nach._, No. 3,271; _Astr. and Astrophysics_, vol.
xiii., p. 716.]
[Footnote 1004: _Month. Not._, vol. xxxviii., p. 41; _Mem. Roy. Astr.
Soc._, vol. xliv., p. 123.]
[Footnote 1005: _Astr. and Astrophysics_, vol. xi., p. 668.]
[Footnote 1006: _Ibid._, p. 850.]
[Footnote 1007: _Comptes Rendus_, t. cxv., p. 379.]
[Footnote 1008: _Astr. Jour._, No. 384; _Publ. Astr. Pac. Soc._, vol.
vi., p. 109. _Cf. Observatory_ vol. xvii., pp. 295-336.]
[Footnote 1009: See Mr. Wentworth Erck's remarks in _Trans. Roy. Dublin
Soc._, vol. i., p. 29.]
[Footnote 1010: _Month. Not._, vol. xxxviii., p. 206.]
[Footnote 1011: _Annals Harvard Coll. Obs._, vol. xi., pt. ii., p. 217.]
[Footnote 1012: Young, _Gen. Astr._, p. 366.]
[Footnote 1013: Campbell, _Publ. Pac. Astr. Soc._, vol. vi., p. 270.]
[Footnote 1014: _Astr. Nach._, No. 3,319.]
[Footnote 1015: _Witch of Atlas_, stanza iii. I am indebted to Dr.
Garnett for the reference.]
[Footnote 1016: Recommended by Chandler, _Astr. Jour._, No. 452.]
[Footnote 1017: _Harvard Circulars_, Nos. 36, 37, 51.]
[Footnote 1018: _Astr. Nach._, No. 3,687.]
[Footnote 1019: Montangerand, _Comptes Rendus_, March 11, 1901.]
[Footnote 1020: Pickering, _Astroph. Jour._, vol. xiii., p. 277.]
[Footnote 1021: _Harvard Circular_, No. 58.]
[Footnote 1022: _Astr. Nach._, No. 752.]
[Footnote 1023: L. Niesten, _Annuaire_, Bruxelles, 1881, p. 269.]
[Footnote 1024: According to Svedstrup (_Astr. Nach._, Nos. 2,240-41),
the inclination to the ecliptic of the "mean asteroid's" orbit is = 6°.]
[Footnote 1025: _Smiths._ Report, 1876, p. 358; _The Asteroids_
(Kirkwood), p. 42, 1888.]
[Footnote 1026: Tisserand, _Annuaire_, Paris, 1891, p. B. 15; Newcomb,
_Astr. Jour._, No. 477; Backlund, _Bull. Astr._, t. xvii., p. 81;
Parmentier, _Bull. Soc. Astr. de France_, March, 1896; Observatory, vol.
xviii., p. 207.]
[Footnote 1027: Berberich, _Astr. Nach._, No. 3,088.]
[Footnote 1028: _Bull. Astr._, t. xviii., p. 39.]
[Footnote 1029: _The Asteroids_, p. 48; _Publ. Astr. Pac. Soc._, vols.
ii., p. 48; iii., p. 95.]
[Footnote 1030: _Comptes Rendus_, t. xxxvii., p. 797.]
[Footnote 1031: _Bull. Astr._, t. v., p. 180.]
[Footnote 1032: _Annuaire_, Bruxelles, 1881, p. 243.]
[Footnote 1033: _Johns Hopkins Un. Circular_, January, 1895;
_Observatory_, vol. xviii., p. 127.]
[Footnote 1034: _Harvard Annals_, vol. xi., part ii., p. 294.]
[Footnote 1035: _Astr. Nach._, Nos. 2,724-5.]
[Footnote 1036: _Month. Not._, vol. lxi., p. 69.]
[Footnote 1037: _Astroph. Jour._, vol. vii., p. 25.]
[Footnote 1038: _Spectra der Planeten_, p. 24.]
[Footnote 1039: Tome i., p. 93.]
[Footnote 1040: _Berlinische Monatsschrift_, 1785, p. 211.]
[Footnote 1041: _Month. Not._, vol. xiii., p. 40.]
[Footnote 1042: _Mem. Am. Ac._, vol. viii., p. 221.]
[Footnote 1043: _Photom. Unters._, p. 303.]
[Footnote 1044: _Astr. Nach._, No. 1,851.]
[Footnote 1045: _Mém. de l'Ac._, t. x., p. 514.]
[Footnote 1046: _Ibid._, 1692, p. 7.]
[Footnote 1047: _Month. Not._, vol. xliv., p. 63.]
[Footnote 1048: _Photom. Unters._, pp. 165, 273; _Potsdam Publ._, No.
30.]
[Footnote 1049: Vogel, _Sp. der Planeten_, p. 33, _note_.]
[Footnote 1050: _Proc. Roy. Soc._, vol. xviii., p. 250.]
[Footnote 1051: _Month. Not._, vol. xl., p. 433.]
[Footnote 1052: _Sitzungsberichte_, Berlin, 1895, ii., p. 15.]
[Footnote 1053: The anomalous shadow-effects recorded by Webb (_Cel.
Objects_, p. 170, 4th ed.) are obviously of atmospheric and optical
origin.]
[Footnote 1054: Engelmann, _Ueber die Helligkeitsverhältnisse der
Jupiterstrabanten_, p. 59.]
[Footnote 1055: _Month. Not._, vol. xxviii., p. 11.]
[Footnote 1056: _Observatory_, vol. vii., p. 175.]
[Footnote 1057: _Month. Not._, vol. xlviii., p. 43.]
[Footnote 1058: _Publ. Astr. Pac. Soc._, vol. ii., p. 296.]
[Footnote 1059: Pickering failed to obtain any photometric evidence of
their variability. _Harvard Annals_, vol. xi., p. 245.]
[Footnote 1060: _Astr. and Astroph._, vol. xii., pp. 194, 481.]
[Footnote 1061: _Annals Lowell Obs._, vol. ii., pt. i.]
[Footnote 1062: _Astr. Nach._, Nos. 2,995, 3,206; _Month. Not._, vols.
li., p. 556; liv., p. 134. Barnard remains convinced that the oval forms
attributed to Jupiter's satellites are illusory effects of their
markings. _Astr. Nach._, Nos. 3,206, 3,453; _Astr. and Astroph._, vol.
xiii., p. 272.]
[Footnote 1063: _Publ. Astr. Pac. Soc._, vol. iii., p. 355.]
[Footnote 1064: _Astr. Nach._, No. 1,017.]
[Footnote 1065: _Publ. Astr. Pac. Soc._, vol. iii., p. 359.]
[Footnote 1066: _Astr. Nach._, No. 3,432.]
[Footnote 1067: _Astr. Jour._, Nos. 275, 325, 367, 472; _Observatory_,
vol. xv., p. 425.]
[Footnote 1068: Tisserand, _Comptes Rendus_, October 8, 1894; Cohn,
_Astr. Nach._, No. 3,404.]
[Footnote 1069: _Bull. Ac. R. Bruxelles_, t. xlviii., p. 607.]
[Footnote 1070: _Astr. Nach._, No. 2,294.]
[Footnote 1071: _Ibid._, No. 2,284.]
[Footnote 1072: Denning, _Month. Not._, vol. xliv., pp. 64, 66;
_Nature_, vol. xxv., p. 226.]
[Footnote 1073: _Sidereal Mess._, December, 1886, p. 289.]
[Footnote 1074: _Astr. Nach._, Nos. 2,280, 2,282.]
[Footnote 1075: _Month. Not._, vol. xlvi., p. 117.]
[Footnote 1076: _Proc. Roy. Soc. N. S. Wales_, vol. xiv., p. 68.]
[Footnote 1077: _Phil. Trans._, vol. i., p. 143.]
[Footnote 1078: For indications relative to the early history of the red
spot, see Holden, _Publ. Astr. Pac. Soc._, vol. ii., p. 77; Noble,
_Month. Not._, vol. xlvii., p. 515; A. S. Williams, _Observatory_, vol.
xiii., p. 338.]
[Footnote 1079: _Astr. and Astrophysics_, vol. xi., p. 192.]
[Footnote 1080: _Month. Not._, vol. l., p. 520.]
[Footnote 1081: _Observatory_, vol. xiii., pp. 297, 326.]
[Footnote 1082: _Trans. R. Dublin Soc._, vol. iv., p. 271, 1889.]
[Footnote 1083: _Publ. Astr. Pac. Soc._, vol. ii., p. 289.]
[Footnote 1084: _Astr. and Astrophysics_, vol. xi., p. 686.]
[Footnote 1085: Denning, _Knowledge_, vol. xxiii., p. 200;
_Observatory_, vol. xxiv., p. 312; _Pop. Astr._, vol. ix., p. 448;
_Nature_, vol. lv., p. 89.]
[Footnote 1086: Williams, _Observatory_, vol. xxiii., p. 282.]
[Footnote 1087: _Month. Not._, vol. lvi., p. 143.]
[Footnote 1088: Bélopolsky, _Astr. Nach._, No. 3,326.]
[Footnote 1089: _Publ. Astr. Pac. Soc._, vol. iv., p. 176.]
[Footnote 1090: _Bull. Astr._, 1900, p. 70.]
[Footnote 1091: _Month. Not._, vol. xxxi., p. 34.]
[Footnote 1092: _Beobachtungen_, Heft ii., p. 99.]
[Footnote 1093: _Ber. Sächs. Ges. der Wiss._, 1871, p. 553.]
[Footnote 1094: _Month. Not._, vol. lix., p. 76.]
[Footnote 1095: _Beziehungen der Sonnenfleckenperiode_, p. 175.]
[Footnote 1096: A. Hall, _Astr. Nach._, No. 2,269.]
[Footnote 1097: _Astr. Jour._ (Gould's), vol. ii., p. 17.]
[Footnote 1098: _Ibid._, p. 5.]
[Footnote 1099: _On the Stability of the Motion of Saturn's Rings_, p.
67.]
[Footnote 1100: _Mém. de l'Ac._, 1715, p. 47; Montucla, _Hist. des
Math._, t. iv., p. 19; _An Original Theory of the Universe_, p. 115.]
[Footnote 1101: _Comptes Rendus_, t. xcviii., p. 718.]
[Footnote 1102: Proctor, _Saturn and its System_ (1865), p. 125.]
[Footnote 1103: Perrotin, _Comptes Rendus_, t. cvi., p. 1716.]
[Footnote 1104: _Abhandl. Akad. der Wiss._, Munich, Bd. xvi., p. 407.]
[Footnote 1105: _Smiths. Report_, 1880 (Holden).]
[Footnote 1106: Quoted by Dr. E. Anding, _Astr. Nach._, No. 2,881.]
[Footnote 1107: _Astr. and Astrophysics_, vol. xi., p. 119; _Month.
Not._, vol. l., p. 108.]
[Footnote 1108: _Astroph. Jour._, vol. i., p. 416.]
[Footnote 1109: _Ibid._, vol. ii., p. 127.]
[Footnote 1110: _Mém. de l'Ac. Imp._ (St. Petersb.), t. vii., 1853, p.
464.]
[Footnote 1111: _Astr. Nach._, No. 2,498.]
[Footnote 1112: _Washington Observations_, App. ii., p. 22]
[Footnote 1113: _Month. Not._, vol. lvi., p. 163.]
[Footnote 1114: T. Lewis, _Observatory_, vol. xviii., p. 379.]
[Footnote 1115: _Harper's Magazine_, June, 1889.]
[Footnote 1116: _Mém. de l'Acad. de Montpellier_, t. viii., p. 296,
1873.]
[Footnote 1117: _Meteoric Astronomy_, chap. xii. He carried the subject
somewhat farther in 1871. See _Observatory_, vol. vi., p. 335.]
[Footnote 1118: _Astr. Nach._, No. 2,527.]
[Footnote 1119: _Amer. Jour. of Sc._, vol. xiv., p. 325.]
[Footnote 1120: _Observatory_, vol. xiv., p. 369.]
[Footnote 1121: _Month. Not._, vol. liv., p. 297.]
[Footnote 1122: _Phil. Trans._, vol. lxxxii., p. 14.]
[Footnote 1123: _Smiths. Report_, 1880.]
[Footnote 1124: _Comptes Rendus_, t. lxiv., p. 1304.]
[Footnote 1125: Huggins, _Proc. R. Soc._, vol. xlvi., p. 231; Keeler,
_Astr. Nach._, No. 2,927; Vogel, _Astroph. Jour._, vol. i., p. 278.]
[Footnote 1126: _Month. Not._, vol. xxxiii., p. 164.]
[Footnote 1127: _Astr. Nach._, No 2,545.]
[Footnote 1128: _Comptes Rendus_, t. xcviii., p. 1419.]
[Footnote 1129: _Comptes Rendus_, t. xcviii., pp. 718, 967.]
[Footnote 1130: _V. J. S. Astr. Ges._, Jahrg. xxiv., p. 267.]
[Footnote 1131: _Publ. Astr. Pac. Soc._, vol. iii., p. 287.]
[Footnote 1132: _Astr. Nach._, No. 2,526.]
[Footnote 1133: _Ibid._, No. 2,730.]
[Footnote 1134: _Astr. Jour._, Nos. 370, 374.]
[Footnote 1135: _Astr. Nach._, No. 3,768.]
[Footnote 1136: _Ann. der Phys._, Bd. clviii., p. 470; _Astroph. Jour._,
vol. i., p. 280.]
[Footnote 1137: _Astr. Nach._, No. 2,927.]
[Footnote 1138: _Month. Not._, vol. xlix., p. 405.]
[Footnote 1139: Astr. Nach., No. 2,927; Scheiner's _Spectralanalyse_, p.
221.]
[Footnote 1140: _Month. Not._, vol. xliv., p. 257.]
[Footnote 1141: _Observatory_, vol. vii., pp. 134, 221, 264.]
[Footnote 1142: _Month. Not._, vol. xlvi., p. 507.]
[Footnote 1143: _Comptes Rendus_, t. cvii., p. 804; _Astr. and
Astroph._, vol. xiii., p. 291; _Astr. Jour._, No. 186.]
[Footnote 1144: _Astr. Jour._, Nos. 342, 436, 508.]
[Footnote 1145: _Astr. Pop._, p. 661; _La Nature_, January 3, 1880.]
[Footnote 1146: _Proc. Roy. Soc. Edinb._, vols. x., p. 429; xi., p. 89.]
[Footnote 1147: _Vierteljahrsschrift. Astr. Ges._, Jahrg. xxi., p. 206.]
[Footnote 1148: _Proc. Roy. Soc. Edinb._, vol. xxiii., p. 370; _Nature_,
vol. lxiv., p. 524.]
[Footnote 1149: _Amer. Jour. of Science_, vol. xx., p. 225.]
CHAPTER IX
_THEORIES OF PLANETARY EVOLUTION_
We cannot doubt that the solar system, as we see it, is the result of
some process of growth--that, during innumerable ages, the forces of
Nature were at work upon its materials, blindly modelling them into the
shape appointed for them from the beginning by Omnipotent Wisdom. To set
ourselves to inquire what that process was may be an audacity, but it is
a legitimate, nay, an inevitable one. For man's implanted instinct to
"look before and after" does not apply to his own little life alone, but
regards the whole history of creation, from the highest to the
lowest--from the microscopic germ of an alga or a fungus to the visible
frame and furniture of the heavens.
Kant considered that the inquiry into the mode of origin of the world
was one of the easiest problems set by Nature; but it cannot be said
that his own solution of it was satisfactory. He, however, struck out in
1755 a track which thought still pursues. In his _Allgemeine
Naturgeschichte_ the growth of sun and planets was traced from the
cradle of a vast and formless mass of evenly diffused particles, and the
uniformity of their movements was sought to be accounted for by the
unvarying action of attractive and repulsive forces, under the dominion
of which their development was carried forward.
In its modern form, the "Nebular Hypothesis" made its appearance in
1796.[1150] It was presented by Laplace with diffidence, as a
speculation unfortified by numerical buttresses of any kind, yet with
visible exultation at having, as he thought, penetrated the birth-secret
of our system. He demanded, indeed, more in the way of postulates than
Kant had done. He started with a sun ready made,[1151] and surrounded
with a vast glowing atmosphere, extending into space out beyond the
orbit of the farthest planet, and endowed with a slow rotatory motion.
As this atmosphere or nebula cooled, it contracted; and as it
contracted, its rotation, by a well-known mechanical law, became
accelerated. At last a point arrived when tangential velocity at the
equator increased beyond the power of gravity to control, and
equilibrium was restored by the separation of a nebulous ring revolving
in the same period as the generating mass. After a time, the ring broke
up into fragments, all eventually reunited in a single revolving and
rotating body. This was the first and farthest planet.
Meanwhile the parent nebula continued to shrink and whirl quicker and
quicker, passing, as it did so, through successive crises of
instability, each resulting in, and terminated by, the formation of a
planet, at a smaller distance from the centre, and with a shorter period
of revolution than its predecessor. In these secondary bodies the same
process was repeated on a reduced scale, the birth of satellites ensuing
upon their contraction, or not, according to circumstances. Saturn's
ring, it was added, afforded a striking confirmation of the theory of
annular separation,[1152] and appeared to have survived in its original
form in order to throw light on the genesis of the whole solar system;
while the four first discovered asteroids offered an example in which
the _débris_ of a shattered ring had failed to coalesce into a single
globe.
This scene of cosmical evolution was a characteristic bequest from the
eighteenth century to the nineteenth. It possessed the self-sufficing
symmetry and entireness appropriate to the ideas of a time of
renovation, when the complexity of nature was little accounted of in
comparison with the imperious orderliness of the thoughts of man. Since
its promulgation, however, knowledge has transgressed many boundaries,
and set at naught much ingenious theorising. How has it fared with
Laplace's sketch of the origin of the world? It has at least not been
discarded as effete. The groundwork of speculation on the subject is
still furnished by it. It is, nevertheless, admittedly inadequate. Of
much that exists it gives no account, or an erroneous one. The march of
events certainly did not everywhere--even if it did anywhere--follow the
exact path prescribed for it. Yet modern science attempts to supplement,
but scarcely ventures to supersede it.
Thought has, in many directions, been profoundly modified by Mayer's and
Joule's discovery, in 1842, of the equivalence between heat and motion.
Its corollary was the grand idea of the "conservation of energy," now
one of the cardinal principles of science. This means that, under the
ordinary circumstances of observation, the old maxim _ex nihilo nihil
fit_ applies to force as well as to matter. The supplies of heat, light,
electricity, must be kept up, or the stream will cease to flow. The
question of the maintenance of the sun's heat was thus inevitably
raised; and with the question of maintenance that of origin is
indissolubly connected.
Dr. Julius Robert Mayer, a physician residing at Heilbronn, was the
first to apply the new light to the investigation of what Sir John
Herschel had termed the "great secret." He showed that if the sun were a
body either simply cooling or in a state of combustion, it must long
since have "gone out." Had an equal mass of coal been set alight four or
five centuries after the building of the Pyramid of Cheops, and kept
burning at such a rate as to supply solar light and heat during the
interim, only a few cinders would now remain in lieu of our undiminished
glorious orb. Mayer looked round for an alternative. He found it in the
"meteoric hypothesis" of solar conservation.[1153] The importance in the
economy of our system of the bodies known as falling stars was then (in
1848) beginning to be recognised. It was known that they revolved in
countless swarms round the sun; that the earth daily encountered
millions of them; and it was surmised that the cone of the zodiacal
light represented their visible condensation towards the attractive
centre. From the zodiacal light, then, Mayer derived the store needed
for supporting the sun's radiations. He proved that, by the stoppage of
their motion through falling into the sun, bodies would evolve from
4,600 to 9,200 times as much heat (according to their ultimate velocity)
as would result from the burning of equal masses of coal, their
precipitation upon the sun's surface being brought about by the
resisting medium observed to affect the revolutions of Encke's comet.
There was, however, a difficulty. The quantity of matter needed to keep,
by the sacrifice of its movement, the hearth of our system warm and
bright would be very considerable. Mayer's lowest estimate put it at
94,000 billion kilogrammes per second, or a mass equal to that of our
moon bi-annually. But so large an addition to the gravitating power of
the sun would quickly become sensible in the movement of the bodies
dependent upon him. Their revolutions would be notably accelerated.
Mayer admitted that each year would be shorter than the previous one by
a not insignificant fraction of a second, and postulated an unceasing
waste of substance, such as Newton had supposed must accompany emission
of the material corpuscles of light, to neutralise continual
reinforcement.
Mayer's views obtained a very small share of publicity, and owned Mr.
Waterston as their independent author in this country. The meteoric, or
"dynamical," theory of solar sustentation was expounded by him before
the British Association in 1853. It was developed with his usual ability
by Lord Kelvin, in the following year. The inflow of meteorites, he
remarked, "is the only one of all conceivable causes of solar heat which
we know to exist from independent evidence."[1154] We know it to exist,
but we now also know it to be entirely insufficient. The supplies
presumed to be contained in the zodiacal light would be quickly
exhausted; a constant inflow from space would be needed to meet the
demand. But if moving bodies were drawn into the sun at anything like
the required rate, the air, even out here at ninety-three millions of
miles distance, would be thick with them; the earth would be red-hot
from their impacts;[1155] geological deposits would be largely
meteoric;[1156] to say nothing of the effects on the mechanism of the
heavens. Lord Kelvin himself urged the inadmissibility of the
"extra-planetary" theory of meteoric supply on the very tangible ground
that, if it were true, the year would be shorter now, actually by six
weeks, than at the opening of the Christian era. The "intra-planetary"
supply, however, is too scanty to be anything more than a temporary
makeshift.
The meteoric hypothesis was naturally extended from the maintenance of
the sun's heat to the formation of the bodies circling round him. The
earth--no less doubtless than the other planets--is still growing.
Cosmical matter in the shape of falling stars and aërolites, to the
amount, adopting Professor Newton's estimate, of 100 tons daily, is
swept up by it as it pursues its orbital round. Inevitably the idea
suggested itself that this process of appropriation gives the key to the
life-history of our globe, and that the momentary streak of fire in the
summer sky represents a feeble survival of the glowing hailstorm by
which in old times it was fashioned and warmed. Mr. E. W. Brayley
supported this view of planetary production in 1864,[1157] and it has
recommended itself to Haidinger, Helmholtz, Proctor, and Faye. But the
negative evidence of geological deposits appears fatal to it.
The theory of solar energy now generally regarded as the true one was
enounced by Helmholtz in a popular lecture in 1854. It depends upon the
same principle of the equivalence of heat and motion which had suggested
the meteoric hypothesis. But here the movement surrendered and
transformed belongs to the particles, not of any foreign bodies, but of
the sun itself. Drawn together from a wide ambit by the force of their
own gravity, their fall towards the sun's centre must have engendered a
vast thermal store, of which 453/454 are computed to be already spent.
Presumably, however, this stream of reinforcement is still flowing. In
the very act of parting with heat, the sun develops a fresh stock. His
radiations, in short, are the direct result of shrinkage through
cooling. A diminution of the solar diameter by 380 feet yearly would
just suffice to cover the present rate of emission, and would for ages
remain imperceptible with our means of observation, since, after the
lapse of 6,000 years, the lessening of angular size would scarcely
amount to one second.[1158] But the process, though not terminated, is
strictly a terminable one. In less than five million years, the sun will
have contracted to half its present bulk. In seven million more, it will
be as dense as the earth. It is difficult to believe that it will then
be a luminous body.[1159] Nor can an unlimited past duration be
admitted. Helmholtz considered that radiation might have gone on with
its actual intensity for twenty-two, Langley allows only eighteen
million years. The period can scarcely be stretched, by the most
generous allowances, to double the latter figure. But this is far from
meeting the demands of geologists and biologists.
An attempt was made in 1881 to supply the sun with machinery analogous
to that of a regenerative furnace, enabling it to consume the same fuel
over and over again, and so to prolong indefinitely its beneficent
existence. The inordinate "waste" of energy, which shocks our thrifty
ideas, was simultaneously abolished. The earth stops and turns variously
to account one 2,250-millionth part of the solar radiations; each of the
other planets and satellites takes a proportionate share; the rest,
being all but an infinitesmal fraction of the whole, is dissipated
through endless space, to serve what purpose we know not. Now, on the
late Sir William Siemens's plan, this reckless expenditure would cease;
the solar incomings and outgoings would be regulated on approved
economic principles, and the inevitable final bankruptcy would be staved
off to remote ages.
But there was a fatal flaw in its construction. He imagined a perpetual
circulation of combustible materials, alternately surrendering and
regaining chemical energy, the round being kept going by the motive
force of the sun's rotation.[1160] This, however, was merely to perch
the globe upon a tortoise, while leaving the tortoise in the air. The
sun's rotation contains a certain definite amount of mechanical
power--enough, according to Lord Kelvin, if directly converted into
heat, to keep up the sun's emission during 116 years and six days--a
mere moment in cosmical time. More economically applied, it would no
doubt go farther. Its exhaustion would, nevertheless, under the most
favourable circumstances, ensue in a comparatively short period.[1161]
Many other objections equally unanswerable have been urged to the
"regenerative" hypothesis, but this one suffices.
Dr. Croll's collision hypothesis[1162] is less demonstrably unsound, but
scarcely less unsatisfactory. By the mutual impact of two dark masses
rushing together with tremendous speed, he sought to provide the solar
nebula with an immense _original_ stock of heat for the reinforcement of
that subsequently evolved in the course of its progressive contraction.
The sun, while still living on its capital, would thus have a larger
capital to live on, and the time-demands of the less exacting geologists
and biologists might be successfully met. But the primitive event,
assumed for the purpose of dispensing them from the inconvenience of
"hurrying up their phenomena," is not one that a sane judgment can
readily admit to have ever, in point of actual fact, happened.
There remains, then, as the only intelligible rationale of solar
sustentation, Helmholtz's shrinkage theory. And this has a very
important bearing upon the nebular view of planetary formation; it may,
in fact, be termed its complement. For it involves the idea that the
sun's materials, once enormously diffused, gradually condensed to their
present volume with development of heat and light, and, it may plausibly
be added, with the separation of dependent globes. The data furnished by
spectrum analysis, too, favour the supposition of a common origin for
sun and planets by showing their community of substance; while gaseous
nebulæ present examples of vast masses of tenuous vapour, such as our
system may plausibly be conjectured to have primitively sprung from.
But recent science raises many objections to the details, if it supplies
some degree of confirmation to the fundamental idea of Laplace's
cosmogony. The detection of the retrograde movement of Neptune's
satellite made it plain that the anomalous conditions of the Uranian
world were due to no extraordinary disturbance, but to a systematic
variety of arrangement at the outskirts of the solar domain. So that,
were a trans-Neptunian planet discovered, we should be fully prepared to
find it rotating, and surrounded by satellites circulating from east to
west. The uniformity of movement, upon the probabilities connected with
which the French geometer mainly based his scheme, thus at once
vanishes.
The excessively rapid revolution of the inner Martian moon is a further
stumbling-block. On Laplace's view, _no_ satellite can revolve in a
shorter time than its primary rotates; for in its period of circulation
survives the period of rotation of the parent mass which filled the
sphere of its orbit at the time of giving it birth. And rotation
quickens as contraction goes on; therefore, the older time of axial
rotation should invariably be the longer. This obstacle can, however, as
we shall presently see, be turned.
More serious is one connected with the planetary periods, pointed out by
Babinet in 1861.[1163] In order to make them fit in with the hypothesis
of successive separation from a rotating and contracting body, certain
arbitrary assumptions have to be made of fluctuations in the
distribution of the matter forming that body at the various epochs of
separation.[1164] Such expedients usually merit the distrust which they
inspire. Primitive and permanent irregularities of density in the solar
nebula, such as Miss Young's calculations suggest,[1165] do not, on the
other hand, appear intrinsically improbable.
Again, it was objected by Professor Kirkwood in 1869[1166] that there
could be no sufficient cohesion in such an enormously diffused mass as
the planets are supposed to have sprung from to account for the wide
intervals between them. The matter separated through the growing excess
of centrifugal speed would have been cast off, not by rarely recurring
efforts, but continually, fragmentarily, _pari passu_ with condensation
and acceleration. Each wisp of nebula, as it found itself unduly
hurried, would have declared its independence, and set about revolving
and condensing on its own account. The result would have been a
meteoric, not a planetary system.
Moreover, it is a question whether the relative ages of the planets do
not follow an order just the reverse of that concluded by Laplace.
Professor Newcomb holds the opinion that the rings which eventually
constituted the planets divided from the main body of the nebula almost
simultaneously, priority, if there were any, being on the side of the
inner and smaller ones;[1167] while in M. Faye's cosmogony,[1168] the
retrograde motion of the systems formed by the two outer planets is
ascribed--on grounds, it is true, of dubious validity--to their
comparatively late origin.
This ingenious scheme was designed, not merely to complete, but to
supersede that of Laplace, which, undoubtedly, through the inclusion by
our system of oppositely directed rotations, forfeits its claim simply
and singly to account for the fundamental peculiarities of its
structure.
M. Faye's leading contention is that, under the circumstances assumed by
Laplace, not the two outer planets alone, but the whole company must
have been possessed of retrograde rotation. For they were formed--_ex
hypothesi_--after the sun; central condensation had reached an advanced
stage when the rings they were derived from separated; the principle of
inverse squares consequently held good, and Kepler's Laws were in full
operation. Now, particles circulating in obedience to these laws can
only--since their velocity decreases outward from the centre of
attraction--coalesce into a globe with a _backward_ axial movement. Nor
was Laplace blind to this flaw in his theory; but his effort to remove
it, though it passed muster for the best part of a century,[1169] was
scarcely successful. His planet-forming rings were made to rotate _all
in one piece_, their outer parts thus necessarily travelling at a
swifter linear rate than their inner parts, and eventually uniting,
equally of necessity, into a _forward_-spinning body. The strength of
cohesion involved may, however, safely be called impossible, especially
when it is considered that nebulous materials were in question.
The reform proposed by M. Faye consists in admitting that all the
planets inside Uranus are of pre-solar origin--that they took globular
form in the bosom of a nearly homogeneous nebula, revolving in a single
period, with motion accelerated from centre to circumference, and hence
agglomerating into masses with a direct rotation. Uranus and Neptune owe
their exceptional characteristics to their later birth. When they came
into existence, the development of the sun was already far advanced,
central force had acquired virtually its present strength, unity of
period had been abolished by its predominance, and motion was retarded
outward.
Thus, what we may call the relative chronology of the solar system is
thrown once more into confusion. The order of seniority of the planets
is now no easier to determine than the "Who first, who last?" among the
victims of Hector's spear. For M. Faye's arrangements, notwithstanding
the skill with which he has presented them, cannot be unreservedly
accepted. The objections to them, thoughtfully urged by M. C. Wolf[1170]
and Professor Darwin,[1171] are grave. Not the least so is his omission
to take account of an agency of change presently to be noticed.
A further valuable discussion of the matter was published by M. du
Ligondès in 1897.[1172] His views are those of Faye, modified to disarm
the criticisms they had encountered; and special attention may be
claimed for his weighty remark that each planet has a life-history of
its own, essentially distinct from those of the others, and, despite
original unity, not to be confounded with them. The drift of recent
investigations seems, indeed, to be to find the embryonic solar system
already potentially complete in the parent nebula, like the oak in an
acorn, and to relegate detailed explanations of its peculiarities to the
dim pre-nebular fore-time.
We now come to a most remarkable investigation--one, indeed, unique in
its profession to lead us back with mathematical certainty towards the
origin of a heavenly body. We refer to Professor Darwin's inquiries into
the former relations of the earth and moon.[1173]
They deal exclusively with the effects of tidal friction, and primarily
with those resulting, not from oceanic, but from "bodily" tides, such as
the sun and moon must have raised in past ages on a liquid or viscous
earth. The immediate effect of either is, as already explained, to
destroy the rotation of the body on which the tide is raised, as regards
the tide-raising body, bringing it to turn always the same face towards
its disturber. This, we can see, has been completely brought about in
the case of the moon. There is, however, a secondary or reactive effect.
Action is always mutual. Precisely as much as the moon pulls the
terrestrial tidal wave backward, the tidal wave pulls the moon forward.
But pulling a body forward in its orbit implies the enlargement of that
orbit; in other words, the moon is, as a consequence of tidal friction,
very slowly receding from the earth. This will go on (other
circumstances remaining unchanged) until the lengthening day overtakes
the more tardily lengthening month, when each will be of about 1,400
hours.[1174] A position of what we may call tidal equilibrium between
earth and moon will (apart from disturbance by other bodies) then be
attained.
If, however, it be true that, in the time to come, the moon will be much
farther from us, it follows that in the time past she was much nearer to
us than she now is. Tracing back her history by the aid of Professor
Darwin's clue, we at length find her revolving in a period of somewhere
between two and four hours, almost in contact with an earth rotating
just at the same rate. This was before tidal friction had begun its work
of grinding down axial velocity and expanding orbital range. But the
position was not one of stable equilibrium. The slightest inequality
must have set on foot a series of uncompensated changes. If the moon had
whirled the least iota faster than the earth spun she must have been
precipitated upon it. Her actual existence shows that the trembling
balance inclined the other way. By a second or two to begin with, the
month exceeded the day; the tidal wave crept ahead of the moon; tidal
friction came into play, and our satellite started on its long spiral
journey outward from the parent globe. This must have occurred, it is
computed, _at least_ fifty-four million years ago.
That this kind of tidal reactive effect played its part in bringing the
moon into its present position, and is still, to some slight extent, at
work in changing it, there can be no doubt whatever. An irresistible
conjecture carried the explorer of its rigidly deducible consequences
one step beyond them. The moon's time of revolution, when so near the
earth as barely to escape contact with it, must have been, by Kepler's
Law, more than two and less than two and a half hours. Now it happens
that the most rapid rate of rotation of a fluid mass of the earth's
average density, consistent with spheroidal equilibrium, is two hours
and twenty minutes. Quicken the movement but by one second and the globe
must fly asunder. Hence the inference that the earth actually _did_ fly
asunder through over-fast spinning, the ensuing disruption representing
the birth-throes of the moon. It is likely that the event was hastened
or helped by solar tidal disturbance.
To recapitulate. Analysis tracks backward the two bodies until it leaves
them in very close contiguity, one rotating and the other revolving in
approximately the same time, and that time certainly not far different
from, and quite possibly identical with, the critical period of
instability for the terrestrial spheroid. "Is this," Professor Darwin
asks, "a mere coincidence, or does it not rather point to the break-up
of the primeval planet into two masses in consequence of a too rapid
rotation?"[1175]
We are tempted, but are not allowed to give an unqualified assent. Mr.
James Nolan of Victoria has made it clear that the moon could not have
subsisted as a continuous mass under the powerful disruptive strain
which would have acted upon it when revolving almost in contact with the
present surface of the earth; and Professor Darwin, admitting the
objection, concedes to our satellite, in its initial stage, the
alternative form of a flock of meteorites.[1176] But such a congregation
must have been quickly dispersed, by tidal action, into a meteoric ring.
The same investigator subsequently fixed 6,500 miles from centre to
centre as the minimum distance at which the moon could have revolved in
its entirety; and he concluded it "necessary to suppose that, after the
birth of a satellite, if it takes place at all in this way, a series of
changes occur which are quite unknown."[1177] The evidence, however, for
the efficiency of tidal friction in bringing about the actual
configuration of the lunar-terrestrial system is not invalidated by this
failure to penetrate its natal mystery. Under its influence the
principal elements of that system fall into interdependent mutual
relations. It connects, casually and quantitatively, the periods of the
moon's revolution and of the earth's rotation, the obliquity of the
ecliptic, the inclination and eccentricity of the lunar orbit. All this
can scarcely be accidental.
Professor Darwin's first researches on this subject were communicated to
the Royal Society, December 18, 1879. They were followed, January 20,
1881,[1178] by an inquiry on the same principles into the earlier
condition of the entire solar system. The results were a warning against
hasty generalisation. They showed that the lunar-terrestrial system, far
from being a pattern for their development, was a singular exception
among the bodies swayed by the sun. Its peculiarity resides in the fact
that the moon is _proportionately_ by far the most massive attendant
upon any known planet. Its disturbing power over its primary is thus
abnormally great, and tidal friction has, in consequence, played a
predominant part in bringing their mutual relations into their present
state.
The comparatively late birth of the moon tends to ratify this inference.
The dimensions of the earth did not differ (according to our present
authority) very greatly from what they now are when her solitary
offspring came, somehow, into existence. This is found not to have been
the case with any other of the planets. It is unlikely that the
satellites of Jupiter, Saturn, or Mars (we may safely add, of Uranus or
Neptune) ever revolved in much narrower orbits than those they now
traverse; it is practically certain that they did not, like our moon,
originate very near the _present_ surfaces of their primaries.[1179]
What follows? The tide-raising power of a body grows with vicinity in a
rapidly accelerated ratio. Lunar tides must then have been on an
enormous scale when the moon swung round at a fraction of its actual
distance from the earth. But no other satellite with which we are
acquainted occupied at any time a corresponding position. Hence no other
satellite ever possessed tide-raising capabilities in the least
comparable to those of the moon. We conclude once more that tidal
friction had an influence here very different from its influence
elsewhere. Quite possibly, however, that influence may be more nearly
spent than in less advanced combinations of revolving globes. Mr. Nolan
concluded in 1895[1180] that it still retains appreciable efficacy in
the several domains of the outer planets. The moons of Jupiter and
Saturn are, by his calculations, in course of sensible retreat, under
compulsion of the perennial ripples raised by them on the surfaces of
their gigantic primaries. He thus connects the interior position of the
fifth Jovian satellite with its small mass. The feebleness of its
tide-raising power obliged it to remain behind its companions; for there
is no sign of its being more juvenile than the Galilean quartette.
The yielding of plastic bodies to the strain of unequal attractions is a
phenomenon of far-reaching consequence. We know that the sun as well as
the moon causes tides in our oceans. There must, then, be solar, no less
than lunar, tidal friction. The question at once arises: What part has
it played in the development of the solar system? Has it ever been one
of leading importance, or has its influence always been, as it now is,
subordinate, almost negligible? To this, too, Professor Darwin supplies
an answer.
It can be stated without hesitation that the sun did _not_ give birth to
the planets, as the earth has been supposed to have given birth to the
moon, by the disruption of its already condensed, though viscous and
glowing mass, pushing them then gradually backward from its surface into
their present places. For the utmost possible increase in the length of
the year through tidal friction is one hour; and five minutes is a more
probable estimate.[1181] So far as the pull of tide-waves raised on the
sun by the planets is concerned, then, the distances of the latter have
never been notably different from what they now are; though that cause
may have converted the paths traversed by them from circles into
ellipses.
Over their _physical_ history, however, it was probably in a large
measure influential. The first vital issue for each of them
was--satellites or no satellites? Were they to be governors as well as
governed, or should they revolve in sterile isolation throughout the
æons of their future existence? Here there is strong reason to believe
that solar tidal friction was the overruling power. It is remarkable
that planetary fecundity increases--at least so far outward as
Saturn--with distance from the sun. Can these two facts be in any way
related? In other words, is there any conceivable way by which tidal
influence could prevent or impede the throwingoff of secondary bodies?
We have only to think for a moment in order to see that this is
precisely one of its direct results.[1182]
Tidal friction, whether solar or lunar, tends to reduce the axial
movement of the body it acts upon. But the separation of satellites
depends--according to the received view--upon the attainment of a
disruptive rate of rotation. Hence, if solar tidal friction were strong
enough to keep down the pace below this critical point, the contracting
mass would remain intact--there would be no satellite-production. This,
in all probability, actually occurred in the case both of Mercury and
Venus. They cooled without dividing, because the solar friction-brake
applied to them was too strong to permit acceleration to pass the limit
of equilibrium. The complete destruction of their relative axial
movement has been rendered probable by recent observations; and that the
process went on rapidly is a reasonable further inference. The earth
barely escaped the fate of loneliness incurred by her neighbours. Her
first and only epoch of instability was retarded until she had nearly
reached maturity. The late appearance of the moon accounts for its large
relative size--through the increased cohesion of an already strongly
condensed parent mass--and for the distinctive peculiarities of its
history and influence on the producing globe.
Solar tidal friction, although it did not hinder the formation of two
minute dependents of Mars, has been invoked to explain the anomalously
rapid revolution of one of them. Phobos, we have seen, completes more
than three revolutions while Mars rotates once. But this was probably
not always so. The two periods were originally nearly equal. The
difference, it is alleged, was brought about by tidal waves raised by
the sun on the semi-fluid spheroid of Mars. Rotatory velocity was
thereby destroyed, the Martian day slowly lengthened, and, as a
secondary consequence, the period of the inner satellite, become shorter
than the augmented day, began progressively to diminish. So that Phobos,
unlike our moon, was in the beginning farther from its primary than now.
But here again Mr. Nolan entered a _caveat_. Applying the simple test of
numerical evaluation, he showed that before solar tidal friction could
lengthen the rotation-period of Mars by so much as one minute, Phobos
should have been precipitated upon its surface.[1183] For the enormous
disparity of mass between it and the sun is so far neutralised by the
enormous disparity in their respective distances from Mars that solar
tidal force there is only fifty times that of the little satellite. But
the tidal effects of a satellite circulating quicker than its primary
rotates exactly reverse those of one moving, like our moon,
comparatively slowly, so that the tides raised by Phobos tend to
_shorten_ both periods. Its orbital momentum, however, is so extremely
small in proportion to the rotational momentum of Mars, that any
perceptible inroad upon the latter is attended by a lavish and ruinous
expenditure of the former. It is as if a man owning a single five-pound
note were to play for equal stakes with a man possessing a million. The
bankruptcy sure to ensue is typified by the coming fate of the Martian
inner satellite. The catastrophe of its fall needs to bring it about
only a very feeble reactive pull compared with the friction which the
sun should apply in order to protract the Martian day by one minute. And
from the proportionate strength of the forces at work, it is quite
certain that one result cannot take place without the other. Nor can
things have been materially different in the past; hence the idea must
be abandoned that the primitive time of rotation of Mars survives in the
period of its inner satellite.
The anomalous shortness of the latter may, however, in M. Wolf's
opinion,[1184] be explained by the "traînées elliptiques" with which
Roche supplemented nebular annulation.[1185] These are traced back to
the descent of separating strata from the _shoulders_ of the great
nebulous spheroid towards its equatorial plane. Their rotational
velocity being thus relatively small, they formed "inner rings," very
much nearer to the centre of condensation than would have been possible
on the unmodified theory of Laplace. Phobos might, in this view, be
called a polar offset of Mars; and the rings of Saturn are thought to
own a similar origin.
Outside the orbit of Mars, solar tidal friction can scarcely be said to
possess at present any sensible power. But it is far from certain that
this was always so. It seems not unlikely that its influence was the
overruling one in determining the direction of planetary rotation. M.
Faye, as we have seen, objected to Laplace's scheme that only retrograde
secondary systems could be produced by it. In this he was anticipated by
Kirkwood, who, however, supplied an answer to his own objection.[1186]
Sun-raised tides must have acted with great power on the diffused masses
of the embryo planets. By their means they doubtless very soon came to
turn (in lunar fashion) the same hemisphere always towards their centre
of motion. This amounts to saying that even if they started with
retrograde rotation, it was, by solar tidal friction, quickly rendered
direct.[1187] For it is scarcely necessary to point out that a planet
turning an invariable face to the sun rotates in the same direction in
which it revolves, and in the same period. As, with the progress of
condensation, tides became feebler and rotation more rapid, the
accelerated spinning necessarily proceeded in the sense thus prescribed
for it. Hence the backward axial movements of Uranus and Neptune may
very well be a survival, due to the inefficiency of solar tides at their
great distance, of a state of things originally prevailing universally
throughout the system.
The general outcome of Mr. Darwin's researches has been to leave
Laplace's cosmogony untouched. He concludes nothing against it, and,
what perhaps tells with more weight in the long run, has nothing to
substitute for it. In one form or the other, if we speculate at all on
the development of the planetary system, our speculations are driven
into conformity with the broad lines of the Nebular Hypothesis--to the
extent, at least, of admitting an original material unity and motive
uniformity. But we can see now, better than formerly, that these supply
a bare and imperfect sketch of the truth. We should err gravely were we
to suppose it possible to reconstruct, with the help of any knowledge
our race is ever likely to possess, the real and complete history of our
admirable system. "The subtlety of nature," Bacon says, "transcends in
many ways the subtlety of the intellect and senses of man." By no mere
barren formula of evolution, indiscriminately applied all round, the
results we marvel at, and by a fragment of which our life is
conditioned, were brought forth; but by the manifold play of interacting
forces, variously modified and variously prevailing, according to the
local requirements of the design they were appointed to execute.
FOOTNOTES:
[Footnote 1150: _Exposition du Système du Monde_, t. ii., p. 295.]
[Footnote 1151: In later editions a retrospective clause was added
admitting a prior condition of all but evanescent nebulosity.]
[Footnote 1152: _Méc. Cél._, lib. xiv., ch. iii.]
[Footnote 1153: _Beiträge zur Dynamik des Himmels_, p. 12.]
[Footnote 1154: _Trans. Roy. Soc. of Edinburgh_, vol. xxi., p. 66.]
[Footnote 1155: Newcomb, _Pop. Astr._, p. 521 (2nd ed.).]
[Footnote 1156: M. Williams, _Nature_, vol. iii., p. 26.]
[Footnote 1157: _Comp. Brit. Almanac_, p. 94.]
[Footnote 1158: Radau, _Bull. Astr._, t. ii., p. 316.]
[Footnote 1159: Newcomb, _Pop. Astr._, pp. 521-525.]
[Footnote 1160: _Proc. Roy. Soc._, vol. xxxiii., p. 393.]
[Footnote 1161: To this hostile argument, as urged by Mr. E. Douglas
Archibald, Sir W. Siemens opposed the increase of rotative velocity
through contraction (_Nature_, vol. xxv., p. 505). But contraction
cannot restore lost momentum.]
[Footnote 1162: _Stellar Evolution, and its Relations to Geological
Time_, 1889.]
[Footnote 1163: _Comptes Rendus_, t. lii., p. 481. See also Kirkwood,
_Observatory_, vol. iii., p. 409.]
[Footnote 1164: Fouché, _Comptes Rendus_, t. xcix., p. 903.]
[Footnote 1165: _Astroph. Jour._, vol. xiii., p. 338.]
[Footnote 1166: _Month. Not._, vol. xxix., p. 96.]
[Footnote 1167: _Pop. Astr._, p. 257.]
[Footnote 1168: _Sur l'Origine du Monde_, 1884.]
[Footnote 1169: Kirkwood adverted to it in 1864, _Am. Jour._, vol.
xxxviii., p. 1.]
[Footnote 1170: _Bull. Astr._, t. ii.]
[Footnote 1171: _Nature_, vol. xxxi., p. 506.]
[Footnote 1172: _Formation Mécanique du Système du Monde; Bull. Astr._,
t. xiv., p. 313 (O. Callandreau). See also, _Le Problème Solaire_, by
l'Abbé Th. Moreux, 1900.]
[Footnote 1173: _Phil. Trans._, vol. clxxi., p. 713.]
[Footnote 1174: Mr. J. Nolan has pointed out (_Nature_, vol. xxxiv., p.
287) that the length of the equal day and month will be reduced to about
1,240 hours by the effects of _solar_ tidal friction.]
[Footnote 1175: _Phil. Trans._, vol. clxxi., p. 835.]
[Footnote 1176: _Nature_, vol. xxxiii., p. 368; see also Nolan, _Ibid._,
vol. xxxiv., p. 286.]
[Footnote 1177: _Phil. Trans._, vol. clxxviii., p. 422.]
[Footnote 1178: _Ibid._, vol. clxxii., p. 491.]
[Footnote 1179: _Ibid._, p. 530.]
[Footnote 1180: _Satellite Evolution_, Melbourne, 1895; _Knowledge_,
vol. xviii., p. 205.]
[Footnote 1181: _Phil. Trans._, vol. clxxii., p. 533.]
[Footnote 1182: This was perceived by M. Ed. Roche in 1872. _Mém. de
l'Acad. des Sciences de Montpellier_, t. viii., p. 247.]
[Footnote 1183: _Nature_, vol. xxxiv., p. 287.]
[Footnote 1184: _Bull. Astr._, t. ii., p. 223.]
[Footnote 1185: _Montpellier Méms._, t. viii., p. 242.]
[Footnote 1186: _Amer. Jour._, vol. xxxviii. (1864), p. 1.]
[Footnote 1187: Wolf, _Bull. Astr._, t. ii., p. 76.]
CHAPTER X
_RECENT COMETS_
On the 2nd of June, 1858, Giambattista Donati discovered at Florence a
feeble round nebulosity in the constellation Leo, about one-tenth the
diameter of the full moon. It proved to be a comet approaching the sun.
But it changed little in apparent place or brightness for some weeks.
The gradual development of a central condensation of light was the first
symptom of coming splendour. At Harvard, in the middle of July, a strong
stellar nucleus was seen; on August 14 a tail began to be thrown out. As
the comet wanted still over six weeks of the time of its
perihelion-passage, it was obvious that great things might be expected
of it. They did not fail of realisation.
Not before the early days of September was it generally recognised with
the naked eye, though it had been detected without a glass at Pulkowa,
August 19. But its growth was thenceforward surprisingly rapid, as it
swept with accelerated motion under the hindmost foot of the Great Bear,
and past the starry locks of Berenice. A sudden leap upward in lustre
was noticed on September 12, when the nucleus shone with about the
brightness of the pole-star, and the tail, notwithstanding large
foreshortening, could be traced with the lowest telescopic power over
six degrees of the sphere. The appendage, however, attained its full
development only after perihelion, September 30, by which time, too, it
lay nearly square to the line of sight from the earth. On October 10 it
stretched in a magnificent scimitar-like curve over a third and upwards
of the visible hemisphere, representing a real extension in space of
fifty-four million miles. But the most striking view was presented on
October 5, when the brilliant star Arcturus became involved in the
brightest part of the tail, and during many hours contributed, its
lustre undiminished by the interposed nebulous screen, to heighten the
grandeur of the most majestic celestial object of which living memories
retain the impress. Donati's comet was, according to Admiral Smyth's
testimony,[1188] outdone "as a mere _sight_-object" by the great comet
of 1811; but what it lacked in splendour, it surely made up in grace,
and variety of what we may call "scenic" effects.
Some of these were no less interesting to the student than impressive to
the spectator. At Pulkowa, on the 16th September, Winnecke,[1189] the
first director of the Strasburg Observatory, observed a faint outer
envelope resembling a veil of almost evanescent texture flung somewhat
widely over the head. Next evening, the first of the "secondary" tails
appeared, possibly as part of the same phenomenon. This was a narrow
straight ray, forming a tangent to the strong curve of the primary tail,
and reaching to a still greater distance from the nucleus. It continued
faintly visible for about three weeks, during part of which time it was
seen in duplicate. For from the chief train itself, at a point where its
curvature abruptly changed, issued, as if through the rejection of some
of its materials, a second beam nearly parallel to the first, the rigid
line of which contrasted singularly with the softly diffused and waving
aspect of the plume of light from which it sprang. Olbers's theory of
unequal repulsive forces was never more beautifully illustrated. The
triple tail seemed a visible solar analysis of cometary matter.
The processes of luminous emanation going on in this body forcibly
recalled the observations made on the comets of 1744 and 1835. From the
middle of September, the nucleus, estimated by Bond to be under five
hundred miles in diameter, was the centre of action of the most
energetic kind. Seven distinct "envelopes" were detached in succession
from the nebulosity surrounding the head, and after rising towards the
sun during periods of from four to seven days, finally cast their
material backward to form the right and left branches of the great
train. The separation of these by an obscure axis--apparently as black,
quite close up to the nucleus, as the sky--indicated for the tail a
hollow, cone-like structure;[1190] while the repetition of certain spots
and rays in the same corresponding situation on one envelope after
another served to show that the nucleus--to some local peculiarity of
which they were doubtless due--had no proper rotation, but merely
shifted sufficiently on an axis to preserve the same aspect towards the
sun as it moved round it.[1191] This observation of Bond's was strongly
confirmatory of Bessel's hypothesis of opposite polarities in such
bodies' opposite sides.
The protrusion towards the sun, on September 25, of a brilliant luminous
fan-shaped sector completed the resemblance to Halley's comet. The
appearance of the head was now somewhat that of a "bat's-wing" gaslight.
There were, however, no oscillations to and fro, such as Bessel had seen
and speculated upon in 1835. As the size of the nucleus contracted with
approach to perihelion, its intensity augmented. On October 2, it
outshone Arcturus, and for a week or ten days was a conspicuous object
half an hour after sunset. Its lustre--setting aside the light derived
from the tail--was, at that date, 6,300 times what it had been on June
15, though _theoretically_--taking into account, that is, only the
differences of distance from sun and earth--it should have been only
1/33 of that amount. Here, it might be thought, was convincing evidence
of the comet itself becoming ignited under the growing intensity of the
solar radiations. Yet experiments with the polariscope were interpreted
in an adverse sense, and Bond's conclusion that the comet sent us
virtually unmixed reflected sunshine was generally acquiesced in. It
was, nevertheless, negatived by the first application of the
spectroscope to these bodies.
Very few comets have been so well or so long observed as Donati's. It
was visible to the naked eye during 112 days; it was telescopically
discernible for 275, the last observation having been made by Mr.
William Mann at the Cape of Good Hope, March 4, 1859. Its course through
the heavens combined singularly with the orbital place of the earth to
favour curious inspection. The tail, when near its greatest development,
lost next to nothing by the effects of perspective, and at the same time
lay in a plane sufficiently inclined to the line of sight to enable it
to display its exquisite curves to the greatest advantage. Even the
weather was, on both sides of the Atlantic, propitious during the period
of greatest interest, and the moon as little troublesome as possible.
The volume compiled by the younger Bond is a monument to the care and
skill with which these advantages were turned to account. Yet this
stately apparition marked no turning-point in the history of cometary
science. By its study knowledge was indeed materially advanced, but
along the old lines. No quick and vivid illumination broke upon its
path. Quite insignificant objects--as we have already partly seen--have
often proved more vitally instructive.
Donati's comet has been identified with no other. Its path is an
immensely elongated ellipse, lying in a plane far apart from that of the
planetary movements, carrying it at perihelion considerably within the
orbit of Venus, and at aphelion out into space to 5-1/2 times the
distance from the sun of Neptune. The entire circuit occupies over 2,000
years, and is performed in a retrograde direction, or against the order
of the Signs. Before its next return, about the year 4000 A.D., the
enigma of its presence and its purpose may have been to some
extent--though we may be sure not completely--penetrated.
On June 30, 1861, the earth passed, for the second time in the century,
through the tail of a great comet. Some of our readers may remember the
unexpected disclosure, on the withdrawal of the sun below the horizon on
that evening, of an object so remarkable as to challenge universal
attention. A golden-yellow planetary disc, wrapt in dense nebulosity,
shone out while the June twilight of these latitudes was still in its
first strength. The number and complexity of the envelopes surrounding
the head produced, according to the late Mr. Webb,[1192] a magnificent
effect. Portions of six distinct emanations were traceable. "It was as
though a number of light, hazy clouds were floating round a miniature
full moon." As the sky darkened the tail emerged to view.[1193] Although
in brightness and sharpness of definition it could not compete with the
display of 1858, its dimensions proved to be extraordinary. It reached
upwards beyond the zenith when the head had already set. By some
authorities its extreme length was stated at 118°, and it showed no
trace of curvature. Most remarkable, however, was the appearance of two
widely divergent rays, each pointing towards the head, though cut off
from it by sky-illumination, of which one was seen by Mr. Webb, and both
by Mr. Williams at Liverpool, a quarter of an hour before midnight.
There seems no doubt that Webb's interpretation was the true one, and
that these beams were, in fact, "the perspective representation of a
conical or cylindrical tail, hanging closely above our heads, and
probably just being lifted up out of our atmosphere."[1194] The cometary
train was then rapidly receding from the earth, so that the sides of the
"outspread fan" of light shown by it when we were right in the line of
its axis must have appeared (as they did) to close up in departure. The
swiftness with which the visually opened fan shut proved its vicinity;
and, indeed, Mr. Hind's calculations showed that we were not so much
near as actually within its folds at that very time.
Already M. Liais, from his observations at Rio de Janeiro, June 11 to
14, and Mr. Tebbutt, by whom the comet was discovered in New South Wales
on May 13, had anticipated such an encounter, while the former
subsequently proved that it must have occurred in such a way as to cause
an immersion of the earth in cometary matter to a depth of 300,000
miles.[1195] The comet then lay between the earth and the sun at a
distance of about fourteen million miles from the former; its tail
stretched outward just along the line of intersection of its own with
the terrestrial orbit to an extent of fifteen million miles; so that our
globe, happening to pass at the time, found itself during some hours
involved in the flimsy appendage.
No perceptible effects were produced by the meeting; it was known to
have occurred by theory alone. A peculiar glare in the sky, thought by
some to have distinguished the evening of June 30, was, at best,
inconspicuous. Nor were there any symptoms of unusual electric
excitement. The Greenwich instruments were, indeed, disturbed on the
following night, but it would be rash to infer that the comet had art or
part in their agitation.
The perihelion-passage of this body occurred June 11, 1861; and its
orbit has been shown by M. Kreutz of Bonn, from a very complete
investigation founded on observations extending over nearly a year, to
be an ellipse traversed in a period of 409 years.[1196]
Towards the end of August, 1862, a comet became visible to the naked eye
high up in the northern hemisphere, with a nucleus equalling in
brightness the lesser stars of the Plough and a feeble tail 20° in
length. It thus occupied quite a secondary position among the members of
its class. It was, nevertheless, a splendid object in comparison with a
telescopic nebulosity discovered by Tempel at Marseilles, December 19,
1865. This, the sole comet of 1866, slipped past perihelion, January 11,
without pomp of train or other appendages, and might have seemed hardly
worth the trouble of pursuing. Fortunately, this was not the view
entertained by observers and computers; since upon the knowledge
acquired of the movements of these two bodies has been founded one of
the most significant discoveries of modern times. The first of them is
now styled the comet (1862 iii.) of the August meteors, the second (1866
i.) that of the November meteors. The steps by which this curious
connection came to be ascertained were many, and were taken in
succession by a number of individuals. But the final result was reached
by Schiaparelli of Milan, and remains deservedly associated with his
name.
The idea prevalent in the eighteenth century as to the nature of
shooting stars was that they were mere aerial _ignes fatui_--inflammable
vapours accidentally kindled in our atmosphere. But Halley had already
entertained the opinion of their cosmical origin; and Chladni in 1794
formally broached the theory that space is filled with minute
circulating atoms, which, drawn by the earth's attraction, and ignited
by friction in its gaseous envelope, produce the luminous effects so
frequently witnessed.[1197] Acting on his suggestion, Brandes and
Benzenberg, two students at the University of Göttingen, began in 1798
to determine the heights of falling stars by simultaneous observations
at a distance. They soon found that they move with planetary velocities
in the most elevated regions of our atmosphere, and by the ascertainment
of this fact laid a foundation of distinct knowledge regarding them.
Some of the data collected, however, served only to perplex opinion, and
even caused Chladni temporarily to renounce his. Many high authorities,
headed by Laplace in 1802, declared for the lunar-volcanic origin of
meteorites; but thought on the subject was turbid, and inquiry seemed
only to stir up the mud of ignorance. It needed one of those amazing
spectacles, at which man assists, no longer in abject terror for his own
frail fortunes, but with keen curiosity and the vivid expectation of new
knowledge, to bring about a clarification.
On the night of November 12-13, 1833, a tempest of falling stars broke
over the earth. North America bore the brunt of its pelting. From the
Gulf of Mexico to Halifax, until daylight with some difficulty put an
end to the display, the sky was scored in every direction with shining
tracks and illuminated with majestic fireballs. At Boston the frequency
of meteors was estimated to be about half that of flakes of snow in an
average snowstorm. Their numbers, while the first fury of their coming
lasted, were quite beyond counting; but as it waned, a reckoning was
attempted, from which it was computed, on the basis of that much
diminished rate, that 240,000 must have been visible during the nine
hours they continued to fall.[1198]
Now there was one very remarkable feature common to the innumerable
small bodies which traversed, or were consumed in our atmosphere that
night. _They all seemed to come from the same part of the sky._ Traced
backward, their paths were invariably found to converge to a point in
the constellation Leo. Moreover, that point travelled with the stars in
their nightly round. In other words, it was entirely independent of the
earth and its rotation. It was a point in inter-planetary space.
The _effective_ perception of this fact[1199] amounted to a discovery,
as Olmsted and Twining, who had "simultaneous ideas" on the subject,
were the first to realize. Denison Olmsted was then Professor of
Mathematics in Yale College. He showed early in 1834[1200] that the
emanation of the showering meteors from a fixed "radiant" proved their
approach to the earth along nearly parallel lines, appearing to diverge
by an effect of perspective; and that those parallel lines must be
sections of orbits described by them round the sun and intersecting that
of the earth. For the November phenomenon was now seen to be a
periodical one. On the same night of the year 1832, although with less
dazzling and universal splendour than in America in 1833, it had been
witnessed over great part of Europe and in Arabia. Olmsted accordingly
assigned to the cloud of cosmical particles (or "comet," as he chose to
call it), by terrestrial encounters with which he supposed the
appearances in question to be produced, a period of about 182 days; its
path a narrow ellipse, meeting, near its farthest end from the sun, the
place occupied by the earth on November 12.
Once for all, then, as the result of the star-fall of 1833, the study of
luminous meteors became an integral part of astronomy. Their membership
of the solar system was no longer a theory or a conjecture--it was an
established fact. The discovery might be compared to, if it did not
transcend in importance, that of the asteroidal group. "C'est un nouveau
monde planétaire," Arago wrote,[1201] "qui commence à se révéler à
nous."
Evidences of periodicity continued to accumulate. It was remembered that
Humboldt and Bonpland had been the spectators at Cumana, after midnight
on November 12, 1799, of a fiery shower little inferior to that of 1833,
and reported to have been visible from the equator to Greenland.
Moreover, in 1834 and some subsequent years, there were waning
repetitions of the display, as if through the gradual thinning-out of
the meteoric supply. The extreme irregularity of its distribution was
noted by Olbers in 1837, who conjectured that we might have to wait
until 1867 to see the phenomenon renewed on its former scale of
magnificence.[1202] This was the first hint of a thirty-three or
thirty-four year period.
The falling stars of November did not alone attract the attention of the
learned. Similar appearances were traditionally associated with August
10 by the popular phrase in which they figured as "the tears of St.
Lawrence." But the association could not be taken on trust from mediæval
authority. It had to be proved scientifically, and this Quetelet of
Brussels succeeded in doing in December, 1836.[1203]
A second meteoric revolving system was thus shown to exist. But its
establishment was at once perceived to be fatal to the "cosmical cloud"
hypothesis of Olmsted. For if it be a violation of probability to
attribute to one such agglomeration a period of an exact year, or
sub-multiple of a year, it would be plainly absurd to suppose the
movements of _two_ or more regulated by such highly artificial
conditions. An alternative was proposed by Adolf Erman of Berlin in
1839.[1204] No longer in _clouds_, but in closed _rings_, he supposed
meteoric matter to revolve round the sun. Thus the mere circumstance of
intersection by a meteoric of the terrestrial orbit, without any
coincidence of period, would account for the earth meeting some members
of the system at each annual passage through the "node" or point of
intersection. This was an important step in advance, yet it decided
nothing as to the forms of the orbits of such annular assemblages; nor
was it followed up in any direction for a quarter of a century.
Hubert A. Newton took up, in 1864,[1205] the dropped thread of inquiry.
The son of a mathematical mother, he attained, at the age of
twenty-five, to the dignity of Professor of Mathematics in Yale
University, and occupied the post until his death in 1896. The diversion
of his powers, however, from purely abstract studies stimulated their
effective exercise, and constituted him one of the founders of meteoric
astronomy.
A search through old records carried the November phenomenon back to the
year 902 A.D., long distinguished as "the year of the stars." For in the
same night in which Taormina was captured by the Saracens, and the cruel
Aghlabite tyrant Ibrahim ibn Ahmed died "by the judgment of God" before
Cosenza, stars fell from heaven in such abundance as to amaze and
terrify beholders far and near. This was on October 13, and recurrences
were traced down through the subsequent centuries, always with a day's
delay in about seventy years. It was easy, too, to derive from the dates
a cycle of 33-1/4 years, so that Professor Newton did not hesitate to
predict the exhibition of an unusually striking meteoric spectacle on
November 13-14, 1866.[1206]
For the astronomical explanation of the phenomena, recourse was had to a
method introduced by Erman of computing meteoric orbits. It was found,
however, that conspicuous recurrences every thirty-three or thirty-four
years could be explained on the supposition of five widely different
periods, combined with varying degrees of extension in the revolving
group. Professor Newton himself gave the preference to the shortest--of
354-1/2 days, but indicated the means of deciding with certainty upon
the true one. It was furnished by the advancing motion of the node, or
that day's delay of the November shower every seventy years, which the
old chronicles had supplied data for detecting. For this is a strictly
measurable effect of gravitational disturbance by the various planets,
the amount of which naturally depends upon the course pursued by the
disturbed bodies. Here the great mathematical resources of Professor
Adams were brought to bear. By laborious processes of calculation, he
ascertained that four out of Newton's five possible periods were
entirely incompatible with the observed nodal displacement, while for
the fifth--that of 33-1/4 years--a perfectly harmonious result was
obtained.[1207] This was the last link in the chain of evidence proving
that the November meteors--or "Leonids," as they had by that time come
to be called--revolve round the sun in a period of 33·27 years, in an
ellipse spanning the vast gulf between the orbits of the earth and
Uranus, the group being so extended as to occupy nearly three years in
defiling past the scene of terrestrial encounters. But before it was
completed in March, 1867, the subject had assumed a new aspect and
importance.
Professor Newton's prediction of a remarkable star-shower in November,
1866, was punctually fulfilled. This time, Europe served as the main
target of the celestial projectiles, and observers were numerous and
forewarned. The display, although, according to Mr. Baxendell's
memory,[1208] inferior to that of 1833, was of extraordinary
impressiveness. Dense crowds of meteors, equal in lustre to the
brightest stars, and some rivalling Venus at her best,[1209] darted from
east to west across the sky with enormous apparent velocities, and with
a certain determinateness of aim, as if let fly with a purpose, and at
some definite object.[1210] Nearly all left behind them trains of
emerald green or clear blue light, which occasionally lasted many
minutes, before they shrivelled and curled up out of sight. The maximum
rush occurred a little after one o'clock on the morning of November 14,
when attempts to count were overpowered by frequency. But during a
previous interval of seven minutes five seconds, four observers at Mr.
Bishop's observatory at Twickenham reckoned 514, and during an hour
1,120.[1211] Before daylight the earth had fairly cut her way through
the star-bearing stratum; the "ethereal rockets" had ceased to fly.
This event brought the subject of shooting stars once more vividly to
the notice of astronomers. Schiaparelli had, indeed, been already
attracted by it. The results of his studies were made known in four
remarkable letters, addressed, before the close of the year 1866, to
Father Secchi, and published in the _Bulletino_ of the Roman
Observatory.[1212] Their upshot was to show, in the first place, that
meteors possess a real velocity considerably greater than that of the
earth, and travel, accordingly, to enormously greater distances from the
sun along tracks resembling those of comets in being very eccentric, in
lying at all levels indifferently, and in being pursued in either
direction. It was next inferred that comets and meteors equally have an
origin foreign to the solar system, but are drawn into it temporarily by
the sun's attraction, and occasionally fixed in it by the backward pull
of some planet. But the crowning fact was reserved for the last. It was
the astonishing one that the August meteors move in the same orbit with
the bright comet of 1862--that the comet, in fact, is but a larger
member of the family named "Perseids" because their radiant point is
situated in the constellation Perseus.
This discovery was quickly capped by others of the same kind. Leverrier
published, January 21, 1867,[1213] elements for the November swarm,
founded on the most recent and authentic observations; at once
identified by Dr. C. F. W. Peters of Altona with Oppolzer's elements for
Tempel's comet of 1866.[1214] A few days later, Schiaparelli, having
recalculated the orbit of the meteors from improved data, arrived at the
same conclusion; while Professor Weiss of Vienna pointed to the
agreement between the orbits of a comet which had appeared in 1861 and
of a star-shower found to recur on April 20 (Lyraïds), as well as
between those of Biela's comet and certain conspicuous meteors of
November 28.[1215]
These instances do not seem to be exceptional. The number of known or
suspected accordances of cometary tracks with meteor streams contained
in a list drawn up in 1878[1216] by Professor Alexander S. Herschel (who
has made the subject peculiarly his own) amounts to seventy-six;
although the four first detected still remain the most conspicuous, and
perhaps the only absolutely sure examples of a relation as significant
as it was, to most astronomers, unexpected.
There had, indeed, been anticipatory ideas. Not that Kepler's comparison
of shooting stars to "minute comets," or Maskelyne's "forse risulterà
che essi sono comete," in a letter to the Abate Cesaris, December 12,
1774,[1217] need count for much. But Chladni, in 1819,[1218] considered
both to be fragments or particles of the same primitive matter,
irregularly scattered through space as nebulæ; and Morstadt of Prague
suggested about 1837[1219] that the meteors of November might be
dispersed atoms from the tail of Biela's comet, the path of which is cut
across by the earth near that epoch. Professor Kirkwood, however, by a
luminous intuition, penetrated the whole secret, so far as it has yet
been made known. In an article published, or rather buried, in the
_Danville Quarterly Review_ for December, 1861, he argued, from the
observed division of Biela, and other less noted instances of the same
kind, that the sun exercises a "divellent influence" on the nuclei of
comets, which may be presumed to continue its action until their
corporate existence (so to speak) ends in complete pulverisation. "May
not," he continued, "our periodic meteors be the débris of ancient but
now disintegrated comets, whose matter has become distributed round
their orbits?"[1220]
The gist of Schiaparelli's discovery could not be more clearly conveyed.
For it must be borne in mind that with the ultimate destiny of comets'
tails this had nothing to do. The tenuous matter composing them is, no
doubt, permanently lost to the body from which it emanated; but science
does not pretend to track its further wanderings through space. It can,
however, state categorically that these will no longer be conducted
along the paths forsaken under solar compulsion. From the central, and
probably solid parts of comets, on the other hand, are derived the
granules by the swift passage of which our skies are seamed with
periodic fires. It is certain that a loosely agglomerated mass (such as
cometary nuclei most likely are) must gradually separate through the
unequal action of gravity on its various parts--through, in short, solar
tidal influence. Thenceforward its fragments will revolve independently
in parallel orbits, at first as a swarm, finally--when time has been
given for the full effects of the lagging of the slower moving particles
to develop--as a closed ring. The first condition is still, more or
less, that of the November meteors; those of August have already arrived
at the second. For this reason, Leverrier pronounced, in 1867, the
Perseid to be of older formation than the Leonid system. He even
assigned a date at which the introduction of the last-named bodies into
their present orbit was probably effected through the influence of
Uranus. In 126 A.D. a close approach must have taken place between the
planet and the parent comet of the November stars, after which its
regular returns to perihelion, and the consequent process of its
disintegration, set in. Though not complete, it is already far advanced.
The view that meteorites are the dust of decaying comets was now to be
put to a definite test of prediction. Biela's comet had not been seen
since its duplicate return in 1852. Yet it had been carefully watched
for with the best telescopes; its path was accurately known; every
perturbation it could suffer was scrupulously taken into account. Under
these circumstances, its repeated failure to come up to time might
fairly be thought to imply a cessation from visible existence. Might it
not, however, be possible that it would appear under another form--that
a star-shower might have sprung from and would commemorate its
dissolution?
An unusually large number of falling stars were seen by Brandes,
December 6, 1798. Similar displays were noticed in the years 1830, 1838,
and 1847, and the point from which they emanated was shown by Heis at
Aix-la-Chapelle to be situated near the bright star Gamma
Andromedæ.[1221] Now this is precisely the direction in which the orbit
of Biela's comet would seem to lie, as it runs down to cut the
terrestrial track very near the place of the earth at the above dates.
The inference was, then, an easy one, that the meteors were pursuing the
same path with the comet; and it was separately arrived at, early in
1867, by Weiss, D'Arrest, and Galle.[1222] But Biela travels in the
opposite direction to Tempel's comet and its attendant "Leonids"; its
motion is direct, or from west to east, while theirs is retrograde.
Consequently, the motion of its node is in the opposite direction too.
In other words, the meeting-place of its orbit with that of the earth
retreats (and very rapidly) along the ecliptic instead of advancing. So
that if the "Andromedes" stood in the supposed intimate relation to
Biela's comet, they might be expected to anticipate the times of their
recurrence by as much as a week in half a century. All doubt as to the
fact may be said to have been removed by Signor Zezioli's observation of
the annual shower in more than usual abundance at Bergamo, November 30,
1867.
The missing comet was next due at perihelion in the year 1872, and the
probability was contemplated by both Weiss and Galle of its being
replaced by a copious discharge of falling stars. The precise date of
the occurrence was not easily determinable, but Galle thought the
chances in favour of November 28. The event anticipated the prediction
by twenty-four hours. Scarcely had the sun set in Western Europe on
November 27, when it became evident that Biela's comet was shedding over
us the pulverised products of its disintegration. The meteors came in
volleys from the foot of the Chained Lady, their numbers at times
baffling the attempt to keep a reckoning. At Moncalieri, about 8 p.m.,
they constituted (as Father Denza said[1223]) a "real rain of fire."
Four observers counted, on an average, four hundred each minute and a
half; and not a few fireballs, equalling the moon in diameter, traversed
the sky. On the whole, however, the stars of 1872, though about equally
numerous, were less brilliant than those of 1866; the phosphorescent
tracks marking their passage were comparatively evanescent and their
movements sluggish. This is easily understood when we remember that the
Andromedes _overtake_ the earth, while the Leonids rush to meet it; the
velocity of encounter for the first class of bodies being under twelve,
for the second above forty-four miles a second. The spectacle was,
nevertheless, magnificent. It presented itself successively to various
parts of the earth, from Bombay and the Mauritius to New Brunswick and
Venezuela, and was most diligently and extensively observed. Here it had
well-nigh terminated by midnight.[1224]
It was attended by a slight aurora, and although Tacchini had
telegraphed that the state of the sun rendered some show of polar lights
probable, it has too often figured as an accompaniment of star-showers
to permit the coincidence to rank as fortuitous. Admiral Wrangel was
accustomed to describe how, during the prevalence of an aurora on the
Siberian coast, the passage of a meteor never failed to extend the
luminosity to parts of the sky previously dark;[1225] and an enhancement
of electrical disturbance may well be associated with the flittings of
such cosmical atoms.
A singular incident connected with the meteors of 1872 has now to be
recounted. The late Professor Klinkerfues, who had observed them very
completely at Göttingen, was led to believe that not merely the débris
strewn along its path, but the comet itself must have been in immediate
proximity to the earth during their appearance.[1226] If so, it might be
possible, he thought, to descry it as it retreated in the diametrically
opposite direction from that in which it had approached. On November 30,
accordingly, he telegraphed to Mr. Pogson, the Madras astronomer, "Biela
touched earth November 27; search near Theta Centauri"--the
"anti-radiant," as it is called, being situated close to that star. Bad
weather prohibited observation during thirty-six hours, but when the
rain clouds broke on the morning of December 2, there a comet was, just
in the indicated position. In appearance it might have passed well
enough for one of the Biela twins. It had no tail, but a decided
nucleus, and was about 45 seconds across, being thus altogether below
the range of naked-eye discernment. It was again observed December 3,
when a short tail was perceptible; but overcast skies supervened, and it
has never since been seen. Its identity accordingly remains in doubt. It
seems tolerably certain, however, that it was _not_ the lost comet,
which ought to have passed that spot twelve weeks earlier, and was
subject to no conceivable disturbance capable of delaying to that extent
its revolution. On the other hand, there is the strongest likelihood
that it belonged to the same system[1227]--that it was a third fragment,
torn from the parent-body of the Andromedes at a period anterior to our
first observations of it.
In thirteen years Biela's comet (or its relics) travels nearly twice
round its orbit, so that a renewal of the meteoric shower of 1872 was
looked for on the same day of the year 1885, the probability being
emphasised by an admonitory circular from Dunecht. Astronomers were
accordingly on the alert, and were not disappointed. In England,
observation was partially impeded by clouds; but at Malta, Palermo,
Beyrout, and other southern stations, the scene was most striking. The
meteors were both larger and more numerous than in 1872. Their numbers
in the densest part of the drift were estimated by Professor Newton at
75,000 per hour, visible from one spot to so large a group of spectators
that practically none could be missed. Yet each of these multitudinous
little bodies was found by him to travel in a clear cubical space of
which the edge measured twenty miles![1228] Thus the dazzling effect of
a luminous throng was produced without jostling or overcrowding, by
particles, it might almost be said, isolated in the void.
Their aspect was strongly characteristic of the Andromede family of
meteors. "They invariably," Mr. Denning wrote,[1229] "traversed short
paths with very slow motions, and became extinct in evolved streams of
yellowish sparks." The conclusion seemed obvious "that these meteors are
formed of very soft materials, which expand while incalescent, and are
immediately crumbled and dissipated into exiguous dust."
The Biela meteors of 1885 did not merely gratify astronomers with a
fulfilled prediction, but were the means of communicating to them some
valuable information. Although their main body was cut through by the
moving earth in six hours, and was not more than 100,000 miles across,
skirmishers were thrown out to nearly a million miles on either side of
the compact central battalions. Members of the system were, on the 26th
of November, recorded by Mr. Denning at the hourly rate of about 130;
and they did not wholly cease to be visible until December 1. They
afforded besides a particularly well-marked example of that diffuseness
of radiation previously observed in some less conspicuous displays.
Their paths seemed to diverge from an area rather than from a point in
the sky. They came so ill to focus that divergences of several degrees
were found between the most authentically determined radiants. These
incongruities are attributed by Professor Newton to the irregular shape
of the meteoroids producing unsymmetrical resistance from the air, and
hence causing them to glance from their original direction on entering
it. Thus, their luminous tracks did not always represent (even apart
from the effects of the earth's attraction) the true prolongation of
their course through space.
The Andromedes of 1872 were laggards behind the comet from which they
sprang; those of 1885 were its avant-couriers. That wasted and disrupted
body was not due at the node until January 26, 1886, sixty days, that
is, after the earth's encounter with its meteoric fragments. These are
now probably scattered over more than five hundred million miles of its
orbits;[1230] yet Professor Newton considers that all must have formed
one compact group with Biela at the time of its close approach to
Jupiter about the middle of 1841. For otherwise both comet and
meteorites could not have experienced, as they seem to have done, the
same kind and amount of disturbance. The rapidity of cometary
disintegration is thus curiously illustrated.
A short-lived persuasion that the missing heavenly body itself had been
recovered, was created by Mr. Edwin Holms's discovery, at London,
November 6, 1892, of a tolerably bright, tailless comet, just in a spot
which Biela's comet must have traversed in approaching the intersection
of its orbit with that of the earth. A hasty calculation by Berberich
assigned elements to the newcomer seeming not only to ratify the
identity, but to promise a quasi-encounter with the earth on November
21. The only effect of the prediction, however, was to raise a panic
among the negroes of the Southern States of America. The comet quietly
ignored it, and moved away from instead of towards the appointed
meeting-place. Its projection, then, on the night of its discovery, upon
a point of the Biela-orbit was by a mere caprice of chance. North
America, nevertheless, was visited on November 23 by a genuine Andromede
shower. Although the meteors were less numerous than in 1885, Professor
Young estimated that 30,000, at the least, of their orange fire-streaks
came, during five hours, within the range of view at Princeton.[1231]
Brédikhine estimated the width of the space containing them at about
2,700,000 miles.[1232] The anticipation of their due time by four days
implied--if they were a prolongation of the main Biela group, the
nucleus of which passed the spot of encounter five months previously--a
recession of the node since 1885 by no less than three degrees. Unless,
indeed, Mr. Denning were right in supposing the display to have
proceeded from "an associated branch of the main swarm through which we
passed in 1872 and 1885."[1233] The existence of separated detachments
of Biela meteors, due to disturbing planetary action, was contemplated
as highly probable by Schiaparelli.[1234] Such may have been the belated
flights met with in 1830, 1838, 1841, and 1847, and such the advance
flight plunged through in 1892. A shower looked for November 23, 1899,
did not fall, and no further display from this quarter is probable until
November 17, 1905, although one is possible a year earlier.[1235]
The Leonids, through the adverse influence of Jupiter and Saturn,
inflicted upon multitudes of eager watchers a still more poignant
disappointment. A dense part of the swarm, having nearly completed a
revolution since 1866, should, travelling normally, have met the earth
November 15, 1899; in point of fact, it swerved sunward, and the
millions of meteorites which would otherwise have been sacrificed for
the illumination of our skies escaped a fiery doom. The contingency had
been forecast in the able calculations of Dr. Johnstone Stoney and Dr.
A. M. W. Downing,[1236] superintendent of the Nautical Almanac Office;
but the verification scarcely compensated the failure. Nor was the
situation retrieved in the following years. Only ragged fringes of the
great tempest-cloud here and there touched our globe. As the same
investigators warned us to expect, the course of the meteorites had been
not only rendered sinuous by perturbation, but also broken and
irregular. We can no longer count upon the Leonids. Their glory, for
scenic purposes, is departed. The comet associated with them also evaded
observation. Although it doubtless kept its tryst with the sun in the
spring of 1899, the attendant circumstances were too unfavourable to
allow it to be seen from the earth.[1237] By an almost fantastic
coincidence, nevertheless, a faint comet was photographed, November 14,
1898,[1238] by Dr. Chase, of the Yale College Observatory, close to the
Leonid radiant, whither a "meteorograph" was directed with a view to
recording trails left by precursors of the main Leonid body. A promising
start, too, was made on the same occasion with meteoric researches from
sensitive plates.[1239] Indeed, Schaeberle and Colton[1240] had already,
in 1896, determined the height of a Leonid by means of photographs taken
at stations on different ridges of Mount Hamilton; and Professor
Pickering has prosecuted similar work at Harvard, with encouraging
results. Everything in this branch of science depends upon how far they
can be carried. Without the meteorograph, rigid accuracy in the
observation of shooting stars is unattainable, and rigid accuracy is the
_sine quâ non_ for obtaining exact knowledge.
Biela does not offer the only example of cometary disruption. Setting
aside the unauthentic reports of early chroniclers, we meet the "double
comet" discovered by Liais at Olinda (Brazil), February 27, 1860, of
which the division appeared recent, and about to be carried
farther.[1241] But a division once established, separation must
continually progress. The periodic times of the fragments will never be
identical; one must drop a little behind the other at each revolution,
until at length they come to travel in remote parts of nearly the same
orbit. Thus the comet predicted by Klinkerfues and discovered by Pogson
had already lagged to the extent of twelve weeks, and we shall meet
instances farther on where the retardation is counted, not by weeks, but
by years. Here original identity emerges only from calculation and
comparison of orbits.
Comets, then, die, as Kepler wrote long ago, _sicut bombyces filo
fundendo_. This certainty, anticipated by Kirkwood in 1861, we have at
least acquired from the discovery of their generative connection with
meteors. Nay, their actual materials become, in smaller or larger
proportions, incorporated with our globe. It is not, indeed, universally
admitted that the ponderous masses of which, according to Daubrée's
estimate,[1242] at least 600 fall annually from space upon the earth,
ever formed part of the bodies known to us as comets. Some follow
Tschermak in attributing to aerolites a totally different origin from
that of periodical shooting-stars. That no clear line of demarcation can
be drawn is no valid reason for asserting that no real distinction
exists; and it is certainly remarkable that a meteoric fusillade may be
kept up for hours without a single solid projectile reaching its
destination. It would seem as if the celestial army had been supplied
with blank cartridges. Yet, since a few detonating meteors have been
found to proceed from ascertained radiants of shooting-stars, it is
difficult to suppose that any generic difference separates them.
Their assimilation is further urged--though not with any demonstrative
force--by two instances, the only two on record, of the tangible descent
of an aerolite during the progress of a star-shower. On April 4, 1095,
the Saxon Chronicle informs us that stars fell "so thickly that no man
could count them," and adds that one of them having struck the ground in
France, a bystander "cast water upon it, which was raised in steam with
a great noise of boiling."[1243] And again, on November 27, 1885, while
the skirts of the Andromede-tempest were trailing over Mexico, "a ball
of fire" was precipitated from the sky at Mazapil, within view of a
ranchman.[1244] Scientific examination proved it to be a "siderite," or
mass of "nickel-iron"; its weight exceeded eight pounds, and it
contained many nodules of graphite. We are not, however, authorised by
the circumstances of its arrival to regard the Mazapil fragment of
cosmic metal as a specimen torn from Biela's comet. In this, as in the
preceding case, the coincidence of the fall with the shower may have
been purely casual, since no hint is given of any sort of agreement
between the tracks followed by the sample provided for curious study,
and the swarming meteors consumed in the upper air.
Professor Newton's inquiries into the tracks pursued by meteorites
previous to their collisions with the earth tend to distinguish them, at
least specifically, from shooting-stars. He found that nearly all had
been travelling with a direct movement in orbits the perihelia of which
lay in the outer half of the space separating the earth from the
sun.[1245] Shooting-stars, on the contrary, are entirely exempt from
such limitations. The Yale Professor concluded "that the larger
meteorites moving in our solar system are allied much more closely with
the group of comets of short period than with the comets whose orbits
are nearly parabolic." They would thus seem to be more at home than
might have been expected amid the planetary family. Father Carbonelle
has, moreover, shown[1246] that meteorites, if explosion-products of the
earth or moon, should, with rare exceptions, follow just the kind of
paths assigned to them, from data of observation, by Professor Newton.
Yet it is altogether improbable that projectiles from terrestrial
volcanoes should, at any geological epoch, have received impulses
powerful enough to enable them, not only to surmount the earth's
gravity, but to penetrate its atmosphere.
A striking--indeed, an almost startling--peculiarity, on the other
hand, divides from their congeners a class of meteors identified by
Mr. Denning during ten years' patient watching of such phenomena at
Bristol.[1247] These are described as "meteors with stationary
radiants," since for months together they seem to come from the same
fixed points in the sky. Now this implies quite a portentous velocity.
The direction of meteor-radiants is affected by a kind of
_aberration_, analogous to the aberration of light. It results from a
composition of terrestrial with meteoric motion. Hence, unless that of
the earth in its orbit be by comparison insignificant, the visual line
of encounter must shift, if not perceptibly from day to day, at any
rate conspicuously from month to month. The fixity, then, of many
systems observed by Mr. Denning seems to demand the admission that
their members travel so fast as to throw the earth's movement
completely out of the account. The required velocity would be, by Mr.
Ranyard's calculation, at least 880 miles a second.[1248] But the
aspect of the meteors justifies no such extravagant assumption. Their
seeming swiftness is very various, and--what is highly significant--it
is notably less when they pursue than when they meet the earth. Yet
the "incredible and unaccountable"[1249] fact of the existence of
these "long radiants," although doubted by Tisserand[1250] because of
its theoretical refractoriness, must apparently be admitted. The first
plausible explanation of them was offered by Professor Turner in
1899.[1251] They represent, in his view, the cumulative effects of the
earth's attraction. The validity of his reasoning is, however, denied
by M. Brédikhine,[1252] who prefers to regard them as a congeries of
separate streams. The enigma they present has evidently not yet
received its definitive solution.
The Perseids afford, on the contrary, a remarkable instance of a
"shifting radiant." Mr. Denning's observations of these yellowish,
leisurely meteors extend over nearly six weeks, from July 8 to August
16; the point of radiation meantime progressing no less than 57° in
right ascension. Doubts as to their common origin were hence freely
expressed, especially by Mr. Monck of Dublin.[1253] But the late Dr.
Kleiber[1254] showed, by strict geometrical reasoning, that the
forty-nine radiants successively determined for the shower were all, in
fact, comprised within one narrowly limited region of space. In other
words, the application of the proper correction for the terrestrial
movement, and the effects of attraction by which each individual
shooting-star is compelled to describe a hyperbola round the earth's
centre, reduces the extended line of radiants to a compact group, with
the cometary radiant for its central point; the cometary radiant being
the spot in the sky met by a tangent to the orbit of the Perseid comet
of 1862 at its intersection with the orbit of the earth. The reality of
the connection between the comet and the meteors could scarcely be more
clearly proved; while the vast dimensions of the stream into which the
latter are found to be diffused cannot but excite astonishment not
unmixed with perplexity.
The first successful application of the spectroscope to comets was by
Donati in 1864.[1255] A comet discovered by Tempel, July 4, brightened
until it appeared like a star somewhat below the second magnitude, with
a feeble tail 30° in length. It was remarkable as having, on August 7,
almost totally eclipsed a small star--a very rare occurrence.[1256] On
August 5 Donati admitted its light through his train of prisms, and
found it, thus analysed, to consist of three bright bands--yellow,
green, and blue--separated by wider dark intervals. This implied a good
deal. Comets had previously been considered, as we have seen, to shine
mainly, if not wholly, by reflected sunlight. They were now perceived to
be self-luminous, and to be formed, to a large extent, of glowing gas.
The next step was to determine what _kind_ of gas it was that was thus
glowing in them; and this was taken by Sir William Huggins in
1868.[1257]
A comet of subordinate brilliancy, known as comet 1868 ii., or sometimes
as Winnecke's, was the subject of his experiment. On comparing its
spectrum with that of an olefiant-gas "vacuum tube" rendered luminous by
electricity, he found the agreement exact. It has since been abundantly
confirmed. All the eighteen comets tested by light analysis, between
1868 and 1880, showed the typical hydro-carbon spectrum[1258] common to
the whole group of those compounds, but probably due immediately to the
presence of acetylene. Some minor deviations from the laboratory
pattern, in the shifting of the maxima of light from the edge towards
the middle of the yellow and blue bands, have been experimentally
reproduced by Vogel and Hasselberg in tubes containing a mixture of
carbonic oxide with olefiant gas.[1259] Their illumination by disruptive
electric discharges was, however, a condition _sine quâ non_ for the
exhibition of the cometary type of spectrum. When a continuous current
was employed, the carbonic oxide bands asserted themselves to the
exclusion of the hydro-carbons. The distinction has great significance
as regards the nature of comets. Of particular interest in this
connection is the circumstance that carbonic oxide is one of the gases
evolved by meteoric stones and irons under stress of heat.[1260] For it
must apparently have formed part of an aeriform mass in which they were
immersed at an earlier stage of their history.
PLATE II.
[Illustration: Great Comet.
Photographed, May 5, 1901, with the thirteen-inch Astrographic Refractor
of the Royal Observatory, Cape of Good Hope.]
In a few exceptional comets the usual carbon-bands have been missed. Two
such were observed by Sir William Huggins in 1866 and 1867
respectively.[1261] In each a green ray, approximating in position to
the fundamental nebular line, crossed an otherwise unbroken spectrum.
And Holmes's comet of 1892 displayed only a faint prismatic band devoid
of any characteristic feature.[1262] Now these three might well be set
down as partially effete bodies; but a brilliant comet, visible in
southern latitudes in April and May, 1901, so far resembled them in the
quality of its light as to give a spectrum mainly, if not purely,
continuous. This, accordingly, is no symptom of decay.
The earliest comet of first-class lustre to present itself for
spectroscopic examination was that discovered by Coggia at Marseilles,
April 17, 1874. Invisible to the naked eye till June, it blazed out in
July a splendid ornament of our northern skies, with a just perceptibly
curved tail, reaching more than half way from the horizon to the zenith,
and a nucleus surpassing in brilliancy the brightest stars in the Swan.
Brédikhine, Vogel, and Huggins[1263] were unanimous in pronouncing its
spectrum to be that of marsh or olefiant gas. Father Secchi, in the
clear sky of Rome, was able to push the identification even closer than
had heretofore been done. The _complete_ hydro-carbon spectrum consists
of five zones of variously coloured light. Three of these only--the
three central ones--had till then been obtained from comets; owing, it
was supposed, to their temperature not being high enough to develop the
others. The light of Coggia's comet, however, was found to contain all
five, traces of the violet band emerging June 4, of the red, July
2.[1264] Presumably, all five would show universally in cometary
spectra, were the dispersed rays strong enough to enable them to be
seen.
The gaseous surroundings of comets are, then, largely made up of a
compound of hydrogen with carbon. Other materials are also present; but
the hydro-carbon element is probably unfailing and predominant. Its
luminosity is, there is little doubt, an effect of electrical
excitement. Zöllner showed in 1872[1265] that, owing to evaporation and
other changes produced by rapid approach to the sun, electrical
processes of considerable intensity must take place in comets; and that
their original light is immediately connected with these, and depends
upon solar radiation, rather through its direct or indirect electrifying
effects, than through its more obvious thermal power, may be considered
a truth permanently acquired to science.[1266] They are not, it thus
seems, bodies incandescent through heat, but glowing by electricity; and
this is compatible, under certain circumstances, with a relatively low
temperature.
The gaseous spectrum of comets is accompanied, in varying degrees, by a
continuous spectrum. This is usually derived most strongly from the
nucleus, but extends, more or less, to the nebulous appendages. In part,
it is certainly due to reflected sunlight; in part, most likely, to the
ignition of minute solid particles.
FOOTNOTES:
[Footnote 1188: _Month. Not._, vol. xix., p. 27.]
[Footnote 1189: _Mém. de l'Ac. Imp._, t. ii., 1859, p. 46.]
[Footnote 1190: _Harvard Annals_, vol. iii., p. 368.]
[Footnote 1191: _Ibid._, p. 371.]
[Footnote 1192: _Month. Not._, vol. xxii., p. 306.]
[Footnote 1193: Stothard in _Ibid._, vol. xxi., p. 243.]
[Footnote 1194: _Intell. Observer_, vol. i., p. 65.]
[Footnote 1195: _Comptes Rendus_, t. lxi., p. 953.]
[Footnote 1196: _Smiths. Report_, 1881 (Holden); _Nature_, vol. xxv., p.
94; _Observatory_, vol. xxi., p. 378 (W. T. Lynn).]
[Footnote 1197: _Ueber den Ursprung der von Pallas gefundenen
Eisenmassen_, p. 24.]
[Footnote 1198: Arago, _Annuaire_, 1836, p. 294.]
[Footnote 1199: Humboldt had noticed the emanation of the shooting stars
of 1799 from a single point, or "radiant," as Greg long afterwards
termed it; but no reasoning was founded on the observation.]
[Footnote 1200: _Am. Journ. of Sc._, vol. xxvi., p. 132.]
[Footnote 1201: _Annuaire_, 1836, p. 297.]
[Footnote 1202: _Ann. de l'Observ._, Bruxelles, 1839, p. 248.]
[Footnote 1203: _Ibid._, 1837, p. 272.]
[Footnote 1204: _Astr. Nach._, Nos. 385, 390.]
[Footnote 1205: _Am. Jour. of Sc._, vol. xxxviii. (2nd ser.), p. 377.]
[Footnote 1206: _Ibid._, vol. xxxviii., p. 61.]
[Footnote 1207: _Month. Not._, vol. xxvii., p. 247.]
[Footnote 1208: _Am. Jour. of Sc._, vol. xliii. (2nd ser.), p. 87.]
[Footnote 1209: Grant, _Month. Not._, vol. xxvii., p. 29.]
[Footnote 1210: P. Smyth, _Ibid._, p. 256.]
[Footnote 1211: Hind, _Ibid._, p. 49.]
[Footnote 1212: Reproduced in _Les Mondes_, t. xiii.]
[Footnote 1213: _Comptes Rendus_, t. lxiv., p. 96.]
[Footnote 1214: _Astr. Nach._, No. 1,626.]
[Footnote 1215: _Ibid._, No. 1,632.]
[Footnote 1216: _Month. Not._, vol. xxxviii., p. 369.]
[Footnote 1217: Schiaparelli, _Le Stelle Cadenti_, p. 54.]
[Footnote 1218: _Ueber Feuer-Meteore_, p. 406.]
[Footnote 1219: _Astr. Nach._, No. 347 (Mädler); see also Boguslawski,
_Die Kometen_, p. 98. 1857.]
[Footnote 1220: _Nature_, vol. vi., p. 148.]
[Footnote 1221: A. S. Herschel, _Month. Not._, vol. xxxii., p. 355.]
[Footnote 1222: _Astr. Nach._, Nos. 1,632, 1,633, 1,635.]
[Footnote 1223: _Nature_, vol. vii., p. 122.]
[Footnote 1224: A. S. Herschel, _Report Brit. Ass._, 1873, p. 390.]
[Footnote 1225: Humboldt, _Cosmos_, vol. i., p. 114 (Otté's trans.).]
[Footnote 1226: _Month. Not._, vol. xxxiii., p. 128.]
[Footnote 1227: Even this was denied by Bruhns, _Astr. Nach._, No.
2,054.]
[Footnote 1228: _Am. Jour._, vol. xxxi., p. 425.]
[Footnote 1229: _Month. Not._, vol. xlvi., p. 69.]
[Footnote 1230: In Schiaparelli's opinion, centuries must have elapsed
while the observed amount of scattering was being produced. _Le Stelle
Cadenti_, 1886, p. 112.]
[Footnote 1231: _Astr. and Astroph._, vol. xi., p. 943.]
[Footnote 1232: _Bull. de l'Acad. St. Petersbourg_, t. xxxv., p. 598.
1894.]
[Footnote 1233: _Observatory_, vol. xvi., p. 55.]
[Footnote 1234: _Le Stelle Cadenti_, p. 133; _Rendiconti dell' Istituto
Lombardo_, t. iii., ser. ii., p. 23.]
[Footnote 1235: Denning, _Memoirs Roy. Astr. Soc._, vol. liii., p. 214;
Abelmann, _Astr. Nach._, No. 3,516.]
[Footnote 1236: _Proc. Roy. Soc._, March 2, 1899; _Nature_, November 9,
1899.]
[Footnote 1237: Berberich, _Astr. Nach._, No. 3,526.]
[Footnote 1238: Elkin, _Astroph. Jour._, vol. ix., p. 22.]
[Footnote 1239: Elkin, _Astroph. Jour._, vol. x., p. 24.]
[Footnote 1240: _Pop. Astr._, September, 1897, p. 232.]
[Footnote 1241: _Month. Not._, vol. xx., p. 336.]
[Footnote 1242: _Revue des deux Mondes_, December 15, 1885, p. 889.]
[Footnote 1243: Palgrave, _Phil. Trans._, vol. cxxv., p. 175.]
[Footnote 1244: W. E. Hidden, _Century Mag._, vol. xxxiv., p. 534.]
[Footnote 1245: _Amer. Jour. of Science_, vol. xxxvi., p. i., 1888.]
[Footnote 1246: _Revue des Questions Scientifiques_, January, 1899, p.
194; Tisserand, _Bull. Astr._, t. viii., p. 460.]
[Footnote 1247: _Month. Not._, vol. xlv., p. 93.]
[Footnote 1248: _Observatory_, vol. viii., p. 4.]
[Footnote 1249: Denning, _Month. Not._, vol. xxxviii., p. 114.]
[Footnote 1250: _Comptes Rendus_, t. cix., p. 344.]
[Footnote 1251: _Month. Not._, vol. lix., p. 140.]
[Footnote 1252: _Bull. de l'Acad. St. Petersb._, t. xii., p. 95.]
[Footnote 1253: _Publ. Astr. Pac. Soc._, vol. iii., p. 114.]
[Footnote 1254: _Month. Not._, vol. lii., p. 341.]
[Footnote 1255: _Astr. Nach._, No. 1,488.]
[Footnote 1256: _Annuaire_, Paris, 1883, p. 185.]
[Footnote 1257: _Phil. Trans._, vol. clviii., p. 556.]
[Footnote 1258: Hasselberg, _Mém. de l'Ac. Imp. de St. Pétersbourg_, t.
xxviii. (7th ser.), No. 2, p. 66.]
[Footnote 1259: Scheiner, _Die Spectralanalyse der Gestirne_, p. 234.
Kayser (_Astr. and Astroph._, vol. xiii., p. 368) refers the anomalies
of the carbon-spectrum in comets wholly to instrumental sources.]
[Footnote 1260: Dewar, _Proc. Roy. Inst._, vol. xi., p. 541.]
[Footnote 1261: _Proc. R. Soc._, vol. xv., p. 5; _Month. Not._, vol.
xxvii., p. 288.]
[Footnote 1262: Keeler, _Astr. and Astrophysics_, vol. xi., p. 929;
Vogel, _Astr. Nach._, No. 3,142.]
[Footnote 1263: _Proc. Roy. Soc._, vol. xxiii., p. 154.]
[Footnote 1264: Hasselberg, _loc. cit._, p. 58.]
[Footnote 1265: _Ueber die Natur der Cometen_, p. 112.]
[Footnote 1266: Hasselberg, _loc. cit._, p. 38.]
CHAPTER XI
_RECENT COMETS_ (_continued_)
The mystery of comets' tails had been to some extent penetrated; so far,
at least, that, by making certain assumptions strongly recommended by
the facts of the case, their forms can be, with very approximate
precision, calculated beforehand. We have, then, the assurance that
these extraordinary appendages are composed of no ethereal or
supersensual stuff, but of matter such as we know it, and subject to the
ordinary laws of motion, though in a state of extreme tenuity.
Olbers, as already stated, originated in 1812 the view that the tails of
comets are made up of particles subject to a force of electrical
repulsion proceeding from the sun. It was developed and enforced by
Bessel's discussion of the appearances presented by Halley's comet in
1835. He, moreover, provided a formula for computing the movement of a
particle under the influence of a repulsive force of any given
intensity, and thus laid firmly the foundation of a mathematical theory
of cometary emanations. Professor W. A. Norton, of Yale College,
considerably improved this by inquiries begun in 1844, and resumed on
the apparition of Donati's comet; and Dr. C. F. Pape at Altona[1267]
gave numerical values for the impulses outward from the sun, which must
have actuated the materials respectively of the curved and straight
tails adorning the same beautiful and surprising object.
The _physical_ theory of repulsion, however, was, it might be said,
still in the air. Nor did it even begin to assume consistency until
Zöllner took it in hand in 1871.[1268] It is perfectly well ascertained
that the energy of the push or pull produced by electricity depends
(other things being the same) upon the _surface_ of the body acted on;
that of gravity upon its _mass_. The efficacy of solar electrical
repulsion relatively to solar gravitational attraction grows,
consequently, as the size of the particle diminishes. Make this small
enough, and it will virtually cease to gravitate, and will
unconditionally obey the impulse to recession.
This principle Zöllner was the first to realise in its application to
comets. It gives the key to their constitution. Admitting that the sun
and they are similarly electrified, their more substantially aggregated
parts will still follow the solicitations of his gravity, while the
finely divided particles escaping from them will, simply by reason of
their minuteness, fall under the sway of his repellent electric power.
They will, in other words, form "tails." Nor is any extravagant
assumption called for as to the intensity of the electrical charge
concerned in producing these effects. Zöllner, in fact, showed[1269]
that it need not be higher than that attributed by the best authorities
to the terrestrial surface.
Forty years have elapsed since M. Brédikhine, director successively of
the Moscow and of the Pulkowa Observatories, turned his attention to
these curious phenomena. His persistent inquiries on the subject,
however, date from the appearance of Coggia's comet in 1874. On
computing the value of the repulsive force exerted in the formation of
its tail, and comparing it with values of the same force arrived at by
him in 1862 for some other conspicuous comets, it struck him that the
numbers representing them fell into three well-defined classes. "I
suspect," he wrote in 1877, "that comets are divisible into groups, for
each of which the repulsive force is perhaps the same."[1270] This idea
was confirmed on fuller investigation. In 1882 the appendages of
thirty-six well-observed comets had been reconstructed theoretically,
without a single exception being met with to the rule of the three
types. A further study of forty comets led, in 1885, only to a
modification of the numerical results previously arrived at.
In the first of these, the repellent energy of the sun is fourteen times
stronger than his attractive energy;[1271] the particles forming the
enormously long straight rays projected outward from this kind of comet
leave the nucleus with a mean velocity of just seven kilometres per
second, which, becoming constantly accelerated, carries them in a few
days to the limit of visibility. The great comets of 1811, 1843, and
1861, that of 1744 (so far as its principal tail was concerned), and
Halley's comet at its various apparitions, belonged to this class. Less
narrow limits were assigned to the values of the repulsive force
employed to produce the second type. For the axis of the tail, it
exceeds by one-tenth (= 1·1) the power of solar gravity; for the
anterior edge, it is more than twice (2·2), for the posterior only half
as strong. The corresponding initial velocity (for the axis) is 1,500
metres a second, and the resulting appendage a scimitar-like or plumy
tail, such as Donati's and Coggia's comets furnished splendid examples
of. Tails of the third type are constructed with forces of repulsion
from the sun ranging from one-tenth to three-tenths that of his gravity,
producing an accelerated movement of attenuated matter from the nucleus,
beginning at the leisurely rate of 300 to 600 metres a second. They are
short, strongly bent, brush-like emanations, and in bright comets seem
to be only found in combination with tails of the higher classes.
Multiple tails, indeed--that is, tails of different types emitted
simultaneously by one comet--are perceived, as experience advances and
observation becomes closer, to be rather the rule than the
exception.[1272]
Now what is the meaning of these three types? Is any translation of them
into physical fact possible? To this question Brédikhine supplied, in
1879, a plausible answer.[1273] It was already a current surmise that
multiple tails are composed of different kinds of matter, differently
acted on by the sun. Both Olbers and Bessel had suggested this
explanation of the straight and curved emanations from the comet of
1807; Norton had applied it to the faint light tracks proceeding from
that of Donati;[1274] Winnecke to the varying deviations of its more
brilliant plumage. Brédikhine defined and ratified the conjecture. He
undertook to determine (provisionally as yet) the several kinds of
matter appropriated severally to the three classes of tails. These he
found to be hydrogen for the first, hydro-carbons for the second, and
iron for the third. The ground of this apportionment is that the atomic
weights of these substances bear to each other the same inverse
proportion as the repulsive forces employed in producing the appendages
they are supposed to form; and Zöllner had pointed out in 1875 that the
"heliofugal" power by which comets' tails are developed would, in fact,
be effective just in that ratio.[1275] Hydrogen, as the lightest known
element--that is, the least under the influence of gravity--was
naturally selected as that which yielded most readily to the
counter-persuasions of electricity. Hydro-carbons had been shown by the
spectroscope to be present in comets, and were fitted by their specific
weight, as compared with that of hydrogen, to form tails of the second
type; while the atoms of iron were just heavy enough to compose those of
the third, and, from the plentifulness of their presence in meteorites,
might be presumed to enter, in no inconsiderable proportion, into the
mass of comets. These three substances, however, were by no means
supposed to be the sole constituents of the appendages in question. On
the contrary, the great breadth of what, for the present, were taken to
be characteristically "iron" tails was attributed to the presence of
many kinds of matter of high and slightly different specific
weights;[1276] while the expanded plume of Donati was shown to be, in
reality, a whole system of tails, made up of many substances, each
spreading into a separate hollow cone, more or less deviating from, and
partially superposed upon the others.
Yet these felicities of explanation must not make us forget that the
chemical composition attributed to the first type of cometary trains
has, so far, received no countenance from the spectroscope. The emission
lines of free, incandescent hydrogen have never been derived from any
part of these bodies. Dissentient opinions, accordingly, were expressed
as to the cause of their structural peculiarities. Ranyard,[1277]
Zenker, and others advocated the agency of heat repulsion in producing
them; Kiaer somewhat obscurely explains them through the evolution of
gases by colliding particles;[1278] Herz of Vienna concludes tails to be
mere illusory appendages produced by electrical discharges through the
rare medium assumed to fill space.[1279] But Hirn[1280] conclusively
showed that no such medium could possibly exist without promptly
bringing ruin upon our "dædal earth" and its revolving companions.
On the whole, modern researches tend to render superfluous the chemical
diversities postulated by Brédikhine. Electricity alone seems competent
to produce the varieties of cometary emanation they were designed to
account for. The distinction of types rests on a solid basis of fact,
but probably depends upon differences rather in the mode of action than
in the kind of substance acted upon. Suggestive sketches of electrical
and "light-pressure" theories of comets have been published respectively
by Mr. Fessenden of Alleghany,[1281] and by M. Arrhenius at
Stockholm.[1282] Although evidently of a tentative character, they
possess great interest.
Brédikhine's hypothesis was promptly and profusely illustrated. Within
three years of its promulgation, five bright comets made their
appearance, each presenting some distinctive peculiarity by which
knowledge of these curious objects was materially helped forward. The
first of these is remembered as the "Great Southern Comet." It was never
visible in these latitudes, but made a short though stately progress
through southern skies. Its earliest detection was at Cordoba on the
last evening of January, 1880; and it was seen on February 1, as a
luminous streak, extending just after sunset from the south-west horizon
towards the pole, in New South Wales, at Monte Video, and the Cape of
Good Hope. The head was lost in the solar rays until February 4, when
Dr. Gould, then director of the National Observatory of the Argentine
Republic at Cordoba, caught a glimpse of it very low in the west; and on
the following evening, Mr. Eddie, at Graham's Town, discovered a faint
nucleus, of a straw-coloured tinge, about the size of the annular nebula
in Lyra. Its condensation, however, was very imperfect, and the whole
apparition showed an exceedingly filmy texture. The tail was enormously
long. On February 5 it extended--large perspective retrenchment
notwithstanding--over an arc of 50°; but its brightness nowhere exceeded
that of the Milky Way in Taurus. There was little curvature perceptible;
the edges of the appendage ran parallel, forming a nebulous causeway
from star to star; and the comparison to an auroral beam was
appropriately used. The aspect of the famous comet of 1843 was forcibly
recalled to the memory of Mr. Janisch, Governor of St. Helena; and the
resemblance proved not merely superficial. But the comet of 1880 was
less brilliant, and even more evanescent. After only eight days of
visibility, it had faded so much as no longer to strike, though still
discoverable by the unaided eye; and on February 20 it was invisible
with the great Cordoba equatoreal pointed to its known place.
But the most astonishing circumstance connected with this body is the
identity of its path with that of its predecessor in 1843. This is
undeniable. Dr. Gould,[1283] Mr. Hind, and Dr. Copeland,[1284] each
computed a separate set of elements from the first rough observations,
and each was struck with an agreement between the two orbits so close as
to render them virtually indistinguishable. "Can it be possible," Mr.
Hind wrote to Sir George Airy, "that there is such a comet in the
system, almost grazing the sun's surface in perihelion, and revolving in
less than thirty-seven years. I confess I feel a difficulty in admitting
it, notwithstanding the above extraordinary resemblance of
orbits."[1285]
Mr. Hind's difficulty was shared by other astronomers. It would, indeed,
be a violation of common-sense to suppose that a celestial visitant so
striking in appearance had been for centuries back an unnoticed
frequenter of our skies. Various expedients, accordingly, were resorted
to for getting rid of the anomaly. The most promising at first sight was
that of the resisting medium. It was hard to believe that a body,
largely vaporous, shooting past the sun at a distance of less than a
hundred thousand miles from his surface, should have escaped powerful
retardation. It must have passed through the very midst of the corona.
It might easily have had an actual encounter with a prominence. Escape
from such proximity might, indeed, very well have been judged beforehand
to be impossible. Even admitting no other kind of opposition than that
dubiously supposed to have affected Encke's comet, the result in
shortening the period ought to be of the most marked kind. It was proved
by Oppolzer[1286] that if the comet of 1843 had entered our system from
stellar space with parabolic velocity it would, by the action of a
medium such as Encke postulated (varying in density inversely as the
square of the distance from the sun), have been brought down, by its
first perihelion passage, to elliptic movement in a period of
twenty-four years, with such rapid diminution that its next return would
be in about ten. But such restricted observations as were available on
either occasion of its visibility gave no sign of such a rapid progress
towards engulfment.
Another form of the theory was advocated by Klinkerfues.[1287] He
supposed that four returns of the same body had been witnessed within
historical memory--the first in 371 b.c., the next in 1668, besides
those of 1843 and 1880; an original period of 2,039 years being
successively reduced by the withdrawal at each perihelion passage of
1/1320 of the velocity acquired by falling from the far extremity of its
orbit towards the sun, to 175 and 37 years. A continuance of the process
would bring the comet of 1880 back in 1897.
Unfortunately, the earliest of these apparitions cannot be identified
with the recent ones unless by doing violence to the plain meaning of
Aristotle's words in describing it. He states that the comet was first
seen "during the frosts and in the clear skies of winter," setting due
west nearly at the same time as the sun.[1288] This implies some
considerable north latitude. But the objects lately observed had
practically _no_ north latitude. They accomplished their entire course
_above_ the ecliptic in two hours and a quarter, during which space they
were barely separated a hand's-breadth (one might say) from the sun's
surface. For the purposes of the desired assimilation, Aristotle's comet
should have appeared in March. It is not credible, however, that even a
native of Thrace should have termed March "winter."
With the comet of 1668 the case seemed more dubious. The circumstances
of its appearance are barely reconcilable with the identity attributed
to it, although too vaguely known to render certainty one way or the
other attainable. It might however, be expected that recent observations
would at least decide the questions whether the comet of 1843 could have
returned in less than thirty-seven, and whether the comet of 1880 was to
be looked for at the end of 17-1/2 years. But the truth is that both
these objects were observed over so small an arc--8° and 3°
respectively--that their periods remained virtually undetermined. For
while the shape and position of their orbits could be and were fixed
with a very close approach to accuracy, the length of those orbits might
vary enormously without any very sensible difference being produced in
the small part of the curves traced out near the sun. Dr. Wilhelm Meyer,
however, arrived, by an elaborate discussion, at a period of
thirty-seven years for the comet of 1880,[1289] while the observations
of 1843 were admittedly best fitted by Hubbard's ellipse of 533 years;
but these Dr. Meyer supposed to be affected by some constant source of
error, such as would be produced by a mistaken estimate of the position
of the comet's centre of gravity. He inferred finally that, in spite of
previous non-appearances, the two comets represented a single regular
denizen of our system, returning once in thirty-seven years along an
orbit of such extreme eccentricity that its movement might be described
as one of precipitation towards and rapid escape from the sun, rather
than of sedate circulation round it.
The _geometrical_ test of identity has hitherto been the only one which
it was possible to apply to comets, and in the case before us it may
fairly be said to have broken down. We may, then, tentatively, and with
much hesitation, try a _physical_ test, though scarcely yet, properly
speaking, available. We have seen that the comets of 1843 and 1880 were
strikingly alike in general appearance, though the absence of a formed
nucleus in the latter, and its inferior brilliancy, detracted from the
convincing effect of the resemblance. Nor was it maintained when tried
by exact methods of inquiry. M. Brédikhine found that the gigantic ray
emitted in 1843 belonged to his type No. 1; that of 1880 to type No.
2.[1290] The particles forming the one were actuated by a repulsive
force ten times as powerful as those forming the other. It is true that
a second noticeably curved tail was seen in Chili, March 1, and at
Madras, March 11, 1843; and the conjecture was accordingly hazarded that
the materials composing on that occasion the principal appendage having
become exhausted, those of the secondary one remained predominant, and
reappeared alone in the "hydro-carbon" train of 1880. But the one known
instance in point is against such a supposition. Halley's comet, the
only _great_ comet of which the returns have been securely authenticated
and carefully observed, has preserved its "type" unchanged through many
successive revolutions. The dilemma presented to astronomers by the
Great Southern Comet of 1880 was unexpectedly renewed in the following
year.
On the 22nd of May, 1881, Mr. John Tebbutt of Windsor, New South Wales,
scanning the western sky, discerned a hazy-looking object which he felt
sure was a strange one. A marine telescope at once resolved it into two
small stars and a comet, the latter of which quickly attracted the keen
attention of astronomers; for Dr. Gould, computing its orbit from his
first observations at Cordoba, found it to agree so closely with that
arrived at by Bessel for the comet of 1807 that he telegraphed to
Europe, June 1, announcing the unexpected return of that body. So
unexpected that theoretically it was not possible before the year 3346;
and Bessel's investigation was one which inspired and eminently deserved
confidence. Here, then, once more the perplexing choice had to be made
between a premature and unaccountable reappearance and the admission of
a plurality of comets moving nearly in the same path. But in this case
facts proved decisive.
Tebbutt's comet passed the sun, June 16, at a distance of sixty-eight
millions of miles, and became visible in Europe six days later. It was,
in the opinion of some, the finest object of the kind since 1861. In
traversing the constellation Auriga on its _début_ in these latitudes,
it outshone Capella. On June 24 and some subsequent nights, it was
unmatched in brilliancy by any star in the heavens. In the telescope,
the "two interlacing arcs of light" which had adorned the head of
Coggia's comet were reproduced; while a curious _dorsal spine_ of strong
illumination formed the axis of the tail, which extended in clear skies
over an arc of 20°. It belonged to the same "type" as Donati's great
plume; the particles composing it being driven _from_ the sun by a force
twice as powerful as that urging them _towards_ it.[1291] But the
appendage was, for a few nights, and by two observers perceived to be
double. Tempel, on June 27, and Lewis Boss, at Albany (N.Y.), June 26
and 28, saw a long straight ray corresponding to a far higher rate of
emission than the curved train, and shown by Brédikhine to be a member
of the (so-called) hydrogen class. It had vanished by July 1, but made a
temporary reappearance July 22.[1292]
The appendages of this comet were of remarkable transparency.
Small stars shone wholly undimmed across the tail, and a very nearly
central transit of the head over one of the seventh magnitude on the
night of June 29, produced--if any change--an increase of brilliancy in
the object of this spontaneous experiment.[1293] Dr. Meyer, indeed, at
the Geneva Observatory, detected apparent signs of refractive action
upon rays thus transmitted;[1294] but his observations remain isolated,
and were presumably illusory.
The track pursued by this comet gave peculiar advantages for its
observation. Ascending from Auriga through Camelopardus, it stood, July
19, on a line between the Pointers and the Pole, within 8° of the
latter, and thus remained for a lengthened period constantly above the
horizon of northern observers. Its brightness, too, was no transient
blaze, but had a lasting quality which enabled it to be kept steadily in
view during nearly nine months. Visible to the naked eye until the end
of August, the last telescopic observation of it was made February 14,
1882, when its distance from the earth considerably exceeded 300 million
miles. Under these circumstances, the knowledge acquired of its orbit
was of more than usual accuracy, and showed conclusively that the comet
was not a simple return of Bessel's; for this would involve a period of
seventy-four years, whereas Tebbutt's comet cannot revisit the sun until
after the lapse of two and a half millenniums.[1295] Nevertheless, the
twin bodies move so nearly in the same path that an original connection
of some kind is obvious; and the recent example of Biela readily
suggested a conjecture as to what the nature of that connection might
have been. The comets of 1807 and 1881 are, then, regarded with much
probability as fragments of a primitive disrupted body, one following in
the wake of the other at an interval of seventy-four years.
Imperfect photographs were taken of Donati's comet both in England and
America;[1296] but Tebbutt's comet was the first to which the process
was satisfactorily applied. The difficulties to be overcome were very
great. The chemical intensity of cometary light is, to begin with,
extraordinarily small. Janssen estimated it at 1/300000 of
moonlight.[1297] Hence, if the ordinary process by which lunar
photographs are taken had been applied to the comet of 1881, an exposure
of at least _three days_ would have been required in order to get an
impression of the head with about a tenth part of the tail. But by that
time a new method of vastly increased sensitiveness had been rendered
available, by which dry gelatine-plates were substituted for the wet
collodion-plates hitherto in use; and this improvement alone reduced the
necessary time of exposure to two hours. It was brought down to half an
hour by Janssen's employment of a reflector specially adapted to give an
image illuminated eight or ten times as strongly as that produced in the
focus of an ordinary telescope.[1298]
The photographic feebleness of cometary rays was not the only obstacle
in the way of success. The proper motion of these bodies is so rapid as
to render the usual devices for keeping a heavenly body steadily in view
quite inapplicable. The machinery by which the diurnal movement of the
sphere is followed, must be especially modified to suit each eccentric
career. This, too, was done, and on June 30, 1881, Janssen secured a
perfect photograph of the brilliant object then visible, showing the
structure of the tail with beautiful distinctness to a distance of
2-1/2° from the head. An impression to nearly 10° was obtained about the
same time by Dr. Henry Draper at New York, with an exposure of 162
minutes.[1299]
Tebbutt's (or comet 1881 iii.) was also the first comet of which the
spectrum was so much as attempted to be chemically recorded. Both
Huggins and Draper were successful in this respect, but Huggins was more
completely so.[1300] The importance of the feat consisted in its
throwing open to investigation a part of the spectrum invisible to the
eye, and so affording an additional test of cometary constitution. The
result was fully to confirm the origin from carbon-compounds assigned to
the visible rays, by disclosing additional bands belonging to the same
series in the ultra-violet; as well as to establish unmistakably the
presence of a not inconsiderable proportion of reflected solar light by
the clear impression of some of the principal Fraunhofer lines. Thus the
polariscope was found to have told the truth, though not the whole
truth.
The photograph so satisfactorily communicative was taken by Sir William
Huggins on the night of June 24; and on the 29th, at Greenwich, the
tell-tale Fraunhofer lines were perceived to interrupt the visible range
of the spectrum. This was at first so vividly continuous, that the
characteristic cometary bands could scarcely be detached from their
bright background. But as the nucleus faded towards the end of June,
they came out strongly, and were more and more clearly seen, both at
Greenwich and at Princeton, to agree, not with the spectrum of
hydro-carbons glowing in a vacuum tube, but with that of the same
substances burning in a Bunsen flame.[1301] It need not, however, be
inferred that cometary materials are really in a state of combustion.
This, from all that we know, may be called an impossibility. The
additional clue furnished was rather to the manner of their electrical
illumination.[1302]
The spectrum of the tail was, in this comet, found to be not essentially
different from that of the head. Professor Wright of Yale College
ascertained a large percentage of its light to be polarized in a plane
passing through the sun, and hence to be reflected sunlight.[1303] A
faint continuous spectrum corresponded to this portion of its radiance;
but gaseous emissions were also present. At Potsdam, on June 30, the
hydro-carbon bands were indeed traced by Vogel to the very end of the
tail;[1304] and they were kept in sight by Young at a greater distance
from the nucleus than the more equably dispersed light. There seems
little doubt that, as in the solar corona, the relative strength of the
two orders of spectra is subject to fluctuations.
The comet of 1881 iii. was thus of signal service to science. It
afforded, when compared with the comet of 1807, the first undeniable
example of two such bodies travelling so nearly in the same orbit as to
leave absolutely no doubt of the existence of a genetic tie between
them. Cometary photography came to its earliest fruition with it; and
cometary spectroscopy made a notable advance by means of it. Before it
was yet out of sight, it was provided with a successor.
At Ann Arbor Observatory, Michigan, on July 14, a comet was discovered
by Dr. Schaeberle, which, as his claim to priority is undisputed, is
often allowed to bear his name, although designated, in strict
scientific parlance, comet 1881 iv. It was observed in Europe after
three days, became just discernible by the naked eye at the end of July,
and brightened consistently up to its perihelion passage, August 22,
when it was still about fifty million miles from the sun. During many
days of that month, the uncommon spectacle was presented of two bright
comets circling together, though at widely different distances, round
the North pole of the heavens. The newcomer, however, never approached
the pristine lustre of its predecessor. Its nucleus, when brightest, was
comparable to the star Cor Caroli, a narrow, perfectly straight ray
proceeding from it to a distance of 10°. This was easily shown by
Brédikhine to belong to the hydrogen type of tails;[1305] while a
"strange, faint second tail, or bifurcation of the first one," observed
by Captain Noble, August 24,[1306] fell into the hydro-carbon class of
emanations. It was seen, August 22 and 24, by Dr. F. Terby of
Louvain,[1307] as a short nebulous brush, like the abortive beginning of
a congeries of curving trains; but appeared no more. Its well-attested
presence was significant of the complex constitution of such bodies, and
the manifold kinds of action progressing in them.
The only peculiarity in the spectrum of Schaeberle's comet consisted in
the almost total absence of continuous light. The carbon-bands were
nearly isolated and very bright. Barely from the nucleus proceeded a
rainbow-tinted streak, indicative of solid or liquid matter, which, in
this comet, must have been of very scanty amount. Its visit to the sun
in 1881 was, so far as is known, the first. The elements of its orbit
showed no resemblance to those of any previous comet, nor any marked
signs of periodicity. So that, although it may be considered probable,
we do not _know_ that it is moving in a closed curve, or will ever again
penetrate the precincts of the solar system. It was last seen from the
southern hemisphere, October 19, 1881.
The third of a quartette of lucid comets visible within sixteen months,
was discovered by Mr. C. S. Wells at the Dudley Observatory, Albany,
March 17, 1882. Two days later it was described by Mr. Lewis Boss as "a
great comet in miniature," so well defined and regularly developed were
its various parts and appendages. Discernible with optical aid early in
May, it was on June 5 observed on the meridian at Albany just before
noon--an astronomical event of extreme rarity. Comet Wells, however,
never became an object so conspicuous as to attract general attention,
owing to its immersion in the evening twilight of our northern June.
But the study of its spectrum revealed new facts of the utmost interest.
All the comets till then examined had been found (with the two
transiently observed exceptions already mentioned) to conform to one
invariable type of luminous emission. Individual distinctions there had
been, but no specific differences. Now all these bodies had kept at a
respectful distance from the sun; for of the great comet of 1880 no
spectroscopic inquiries had been made. Comet Wells, on the other hand,
approached its surface within little more than five million miles on
June 10, 1882; and the vicinity had the effect of developing a novel
feature in its incandescence.
During the first half of April its spectrum was of the normal type,
though the carbon bands were unusually weak; but with approach to the
sun they died out, and the entire light seemed to become concentrated
into a narrow, unbroken, brilliant streak, hardly to be distinguished
from the spectrum of a star. This unusual behaviour excited attention,
and a strict watch was kept. It was rewarded at the Dunecht Observatory,
May 27, by the discernment of what had never before been seen in a
comet--the yellow ray of sodium.[1308] By June 1, this had kindled into
a blaze overpowering all other emissions. The light of the comet was
practically monochromatic; and the image of the entire head, with the
root of the tail, could be observed, like a solar prominence, depicted,
in its new saffron vesture of vivid illumination, within the jaws of an
open slit.
At Potsdam, the bright yellow line was perceived with astonishment by
Vogel on May 31, and was next evening identified with Fraunhofer's "D."
Its character led him to infer a very considerable density in the
glowing vapour emitting it.[1309] Hasselberg founded an additional
argument in favour of the electrical origin of cometary light on the
changes in the spectrum of comet Wells.[1310] For they were closely
paralleled by some earlier experiments of Wiedemann, in which the
gaseous spectra of vacuum tubes were at once effaced on the introduction
of metallic vapours. It seemed as if the metal had no sooner been
rendered volatile by heat, than it usurped the entire office of carrying
the discharge, the resulting light being thus exclusively of its
production. Had simple incandescence by heat been in question, the
effect would have been different; the two spectra would have been
superposed without prejudice to either. Similarly, the replacement of
the hydro-carbon bands in the spectrum of the comet by the sodium line
proved electricity to be the exciting agent. For the increasing thermal
power of the sun might, indeed, have ignited the sodium, but it could
not have extinguished the hydro-carbons.
Sir William Huggins succeeded in photographing the spectrum of comet
Wells by an exposure of one hour and a quarter.[1311] The result was to
confirm the novelty of its character. None of the ultra-violet carbon
groups were apparent; but certain bright rays, as yet unidentified, had
imprinted themselves. Otherwise the spectrum was strongly continuous,
uninterrupted even by the Fraunhofer lines detected in the spectrum of
Tebbutt's comet. Hence it was concluded that a smaller proportion of
reflected light was mingled with the native emissions of the later
arrival.
All that is certainly known about the _extent_ of the orbit traversed by
the first comet of 1882 is that it came from, and is now retreating
towards, vastly remote depths of space. An American computer[1312] found
a period indicated for it of no less than 400,000 years; A. Thraen of
Dingelstädt arrived at one of 3617.[1313] Both are perhaps equally
insecure.
We have now to give some brief account of one of the most remarkable
cometary apparitions on record, and--with the single exception of that
identified with the name of Halley--the most instructive to astronomers.
The lessons learned from it were as varied and significant as its aspect
was splendid; although from the circumstance of its being visible in
general only before sunrise, the spectators of its splendour were
comparatively few.
The discovery of a great comet at Rio Janeiro, September 11, 1882,
became known in Europe through a telegram from M. Cruls, director of the
observatory at that place. It had, however (as appeared subsequently),
been already seen on the 8th by Mr. Finlay of the Cape Observatory, and
at Auckland as early as September 3. A later, but very singularly
conditioned detection, quite unconnected with any of the preceding, was
effected by Dr. Common at Ealing. Since the eclipse of May 17, when a
comet--named "Tewfik" in honour of the Khedive of Egypt--was caught on
Dr. Schuster's photographs, entangled, one might almost say, in the
outer rays of the corona, he had scrutinized the neighbourhood of the
sun on the infinitesimal chance of intercepting another such body on its
rapid journey thence or thither. We record with wonder that, after an
interval of exactly four months, that infinitesimal chance turned up in
his favour.
On the forenoon of Sunday, September 17, he saw a great comet close to,
and rapidly approaching the sun. It was, in fact, then within a few
hours of perihelion. Some measures of position were promptly taken; but
a cloud-veil covered the interesting spectacle before mid-day was long
past. Mr. Finlay at the Cape was more completely fortunate. Divided from
his fellow-observer by half the world, he unconsciously finished, under
a clearer sky, his interrupted observation. The comet, of which the
silvery radiance contrasted strikingly with the reddish-yellow glare of
the sun's margin it drew near to, was followed "continuously right into
the boiling of the limb"--a circumstance without precedent in cometary
history.[1314] Dr. Elkin, who watched the progress of the event with
another instrument, thought the intrinsic brilliancy of the nucleus
scarcely surpassed by that of the sun's surface. Nevertheless it had no
sooner touched it than it vanished as if annihilated. So sudden was the
disappearance (at 4h. 50m. 58s., Cape mean time), that the comet was at
first believed to have passed _behind_ the sun. But this proved not to
have been the case. The observers at the Cape had witnessed a genuine
transit. Nor could non-visibility be explained by equality of
lustre. For the gradations of light on the sun's disc are amply
sufficient to bring out against the dusky background of the limb any
object matching the brilliancy of the centre; while an object just
equally luminous with the limb must inevitably show dark at the centre.
The only admissible view, then, is that the bulk of the comet was of too
filmy a texture, and its presumably solid nucleus too small, to
intercept any noticeable part of the solar rays--a piece of information
worth remembering.
PLATE III.
[Illustration: The Great Comet of September, 1882.
Photographed at the Royal Observatory, Cape of Good Hope]
On the following morning, the object of this unique observation showed
(in Sir David Gill's words) "an astonishing brilliancy as it rose behind
the mountains on the east of Table Bay, and seemed in no way diminished
in brightness when the sun rose a few minutes afterward. It was only
necessary to shade the eye from direct sunlight with the hand at arm's
length, to see the comet, with its brilliant white nucleus and dense
white, sharply bordered tail of quite half a degree in length."[1315]
All over the world, wherever the sky was clear during that day,
September 18, it was obvious to ordinary vision. Since 1843 nothing had
been seen like it. From Spain, Italy, Algeria, Southern France,
despatches came in announcing the extraordinary appearance. At Cordoba,
in South America, the "blazing star near the sun" was the one topic of
discourse.[1316] Moreover--and this is altogether extraordinary--the
records of its daylight visibility to the naked eye extend over three
days. At Reus, near Tarragona, it showed bright enough to be seen
through a passing cloud when only three of the sun's diameters from his
limb, just before its final rush past perihelion on September 17; while
at Carthagena in Spain, on September 19, it was kept in view during two
hours before and two hours after noon, and was similarly visible in
Algeria on the same day.[1317]
But still more surprising than the appearance of the body itself were
the nature and relations of the path it moved in. The first rough
elements computed for it by Mr. Tebbutt, Dr. Chandler, and Mr. White,
assistant at the Melbourne Observatory, showed at once a striking
resemblance to those of the twin comets of 1843 and 1880. This
suggestive fact became known in this country, September 27, through the
medium of a Dunecht circular. It was fully confirmed by subsequent
inquiries, for which ample opportunities were luckily provided. The
likeness was not, indeed, so absolutely perfect as in the previous case;
it included some slight, though real differences; but it bore a strong
and unmistakable stamp, broadly challenging explanation.
Two hypotheses only were really available. Either the comet of 1882 was
an accelerated return of those of 1843 and 1880, or it was a fragment of
an original mass to which they also had belonged. For the purposes of
the first view the "resisting medium" was brought into full play; the
opinion of its efficacy was for some time both prevalent and popular,
and formed the basis, moreover, of something of a sensational panic. For
a comet which, at a single passage through the sun's atmosphere,
encountered sufficient resistance to shorten its period from
thirty-seven to two years and eight months, must, in the immediate
future, be brought to rest on his surface; and the solar conflagration
thence ensuing was represented in some quarters, with more licence of
imagination than countenance from science, as likely to be of
catastrophic import to the inhabitants of our little planet.
But there was a test available in 1882 which it had not been possible to
apply either in 1843 or in 1880. The two bodies visible in those years
had been observed only after they had already passed perihelion;[1318]
the third member of the group, on the other hand, was accurately
followed for a week before that event, as well as during many months
after it. Finlay's and Elkin's observation of its disappearance at the
sun's edge formed, besides, a peculiarly delicate test of its motion.
The opportunity was thus afforded, by directly comparing the comet's
velocity before and after its critical plunge through the solar
surroundings, of ascertaining with approximate certainty whether any
considerable retardation had been experienced in the course of that
plunge. The answer distinctly given was that there had not. The computed
and observed places on both sides of the sun fitted harmoniously
together. The effect, if any were produced, was too small to be
perceptible.
This result is, in itself, a memorable one. It seems to give the _coup
de grâce_ to Encke's theory--discredited, in addition, by Backlund's
investigation--of a resisting medium growing rapidly denser inwards. For
the perihelion distance of the comet of 1882, though somewhat greater
than that of its predecessors, was nevertheless extremely small. It
passed at less than 300,000 miles of the sun's surface. But the ethereal
substance long supposed to obstruct the movement of Encke's comet would
there be nearly 2,000 times denser than at the perihelion of the smaller
body, and must have exerted a conspicuous retarding influence. That none
such could be detected seems to argue that no such medium exists.
Further evidence of a decisive kind was not wanting on the question of
identity. The "Great September Comet" of 1882 was in no hurry to
withdraw itself from curious terrestrial scrutiny. It was discerned with
the naked eye at Cordoba as late as March 7, 1883, and still showed in
the field of the great equatoreal on June 1 as an "excessively faint
whiteness."[1319] It was then about 480 millions of miles from the
earth--a distance to which no other comet--not even excepting the
peculiar one of 1729--had been pursued.[1320] Moreover, an arc of 340
out of the entire 360 degrees of its circuit had been described under
the eyes of astronomers; so that its course came to be very well known.
That its movement is in a very eccentric ellipse, traversed in several
hundred years, was ascertained.[1321] The later inquiries of Dr.
Kreutz,[1322] completed in a volume published in 1901,[1323]
demonstrated the period to be of about 800 years, while that of its
predecessor in 1843 might possibly agree with it, but is much more
probably estimated at 512 years. The hypothesis that they, or any of the
comets associated with them, were returns of an individual body is
peremptorily excluded. They may all, however, have been separated from
one original mass by the divellent action of the sun at close quarters.
Each has doubtless its own period, since each has most likely suffered
retardations or accelerations special to itself, which, though trifling
in amount, would avail materially to alter the length of the major axis,
while leaving the remaining elements of the common orbit virtually
unchanged.[1324]
A fifth member was added to the family in 1887. On the 18th of January
in that year, M. Thome discovered at Cordoba a comet reproducing with
curious fidelity the lineaments of that observed in the same latitudes
seven years previously. The narrow ribbon of light, contracting towards
the sun, and running outward from it to a distance of thirty-five
degrees; the unsubstantial head--a veiled nothingness, as it appeared,
since no distinct nucleus could be made out; the quick fading into
invisibility, were all accordant peculiarities, and they were confirmed
by some rough calculations of its orbit, showing geometrical affinity to
be no less unmistakable than physical likeness. The observations secured
were indeed, from the nature of the apparition, neither numerous nor
over-reliable; and the earliest of them dated from a week after
perihelion, passed, almost by a touch-and-go escape, January 11. On
January 27, this mysterious object could barely be discerned
telescopically at Cordoba.[1325] That it belonged to the series of
"southern comets" can scarcely be doubted; but the inference that it was
an actual return of the comet of 1880, improbable in itself, was
negatived by its non-appearance in 1894. Meyer's incorporation with this
extraordinary group of the "eclipse-comet" of 1882[1326] has been
approved by Kreutz, after searching examination.
The idea of cometary systems was first suggested by Thomas Clausen in
1831.[1327] It was developed by the late M. Hoek, director of the
Utrecht Observatory, in 1865 and some following years.[1328] He found
that in quite a considerable number of cases, the paths of two or three
comets had a common point of intersection far out in space, indicating
with much likelihood a community of origin. This consisted, according to
his surmise, in the disruption of a parent mass during its sweep round
the star latest visited. Be this as it may, the fact is undoubted that
numerous comets fall into groups, in which similar conditions of motion
betray a pre-existent physical connection. Never before, however, had
geometrical relationship been so notorious as between the comets now
under consideration; and never before, in a comet still, it might be
said, in the prime of life, had physical peculiarities tending to
account for that affinity been so obvious as in the chief member of the
group.
Observation of a granular structure in cometary nuclei dates far back
into the seventeenth century, when Cysatus and Hevelius described the
central parts of the comets of 1618 and 1652 respectively as made up of
a congeries of minute stars. Analogous symptoms of a loose state of
aggregation have of late been not unfrequently detected in telescopic
comets, besides the instances of actual division offered by those
connected with the names of Biela and Liais. The forces concerned in
producing these effects seem to have been peculiarly energetic in the
great comet of 1882.
The segmentation of the nucleus was first noticed in the United States
and at the Cape of Good Hope, September 30. It proceeded rapidly. At
Kiel, on October 5 and 7, Professor Krüger perceived two centres of
condensation. A definite and progressive separation into _three_ masses
was observed by Professor Holden, October 13 and 17.[1329] A few days
later, M. Tempel found the head to consist of _four_ lucid aggregations,
ranged nearly along the prolongation of the caudal axis;[1330] and Dr.
Common, January 27, 1883, saw _five_ nuclei in a line "like pearls on a
string."[1331] This remarkable character was preserved to the last
moment of the comet's distinct visibility. It was a consequence,
according to Dr. Kreutz, of violent interior action in the comet itself
While close to the sun.
There were, however, other curious proofs of a disaggregative tendency
in this body. On October 9, Schmidt discovered at Athens a nebulous
object 4° south-west of the great comet, and travelling in the same
direction. It remained visible for a few days, and, from Oppenheim's and
Hind's calculations, there can be little doubt that it was really the
offspring by fission of the body it accompanied.[1332] This is rendered
more probable by the unexampled spectacle offered, October 14, to
Professor Barnard, then of Nashville, Tennessee, of _six or eight_
distinct cometary masses within 6° south by west of the comet's head,
none of which reappeared on the next opportunity for a search.[1333] A
week later, however, one similar object was discerned by Mr. W. R.
Brooks, in the opposite direction from the comet. Thus space appeared to
be strewn with the filmy débris of this beautiful but fragile structure
all along the track of its retreat from the sun.
Its tail was only equalled (if it were equalled) in length by that of
the comet of 1843. It extended in space to the vast distance of 200
millions of miles from the head; but, so imperfectly were its
proportions displayed to terrestrial observers, that it at no time
covered an arc of the sky of more than 30°. This apparent extent was
attained, during a few days previous to September 25, by a faint, thin,
rigid streak, noticed only by a few observers--by Elkin at the Cape
Observatory, Eddie at Grahamstown, and Cruls at Rio Janeiro. It diverged
at a low angle from the denser curved train, and was produced, according
to Brédikhine,[1334] by the action of a repulsive force twelve times as
strong as the counter-pull of gravity. It belonged, that is, to type 1;
while the great bifurcate appendage, obvious to all eyes, corresponded
to the lower rate of emission characteristic of type 2. This was
remarkable for the perfect definiteness of its termination, for its
strongly-forked shape, and for its unusual permanence. Down to the end
of January, 1883, its length, according to Schmidt's observations, was
still 93 million miles; and a week later it remained visible to the
naked eye, without notable abridgment.
Most singular of all was an anomalous extension of the appendage
_towards_ the sun. During the greater part of October and November, a
luminous "tube" or "sheath," of prodigious dimensions, seemed to
surround the head, and project in a direction nearly opposite to that of
the usual outpourings of attentuated matter. (See Plate III.) Its
diameter was computed by Schmidt to be, October 15, no less than four
million miles, and it was described by Cruls as a "truncated cone of
nebulosity," stretching 3° or 4° sunwards.[1335] This, and the entire
anterior part of the comet, were again surrounded by a thin, but
enormously voluminous paraboloidal envelope, observed by Schiaparelli
for a full month from October 19.[1336] There can be little doubt that
these abnormal effluxes were a consequence of the tremendous physical
disturbance suffered at perihelion; and it is worth remembering that
something analogous was observed in the comet of 1680 (Newton's), also
noted for its excessively close approach to the sun, and possibly moving
in a related orbit. The only plausible hypothesis as to the mode of
their production is that of an opposite state of electrification in the
particles composing the ordinary and extraordinary appendages.
The spectrum of the great comet of 1882 was, in part, a repetition of
that of its immediate predecessor, thus confirming the inference that
the previously unexampled sodium-blaze was in both a direct result of
the intense solar action to which they were exposed. But the D line was,
this time, not seen alone. At Dunecht, on the morning of September 18,
Drs. Copeland and J. G. Lohse succeeded in identifying six brilliant
rays in the green and yellow with as many prominent iron-lines;[1337] a
very significant addition to our knowledge of cometary constitution, and
one which lent countenance to Brédikhine's assumption of various kinds
of matter issuing from the nucleus with velocities inversely as their
atomic weights. All the lines equally showed a slight displacement,
indicating a recession from the earth of the radiating body at the rate
of 37 to 46 miles a second. A similar observation, made by M. Thollon at
Nice on the same day, gave emphatic sanction to the spectroscopic method
of estimating movement in the line of sight. Before anything was as yet
known of the comet's path or velocity, he announced, from the position
of the double sodium-line alone, that at 3 p.m. on September 18 it was
increasing its distance from our planet by from 61 to 76 kilometres per
second.[1338] M. Bigourdan's subsequent calculations showed that its
actual swiftness of recession was at that moment 73 kilometres.
Changes in the inverse order to those seen in the spectrum of comet
Wells soon became apparent. In the earlier body, carbon bands had died
out with _approach_ to perihelion, and had been replaced by sodium
emissions; in its successor, sodium emissions became weakened and
disappeared with _retreat_ from perihelion, and found their substitute
in carbon bands. Professor Riccò was, in fact, able to infer, from the
sequence of prismatic phenomena, that the comet had already passed the
sun; thus establishing a novel criterion for determining the position of
a comet in its orbit by the varying quality of its radiations.
Recapitulating what was learnt from the five conspicuous comets of
1880-82, we find that the leading facts acquired to science were these
three. First, that comets may be met with pursuing each other, after
intervals of many years, in the same, or nearly the same, track; so that
identity of orbit can no longer be regarded as a sure test of individual
identity. Secondly, that at least the outer corona may be traversed by
such bodies with perfect apparent impunity. Finally, that their chemical
constitution is highly complex, and that they possess, in some cases at
least, a metallic core resembling the meteoric masses which occasionally
reach the earth from planetary space.
A group of five comets, including Halley's, own a sort of cliental
dependence upon the planet Neptune. They travel out from the sun just to
about his distance from it, as if to pay homage to a powerful protector,
who gets the credit of their establishment as periodical visitors to the
solar system. The second of these bodies to affect a looked-for return
was a comet--the sixteenth within ten years--discovered by Pons, July
20, 1812, and found by Encke to revolve in an elliptic orbit, with a
period of nearly 71 years. It was not, however, until September 1, 1883,
that Mr. Brooks caught its reappearance; it passed perihelion January
25, and was last seen June 2, 1884. At its brightest, it had the
appearance of a second magnitude star, furnished with a poorly developed
double tail, and was fairly conspicuous to the naked eye in Southern
Europe, from December to March. One exceptional feature distinguished
it. Its fluctuations in form and luminosity were unprecedented in
rapidity and extent. On September 21, Dr. Chandler[1339] observed it at
Harvard as a very faint, diffused nebulosity, with slight central
condensation. On the next night, there was found in its place a bright
star of the eighth magnitude, scarcely marked out, by a bare trace of
environing haze, from the genuine stars it counterfeited. The change was
attended by an eight-fold augmentation of light, and was proved by
Schiaparelli's confirmatory observations[1340] to have been accomplished
within a few hours. The stellar disguise was quickly cast aside. The
comet appeared on September 23 as a wide nebulous disc, and soon after
faded down to its original dimness. Its distance from the sun was then
no less than 200 million miles, and its spectrum showed nothing unusual.
These strange variations recurred slightly on October 15, and with
marked emphasis on January 1, when they were witnessed with amazement,
and photometrically studied by Müller of Potsdam.[1341] The entire cycle
this time was run through in less than four hours--the comet having, in
that brief space, condensed, with a vivid outburst of light, into a
seeming star, and the seeming star having expanded back again into a
comet. Scarcely less transient, though not altogether similar, changes
of aspect were noted by M. Perrotin,[1342] January 13 and 19, 1884. On
the latter date, the continuous spectrum given by a reddish-yellow disc
surrounding the true nucleus seemed intensified by bright knots
corresponding to the rays of sodium.
A comet discovered by Mr. Sawerthal at the Royal Observatory, Cape of
Good Hope, February 19, 1888, distinguished itself by blazing up, on May
19, to four or five times its normal brilliancy, at the same time
throwing out from the head two lustrous lateral branches.[1343] These
had, on June 1, spread backward so as to join the tail, with an effect
like the playing of a fountain; ten or eleven days later, they had
completely disappeared, leaving the comet in its former shape and
insignificance. Its abrupt display of vitality occurred two full months
after perihelion.
On the morning of July 7, 1889, Mr. W. R. Brooks, of Geneva, New York,
eminent as a successful comet-hunter, secured one of his customary
trophies. The faint object in question was moving through the
constellation Cetus, and turned out to be a member of Jupiter's numerous
family of comets, revolving round the sun in a period of seven years.
Its past history came then, to a certain extent, within the scope of
investigation, and proved to have been singularly eventful; nor had the
body escaped scatheless from the vicissitudes to which it had been
exposed. Observing from Mount Hamilton, August 2 and 5, Professor
Barnard noticed this comet (1889, v.) to be attended in its progress
through space by four _outriders_, "The two brighter companions" (the
fainter pair survived a very short time) "were perfect miniatures,"
Professor Barnard tells us,[1344] "of the larger comet, each having a
small, fairly defined head and nucleus, with a faint, hazy tail, the
more distant one being the larger and less developed. The three comets
were in a straight line, nearly east and west, their tails lying along
this line. There was no connecting nebulosity between these objects, the
tails of the two smaller not reaching each other, or the large comet. To
all appearance they were absolutely independent comets." Nevertheless,
Spitaler, at Vienna, in the early days of August, perceived, as it were,
a thin cocoon of nebulosity woven round the entire trio.[1345] One of
them faded from view September 5; the other actually outshone the
original comet on August 31, but was plainly of inferior vitality. It
was last seen by Barnard on November 25, with the thirty-six inch
refractor, while its primary afforded an observation for position with
the twelve-inch, March 20, 1890.[1346] A cause for the disruption it had
presumably undergone had, before then, been plausibly assigned.
The adventures of Lexell's comet have long served to exemplify the
effects of Jupiter's despotic sway over such bodies. Although bright
enough in 1770 to be seen with the naked eye, and ascertained to be
circulating in five and a half years, it had never previously been seen,
and failed subsequently to present itself. The explanation of this
anomaly, suggested by Lexell, and fully confirmed by the analytical
inquiries both of Laplace and Leverrier,[1347] was that a very close
approach to Jupiter in 1767 had completely changed the character of its
orbit, and brought it within the range of terrestrial observation; while
in 1779, after having only twice traversed its new path (at its second
return it was so circumstanced as to be invisible from the earth), it
was, by a fresh encounter, diverted into one entirely different. Yet the
possibility was not lost sight of that the great planet, by inverting
its mode of action, might undo its own work, and fling the comet once
more into the inner part of the solar system. This possibility seemed to
be realized by Chandler's identification of Brooks's and Lexell's
comet.[1348] An exceedingly close approach to Jupiter in 1886 had, he
found reason to believe, produced such extensive alterations in the
elements of its motion as to bring the errant body back to our
neighbourhood in 1889. But his inference, though ratified by Mr. Charles
Lane Poor's preliminary calculations, proved dubious on closer inquiry,
and was rendered wholly inadmissible by the circumstances attending the
return of Brooks's comet in 1896.[1349] The companion-objects watched by
Barnard in 1889 had by that time, perhaps, become dissipated in space,
for they were not redetected. They represented, in all likelihood,
wreckage from a collision with Jupiter, dating, perhaps, so far back as
1791, when Mr. Lane Poor found that one of the fateful meetings to which
short-period comets are especially subject had taken place.
The Lexell-Brooks case was almost duplicated by the resemblance to De
Vico's lost comet of 1844[1350] of one detected November 20, 1894, by
Edward, son of Lewis Swift. Schulhof[1351] announced the identity, and
Chandler,[1352] under reserve, vouched for it. Had the comet continued
to pursue the track laboriously laid down for it at Boston, and shown
itself at the due epoch in 1900, its individuality might have been
considered assured; but the formidable vicegerent of the sun once more
interposed, and, in 1897, swept it out of the terrestrial range of view.
Hence the recognition remains ambiguous.
On the morning of March 7, 1892, Professor Lewis Swift discovered the
brightest comet that had been seen by northern observers since 1882.
About the time of perihelion, which occurred on April 6, it was
conspicuous, as it crossed the celestial equator from Aquarius towards
Pegasus, with a nucleus equal to a third magnitude star, and a tail
twenty degrees long. This tail was multiple, and multiple in a most
curiously variable manner. It divided up into many thin nebulous
streaks, the number and relative lustre of which underwent rapid and
marked changes. Their permanent record on Barnard's and W. H.
Pickering's plates marked a noteworthy advance in cometary photography.
Plate IV. reproduces two of the Lick pictures, taken with a six-inch
camera, on April 5 and 7 respectively, with, in each case, an exposure
of about one hour. The tail is in the first composed of three main
branches, the middle one having sprung out since the previous morning,
and the branches are, in their turn, split up into finer rays, to the
number of perhaps a dozen in all. In the second a very different state
of things is exhibited. "The southern component," Professor Barnard
remarked, "which was the brightest on the 5th, had become diffused and
fainter, while the middle tail was very bright and broad. Its southern
side, which was the best defined, was wavy in numerous places, the tail
appearing as if disturbing currents were flowing at right angles to it.
At 42° from the head the tail made an abrupt bend towards the south, as
if its current was deflected by some obstacle. In the densest portion of
the tail, at the point of deflection, are a couple of dark holes,
similar to those seen in some of the nebulæ. The middle portion of the
tail is brighter, and looks like crumpled silk in places."[1353] Next
morning the southern was the prominent branch, and it was loaded, at 1°
42' from the head, with a strange excrescence, suggesting the
budding-out of a fresh comet in that incongruous situation.[1354] Some
of these changes, Professor Barnard thought, might possibly be explained
by a rotation of the tail on an axis passing through the nucleus, and
Pickering, who formed a similar opinion on independent grounds, assigned
about 94 hours as the period of the gyrating movement.[1355] He,
moreover, determined accelerative velocities outward from the sun of
definite condensations in the tail, indicating for its materials, on
Brédikhine's theory, a density less than one half that of
hydrogen.[1356] This conclusion applied also to Rordame's comet, which
exhibited a year later phenomena analogous to those remarked in Swift's.
Their photographic study led Professor Hussey[1357] to significant
inferences as to the structure and rapid changes of cometary appendages.
PLATE IV.
[Illustration:
Photographs of Swift's Comet.
By Professor E. E. Barnard.
No. 1. Taken April 4, 1892; exposure 1 h.
No. 2. Taken April 6, 1892; exposure 1 h. 5 m ]
Seven comets were detected in 1892, and all, strange to say, were
visible together towards the close of the year.[1358] Among them was a
faint object, which unexpectedly left a trail on a plate exposed by
Professor Barnard to the stars in Aquila[1359] on October 12. This was
the first comet actually discovered by photography, the Sohag comet
having been simultaneously seen and pictured. It has a period of about
six years. Holmes's comet is likewise periodical, in rather less than
seven years. Its path, which is wholly comprised between the orbits of
Mars and Jupiter, is less eccentric than that of any other known comet.
Subsequently to its discovery, on November 6, it underwent some curious
vicissitudes. At first bright and condensed, it expanded rapidly with
increasing distance from the sun (to which it had made its nearest
approach on June 13), until, by the middle of December, it was barely
discernible with powerful telescopes as "a feebly luminous mist on the
face of the sky."[1360] But on January 16, 1893, observers in Europe and
America were bewildered to find, as if substituted for it, a yellow star
of the seventh magnitude, enveloped in a thin nebulous husk, which
enclosed a faint miniature tail.[1361] This condensation and recovery of
light lasted in its full intensity only a couple of days. The almost
evanescent faintness of Holmes's comet at its next return accounted for
its invisibility previous to 1892, when it was evidently in a state of
peculiar excitement. Mr. Perrine was barely able, with the Lick 36-inch,
to find the vague nebulous patch which occupied its predicted place on
June 10, 1899.
The origin of comets has been long and eagerly inquired into, not
altogether apart from the cheering guidance of ascertained facts. Sir
William Herschel regarded them as fragments of nebulæ[1362]--scattered
débris of embryo worlds; and Laplace approved of and adopted the
idea.[1363] But there was a difficulty. No comet has yet been observed
to travel in a decided hyperbola. The typical cometary orbit, apart from
disturbance, is parabolic--that is to say, it is indistinguishable from
an enormously long ellipse. But this circumstance could only be
reconciled with the view that the bodies thus moving were casual
visitors from outer space, by making, as Laplace did, the tacit
assumption that the solar system was at rest. His reasoning was, indeed,
thereby completely vitiated, as Gauss pointed out in 1815;[1364] and the
objections then urged were reiterated by Schiaparelli,[1365] who
demonstrated in 1871 that a large preponderance of well-marked
hyperbolic orbits should result if comets were picked up _en route_ by a
swiftly-advancing sun. The fact that their native movement is
practically parabolic shows it to have been wholly imparted from
without. They passively obeyed the pull exerted upon them. In other
words, their condition previous to being attracted by the sun was one
very nearly of relative repose.[1366] They shared, accordingly, the
movement of translation through space of the solar system.
This significant conclusion had been indicated, on other grounds, as the
upshot of researches undertaken independently by Carrington[1367] and
Mohn[1368] in 1860, with a view to ascertaining the anticipated
existence of a relationship between the general _lie_ of the paths of
comets and the direction of the sun's journey. It is tolerably obvious
that if they wander at haphazard through interstellar regions their
apparitions should markedly aggregate towards the vicinity of the
constellation Lyra; that is to say, we should meet considerably more
comets than would overtake us, for the very same reason that falling
stars are more numerous after than before midnight. Moreover, the comets
met by us should be, apparently, swifter-moving objects than those
coming up with us from behind; because, in the one case, our own real
movement would be added to, in the other subtracted from, theirs. But
nothing of all this can be detected. Comets approach the sun
indifferently from all quarters, and with velocities quite independent
of direction.
We conclude, then, that the "cosmical current" which bears the solar
system towards its unknown goal carries also with it nebulous masses of
undefined extent, and at an undefined remoteness, fragments detached
from which, continually entering the sphere of the sun's attraction,
flit across our skies under the form of comets. These are, however,
almost certainly so far strangers to our system that they had no part in
the long processes of development by which its present condition was
attained. They are, perhaps, survivals of an earlier, and by us scarcely
and dimly conceivable state of things, when the swirling chaos from
which sun and planets were, by a supreme edict, to emerge, had not as
yet separately begun to be.
FOOTNOTES:
[Footnote 1267: _Astr. Nach._, Nos. 1,172-4.]
[Footnote 1268: _Berichte Sächs. Ges._, 1871, p. 174.]
[Footnote 1269: _Natur der Cometen_, p. 124; _Astr. Nach._, No. 2,086.]
[Footnote 1270: _Annales de l'Obs. de Moscou_, t. iii., pt. i., p. 37.]
[Footnote 1271: _Bull. Astr._, t. iii., p. 598. The value of the
repellent force for the comet of 1811 (which offered peculiar facilities
for its determination) was found = 17·5.]
[Footnote 1272: Faye, _Comptes Rendus_, t. xciii., p. 13.]
[Footnote 1273: _Annales_, t. v., pt. ii., p. 137.]
[Footnote 1274: _Am. Jour. of Sc._, vol. xxxii. (2nd ser.), p. 57.]
[Footnote 1275: _Astr. Nach._, No. 2,082.]
[Footnote 1276: _Annales de l'Obs. de Moscou_, t. vi., pt. i., p. 60.]
[Footnote 1277: _Astr. Register_, March, 1883.]
[Footnote 1278: _Astr. Nach._, No. 3,018.]
[Footnote 1279: _Ibid._, No. 3,093.]
[Footnote 1280: _Constitution de l'Espace Céleste_, p. 224.]
[Footnote 1281: _Astroph. Jour._, vol. iii., p. 36.]
[Footnote 1282: _Physikalische Zeitschrift_, November 10 and 17, 1900;
_Astroph. Jour._, vol. xiii., p. 344. _Cf._ Schwarzschild, _Sitzungsb._,
München, 1901, Heft iii.; J. Hahn, _Nature_, vols. lxv., p. 415; lxvi.,
p. 55.]
[Footnote 1283: _Astr. Nach._, No. 2,307.]
[Footnote 1284: _Ibid._, No. 2,304.]
[Footnote 1285: _Observatory_, vol. iii., p. 390.]
[Footnote 1286: _Astr. Nach._, No. 2,319.]
[Footnote 1287: _Ueber die Kometen von 371 v. Chr._, 1668, 1843, I. und
1880 I. Göttingen, 1880.]
[Footnote 1288: _Meteor._, lib. i., cap. 6.]
[Footnote 1289: _Mém. Soc. Phys. de Genève_, t. xxviii., p. 23.]
[Footnote 1290: _Annales de l'Obs. de Moscou_, t. vii., pt. i., p. 60.]
[Footnote 1291: Brédikhine, _Annales_, t. viii., p. 68.]
[Footnote 1292: _Am. Jour. of Sc._, vol. xxii., p. 305.]
[Footnote 1293: Messrs. Burton and Green observed a dilatation of the
stellar image into a nebulous patch by the transmission of its rays
through a nuclear jet of the comet. _Am. Jour. of Sc._, vol. xxii., p.
163.]
[Footnote 1294: _Archives des Sciences_, t. viii., p. 535. _Cf._
Perrine's negative results for Swift's comet in 1899, _Astr. Nach._, No.
3,602.]
[Footnote 1295: Riem concluded in 1896 for a definitive period of 2,429
years; _Observatory_, vol. xix., p. 282.]
[Footnote 1296: Holden, _Publ. Astr. Pac. Soc._, vol. ix., p. 89.]
[Footnote 1297: _Annuaire_, Paris, 1882, p. 781.]
[Footnote 1298: _Annuaire_, 1882, p. 766.]
[Footnote 1299: _Am. Jour. of Sc._, vol. xxii., p. 134.]
[Footnote 1300: _Report Brit. Assoc._, 1881, p. 520.]
[Footnote 1301: _Month. Not._, vol. xlii., p. 14; _Am. Jour. of Sc._,
vol. xxii., p. 136.]
[Footnote 1302: Piazzi Smyth, _Nature_, vol. xxiv., p. 430.]
[Footnote 1303: _Astr. Nach._, No. 2,395.]
[Footnote 1304: _Ibid._]
[Footnote 1305: _Astr. Nach._, No. 2,411.]
[Footnote 1306: _Month. Not._, vol. xlii., p. 49.]
[Footnote 1307: _Astr. Nach._, No. 2,414.]
[Footnote 1308: _Copernicus_, vol. ii., p. 229.]
[Footnote 1309: _Astr. Nach._, Nos. 2,434, 2,437.]
[Footnote 1310: _Ibid._, No. 2,441.]
[Footnote 1311: _Report Brit. Assoc._, 1882, p. 442.]
[Footnote 1312: J. J. Parsons, _Am. Jour. of Science_, vol. xxvii., p.
34.]
[Footnote 1313: _Astr. Nach._, No. 2,441.]
[Footnote 1314: _Observatory_, vol. v., p. 355. The transit had been
foreseen by Mr. Tebbutt, but it occurred after sunset in New South
Wales.]
[Footnote 1315: _Observatory_, vol. v., p. 354.]
[Footnote 1316: Gould, _Astr. Nach._, No. 2,481.]
[Footnote 1317: Flammarion, _Comptes Rendus_, t. xcv., p. 558.]
[Footnote 1318: Captain Ray's sextant observation of the comet of 1843,
a few hours before perihelion, was too rough to be of use.]
[Footnote 1319: _Astr. Nach._, No. 2,538.]
[Footnote 1320: _Nature_, vol. xxix., p. 135.]
[Footnote 1321: _Astr. Nach._, No. 2,482.]
[Footnote 1322: _Vierteljahrsschrift Astr. Ges._, Jahrg. xxiv., p. 308;
_Bull. Astr._, t. vii., p. 513.]
[Footnote 1323: _Observatory_, vol. xxiv., p. 167.]
[Footnote 1324: The attention of the author was kindly directed to this
point by Professor Young of Princeton (N. J.). _Cf._ Rebeur-Paschwitz,
_Sirius_, Bd. xvi., p. 233.]
[Footnote 1325: Oppenheim, _Astr. Nach._, No. 2,902.]
[Footnote 1326: _Astr. Nach._, No. 2,717.]
[Footnote 1327: Gruithuisen's _Analekten_, Heft 7, p. 48.]
[Footnote 1328: _Month. Not._, vols. xxv., xxvi., xxviii. _Cf._ Plummer,
_Observatory_, vol. xiii., p. 263.]
[Footnote 1329: _Nature_, vol. xxvii., p. 246.]
[Footnote 1330: _Astr. Nach._, No. 2,468.]
[Footnote 1331: _Athenæum_, February 3, 1883.]
[Footnote 1332: _Astr. Nach._, Nos. 2,462, 2,466.]
[Footnote 1333: _Ibid._, No. 2,489.]
[Footnote 1334: _Annales_, Moscow, t. ix., pt. ii., p. 52.]
[Footnote 1335: _Comptes Rendus_, t. xcvii., p. 797.]
[Footnote 1336: _Astr. Nach._, No. 2,966.]
[Footnote 1337: _Copernicus_, vol. ii., p. 235.]
[Footnote 1338: _Comptes Rendus_, t. xcvi., p. 371.]
[Footnote 1339: _Astr. Nach._, No. 2,553.]
[Footnote 1340: _Ibid._]
[Footnote 1341: _Astr. Nach._, No. 2,568.]
[Footnote 1342: _Annales de l'Observatoire de Nice_, t. ii., c. 53.]
[Footnote 1343: Fényi, _Astr. Nach._, No. 2,844; Kammermann, _Ibid._,
No. 2,849.]
[Footnote 1344: _Publ. Astr. Pac. Soc._, vol. i., p. 72.]
[Footnote 1345: _Annuaire_, Paris, 1891, p. 301.]
[Footnote 1346: _Astr. Nach._, No. 2,989.]
[Footnote 1347: _Comptes Rendus_, t. xxv., p. 564.]
[Footnote 1348: _Astr. Journ._, Nos. 205, 231.]
[Footnote 1349: _Ibid._, Nos. 228, 244, 380.]
[Footnote 1350: _Observatory_, vol. xviii., pp. 60, 163 (Denning and
Lynn).]
[Footnote 1351: _Astr. Nach._, No. 3,267; Plummer, _Knowledge_, vol.
xix., p. 156.]
[Footnote 1352: _Astr. Jour._, Nos. 333, 338.]
[Footnote 1353: _Astr. and Astroph._, vol. xi., p. 387.]
[Footnote 1354: _Knowledge_, vol. xv., p. 299.]
[Footnote 1355: _Harvard Annals_, vol. xxxii., pt. ii., p. 272.]
[Footnote 1356: _Ibid._, p. 287.]
[Footnote 1357: _Publ. Astr. Pac. Soc._, vol. vii., p. 161.]
[Footnote 1358: H. C. Wilson, _Astr. and Astroph._, vol. xii., p. 121.]
[Footnote 1359: _Observatory_, vol. xvi., p. 92.]
[Footnote 1360: Barnard, _Astr. and Astroph._, vol. xii., p. 180;
_Astroph. Jour._, vol. iii., p. 41.]
[Footnote 1361: Palisa, _Astr. Nach._, No. 3,147; Denning,
_Observatory_, vol. xvi., p. 142.]
[Footnote 1362: _Phil. Trans._, vol. ci., p. 306.]
[Footnote 1363: _Conn. des Temps_, 1816, p. 213.]
[Footnote 1364: _OEuvres_, t. vi., p. 581.]
[Footnote 1365: _Mem. dell' Istit. Lombardo_, t. xii., p. 164;
_Rendiconti_, t. vii., p. 77, 1874.]
[Footnote 1366: W. Förster, _Pop. Mitth._, 1879, p. 7; Fabry, _Étude sur
la Probabilité des Comètes Hyperboliques_, Marseille, 1893, p. 158.]
[Footnote 1367: _Mem. R. A. Soc._, vol. xxix., p. 335.]
[Footnote 1368: _Month. Not._, vol. xxiii., p. 203.]
CHAPTER XII
_STARS AND NEBULÆ_
That a science of stellar chemistry should not only have become
possible, but should already have made material advances, is assuredly
one of the most amazing features in the swift progress of knowledge our
age has witnessed. Custom can never blunt the wonder with which we must
regard the achievement of compelling rays emanating from a source devoid
of sensible magnitude through immeasurable distance, to reveal, by its
distinctive qualities, the composition of that source. The discovery of
revolving double stars assured us that the great governing force of the
planetary movements, and of our own material existence, sways equally
the courses of the farthest suns in space; the application of prismatic
analysis certified to the presence in the stars of the familiar
materials, no less of the earth we tread, than of the human bodies built
up out of its dust and circumambient vapours.
We have seen that, as early as 1823, Fraunhofer ascertained the generic
participation of stellar light in the peculiarity by which sunlight,
spread out by transmission through a prism, shows numerous transverse
rulings of interrupting darkness. No sooner had Kirchhoff supplied the
key to the hidden meaning of those ciphered characters than it was
eagerly turned to the interpretation of the dim scrolls unfolded in the
spectra of the stars. Donati made at Florence in 1860 the first efforts
in this direction; but with little result, owing to the imperfections of
the instrumental means at his command. His comparative failure, however,
was a prelude to others' success. Almost simultaneously, in 1862, the
novel line of investigation was entered upon by Huggins near London, by
Father Secchi at Rome, and by Lewis M. Rutherfurd in New York.
Fraunhofer's device of using a cylindrical lens for the purpose of
giving a second dimension to stellar spectra was adopted by all, and
was, indeed, indispensable. For a luminous point, such as a star
appears, becomes, when viewed through a prism, a variegated line, which,
until broadened into a band by the intervention of a cylindrical lens,
is all but useless for purposes of research. This process of _rolling
out_ involves, it is true, much loss of light--a scanty and precious
commodity, as coming from the stars; but the loss is an inevitable one.
And so fully is it compensated by the great light-grasping power of
modern telescopes that important information can now be gained from the
spectroscopic examination of stars far below the range of the unarmed
eye.
The effective founders of stellar spectroscopy, then (since Rutherfurd
shortly turned his efforts elsewhither), were Father Secchi, the eminent
Jesuit astronomer of the Collegio Romano, where he died, February 26,
1878, and Sir William Huggins, with whom the late Professor W. A. Miller
was associated. The work of each was happily directed so as to
supplement that of the other. With less perfect appliances, the Roman
astronomer sought to render his extensive rather than precise; at Tulse
Hill searching accuracy over a narrow range was aimed at and attained.
To Father Secchi is due the merit of having executed the first
spectroscopic survey of the heavens. Above 4,000 stars were passed in
review by him, and classified according to the varying qualities of
their light. His provisional establishment (1863-67) of four types of
stellar spectra[1369] has proved a genuine aid to knowledge through the
facilities afforded by it for the arrangement and comparison of rapidly
accumulating facts. Moreover, it is scarcely doubtful that these
spectral distinctions correspond to differences in physical condition of
a marked kind.
The first order comprises more than half the visible and probably an
overwhelming proportion of the faintest stars. Sirius, Vega, Regulus,
Altair, are amongst its leading members. Their spectra are distinguished
by the breadth and intensity of the four dark bars due to the absorption
of hydrogen, and by the extreme faintness of the metallic lines, of
which, nevertheless, hundreds are disclosed by careful examination. The
light of these "Sirian" orbs is white or bluish; and it is found to be
rich in ultra-violet rays.
Capella and Arcturus belong to the second, or solar type of stars, which
is about one-sixth less numerously represented than the first. Their
spectra are quite closely similar to that of sunlight, in being ruled
throughout by innumerable fine dark lines; and they share its yellowish
tinge.
The third class includes most red and variable stars (commonly
synonymous), of which Betelgeux in the shoulder of Orion, and "Mira" in
the Whale, are noted examples. Their characteristic spectrum is of the
"fluted" description. It shows like a strongly illuminated range of
seven or eight variously tinted columns seen in perspective, the light
falling from the red end towards the violet. This _kind_ of absorption
is produced by the vapours of metalloids or of compound substances.
To the fourth order of stars belongs also a colonnaded spectrum, but
_reversed_; the light is thrown the other way. The three broad zones of
absorption which interrupt it are sharp towards the red, insensibly
gradated towards the violet end. The individuals composing Class IV. are
few and apparently insignificant, the brightest of them not exceeding
the fifth magnitude. They are commonly distinguished by a deep red tint,
and gleam like rubies in the field of the telescope. Father Secchi, who
in 1867 detected the peculiarity of their analyzed light, ascribed it to
the presence of carbon in some form in their atmospheres; and this was
confirmed by the researches of H. C. Vogel,[1370] director of the
Astro-physical Observatory at Potsdam. The hydro-carbon bands, in fact,
seen bright in comets, are dark in these singular objects--the only ones
in the heavens (save one bright-line star and a rare meteor)[1371] which
display a cometary analogy of the fundamental sort revealed by the
spectroscope.
The members of all four orders are, however, emphatically suns. They
possess, it would appear, photospheres radiating all kinds of light, and
differ from each other mainly in the varying qualities of their
absorptive atmospheres. The principle that the colours of stars depend,
not on the intrinsic nature of their light, but on the kinds of vapours
surrounding them, and stopping out certain portions of that light, was
laid down by Huggins in 1864.[1372] The phenomena of double stars seem
to indicate a connection between the state of the investing atmospheres,
by the action of which their often brilliantly contrasted tints are
produced, and their mutual physical relations. A tabular statement put
forward by Professor Holden in June, 1880,[1373] made it, at any rate,
clear that inequality of magnitude between the components of binary
systems accompanies unlikeness in colour, and that stars more equally
matched in one respect are pretty sure to be so in the other. Besides,
blue and green stars of a decided tinge are never solitary; they
invariably form part of systems. So that association has undoubtedly a
predominant influence upon colour.
Nevertheless, the crude notion thrown out by Zöllner in 1865,[1374] that
yellow and red stars are simply white stars in various stages of
cooling, obtained for a time undeserved currency. D'Arrest, indeed,
protested against it, and Ångström, in 1868,[1375] substituted
atmospheric quality for mere colour[1376] as a criterion of age and
temperature. His lead was followed by Lockyer in 1873,[1377] and by
Vogel in 1874.[1378] The scheme of classification due to the Potsdam
astro-physicist differed from Father Secchi's only in presenting his
third and fourth types as subdivisions of the same order, and in
inserting three subordinate categories; but their variety was
"rationalised" by the addition of the seductive idea of progressive
development. Thus, the white Sirian stars were represented as the
_youngest_ because the hottest of the sidereal family; those of the
solar pattern as having already wasted much of their store by radiation,
and being well advanced in middle life; while the red stars with banded
spectra figured as effete suns, hastening rapidly down the road to final
extinction.
Vogel's scheme is, however, incomplete. It traces the downward curve of
decay, but gives no account of the slow ascent to maturity. The present
splendour of Vega, for instance, was prepared, according to all creative
analogy, by almost endless processes of gradual change. What was its
antecedent condition? The question has been variously answered. Dr.
Johnstone Stoney advocated, in 1867, the comparative youth of red
stars;[1379] A. Ritter, of Aix-la-Chapelle, divided them, in 1883,[1380]
into two squadrons, posted, the one on the ascending, the other on the
descending branch of the temperature-curve, and corresponding,
presumably, with Secchi's third and fourth orders of stars with banded
spectra. Whether, in the interim, they should display spectra of the
Sirian or of the solar type was made to depend on their greater or less
massiveness.[1381] But the relation actually existing perhaps inverts
that contemplated by Ritter. Certainly, the evidence collected by Mr.
Maunder in 1891 strongly supports the opinion that the average solar
star is a weightier body than the average Sirian star.[1382]
On November 17, 1887, Sir Norman Lockyer communicated to the Royal
Society the first of a series of papers embodying his "Meteoritic
Hypothesis" of cosmical constitution, stated and supported more at large
in a separate work bearing that name, published in 1890. The fundamental
proposition wrought out in it was that "all self-luminous bodies in the
celestial space are composed either of swarms of meteorites or of masses
of meteoric vapour produced by heat."[1383] On the basis of this
supposed community of origin, sidereal objects were distributed in seven
groups along a temperature-curve ascending from nebulæ and gaseous, or
bright-line stars, through red stars of the third type, and a younger
division of solar stars, to the high Sirian level; then descending
through the more strictly solar stars to red stars of the fourth type
("carbon-stars"), below which lay only the _caput mortuum_ entitled
Group vii. The ground-work of this classification was, however,
insecure, and has given way. Certain spectroscopic coincidences,
avowedly only approximate, suggesting that stars and nebulæ of every
species might be formed out of variously aggregated meteorites, failed
of verification by exact inquiry. And spectroscopic coincidences admit
of no compromise. Those that are merely approximate are, as a rule,
unmeaning.
In his Presidential Address at the Cardiff Meeting of the British
Association in 1891, Dr. Huggins adhered in the main to the line of
advance traced by Vogel. The inconspicuousness of metallic lines in the
spectra of the white stars he attributed, not to the paucity, but to the
high temperature of the vapours producing them, and the consequent
deficiency of contrast between their absorption-rays and the continuous
light of the photospheric background. "Such a state of things would more
probably," in his opinion, "be found in conditions anterior to the solar
stage," while "a considerable cooling of the sun would probably give
rise to banded spectra due to compounds." He adverted also to the
influential effects upon stellar types of varying surface gravity, which
being a function of both mass and bulk necessarily gains strength with
wasting heat and consequent shrinkage. The same leading ideas were more
fully worked out in "An Atlas of Representative Stellar Spectra,"
published by Sir William and Lady Huggins in 1899. They were, moreover,
splendidly illustrated by a set of original spectrographic plates, while
precision was added to the adopted classification by the separation of
helium from hydrogen stars. The spectrum of the exotic substance
terrestrially captured in 1895 is conspicuous by absorption, as Vogel,
Lockyer, and Deslandres promptly recognised in a considerable number of
white stars, among them the Pleiades and most of the brilliants in
Orion. Mr. McClean, whose valuable spectrographic survey of the heavens
was completed at the Cape in 1897, found reason to conclude that they
are in the first stage of development from gaseous nebulæ;[1384] and in
this the Tulse Hill investigators unhesitatingly concur.
The strongest evidence for the primitive state of white stars is found
in their nebular relations. The components of groups, still involved and
entangled with "silver braids" of cosmic mist, show, perhaps invariably,
spectra of the helium type, occasionally crossed by bright rays.
Possibly all such stars have passed through a bright-line stage; but
further evidence on the point is needed. Relative density furnishes
another important test of comparative age, and Sirian stars are, on the
whole, undoubtedly more bulky proportionately to their mass than solar
stars. The rule, however, seems to admit of exceptions; hence the change
from one kind of spectrum to the other is not inevitably connected with
the attainment of a particular degree of condensation. There is reason
to believe that it is anticipated in the more massive globes, despite
their comparatively slow cooling, as a consequence of the greater power
of gravity over their investing vaporous envelopes. This conclusion is
enforced by the relations of double-star spectra. The fact that, in
unequal pairs, the chief star most frequently shows a solar, its
companion a Sirian, spectrum can scarcely be otherwise explained than by
admitting that, while the sequence of types is pursued in an invariable
order, it is pursued much more rapidly in larger than in small orbs. It
need not, indeed, be supposed that all stars are identical in
constitution, and present identical life-histories.[1385]
Individualities in the one, and divergencies in the other, must be
allowed for. Yet the main track is plainly continuous, and leads by
insensible gradations from nebulæ through helium stars to the Sirian,
and onward to the solar type, whence, by an inevitable transition,
fluted, or "Antarian,"[1386] spectra develop.
The first-known examples of the class of gaseous stars--Beta Lyræ
and Gamma Cassiopeiæ--were noticed by Father Secchi at the outset
of his spectroscopic inquiries. Both show _bright_ lines of hydrogen and
helium, so that the peculiarity of their condition probably consists in
the intense ignition of their chromospheric surroundings. Their entire
radiating surfaces might be described as _faculous_. That is to say,
brilliant formations, such as have been photographed by Professor Hale
on the sun's disc,[1387] cover, perhaps, the whole, instead of being
limited to a small portion of the photospheric area. But this state of
things is more or less inconstant. Some at least of the bright rays
indicative of it are subject to temporary extinctions. Already in
1871-72, Dr. Vogel[1388] suspected the prevalence of such vicissitudes;
and their reality was ascertained by M. Eugen von Gothard. After the
completion of his new astrophysical observatory at Herény in the autumn
of 1881, he repeatedly observed the spectra of both stars without
perceiving a trace of bright lines; and was thus taken quite by surprise
when he caught a twinkling of the crimson C in Gamma Cassiopeiæ,
August 13, 1883.[1389] A few days later, the whole range including D_3
was lustrous. Duly apprised of the recurrence of a phenomenon he had
himself vainly looked for during some years, M. von Konkoly took the
opportunity of the great Vienna refractor being placed at his disposal
to examine with it the relighted spectrum on August 27.[1390] In its
wealth of light C was dazzling; D_3 and the green and blue hydrogen rays
shone somewhat less vividly; D and the group _b_ showed faintly dark;
while three broad absorption-bands, sharply terminated towards the red,
diffuse towards the violet, shaded the spectrum near its opposite
extremities.
The previous absence of bright lines from the spectrum of this star was,
however, by no means so protracted or complete as M. von Gothard
supposed. At Dunecht, C was "superbly visible" December 20, 1879[1391];
F was seen bright on October 28 of the same year, and frequently at
Greenwich in 1880-81. The curious fact has, moreover, been adverted to
by Dr. Copeland, that C _is much more variable than F_. To Vogel, June
18, 1872, the first was invisible, while the second was bright; at
Dunecht, January 11, 1887, the conditions were so far inverted that C
was resplendent, F comparatively dim.
No spectral fluctuations were detected in Gamma Cassiopeiæ by
Keeler in 1889; but even with the giant telescope of Mount Hamilton, the
helium-ray was completely invisible.[1392] It made, nevertheless,
capricious appearances at South Kensington during that autumn, and again
October 21, 1894,[1393] while in September, 1892, Bélopolsky could
obtain no trace of it on orthochromatic plates exposed with the 30-inch
Pulkowa refractor.[1394] Still more noteworthy is the circumstance that
the well-known green triplet of magnesium (_b_), recorded as dark by
Keeler in 1889, came out bright on fifty-two spectrographs of the star
taken by Father Sidgreaves during the years 1891-99.[1395] No
fluctuations in the hydrogen-spectrum were betrayed by them; but
subordinate lines of unknown origin showed alternate fading and
vivification.
The spectrum of Beta Lyræ undergoes transitions to some extent
analogous, yet involving a different set of considerations. First
noticed by Von Gothard in 1882,[1396] they were imperfectly made out,
two years later, to be of a cyclical character.[1397] This, however,
could only be effectively determined by photographic means. Beta Lyræ is
a "short-period variable." Its light changes with great regularity from
3·4 to 4·4 magnitude every twelve days and twenty-two hours, during
which time it attains a twofold maximum, with an intervening secondary
minimum. The question, then, is of singular interest, whether the
changes of spectral quality visible in this object correspond to its
changes in visual brightness. A distinct answer in the affirmative was
supplied through Mrs. Fleming's examination of the Harvard plates of the
star's spectrum, upon which, in 1891, she found recorded diverse complex
changes of bright and dark lines obviously connected with the phases of
luminous variation, and obeying, in the long-run, precisely the same
period.[1398] Something more will be said presently as to the import of
this discovery.
Bright hydrogen lines have so far been detected--for the most part
photographically at Harvard College--in about sixty stars, including
Pleione, the surmised lost Pleiad, P Cygni, noted for instability of
light in the seventeenth century, and the extraordinary southern
variable, Eta Carinæ. In most of these objects other vivid rays
are associated with those due to hydrogen. A blaze of hydrogen,
moreover, accompanies the recurring outbursts of about one hundred and
fifty "long-period variables," giving banded spectra of the third type.
Professor Pickering discovered the first example of this class, towards
the close of 1886, in Mira Ceti; further detections were made visually
by Mr. Espin; and the conjunction of bright hydrogen-lines with dusky
bands has been proved by Mrs. Fleming's long experience in studying the
Harvard photographs, to indicate unerringly the subjection of the stars
thus characterised to variations of lustre accomplished in some months.
A third variety of gaseous star is named after MM. Wolf and Rayet, who
discovered, at Paris in 1867,[1399] its three typical representatives,
close together in the constellation Cygnus. Six further specimens were
discovered by Dr. Copeland, five of them in the course of a trip for the
exploration of visual facilities in the Andes in 1883;[1400] and a large
number have been made known through spectral photographs taken in both
hemispheres under Professor Pickering's direction. At the close of the
nineteenth century, over a hundred such objects had been registered,
none brighter than the sixth magnitude, with the single exception of
Gamma Argûs, the resplendent continuous spectrum of which, first
examined by Respighi and Lockyer in 1871, is embellished with the yellow
and blue rays distinctive of the type.[1401] Here, then, we have a
stellar globe apparently at the highest point of sunlike incandescence,
sharing the peculiarities of bodies verging towards the nebulous state.
Examined with instruments of adequate power, their spectra are seen to
be highly complex. They include a fairly strong continuous element, a
numerous set of absorption-lines, and a range of emission-lines, more or
less completely represented in different stars. Especially conspicuous
is a broad effluence of azure light, found by Dr. Vogel in 1883,[1402]
and by Sir William and Lady Huggins in 1890,[1403] to be of multiple
structure, and hence to vary in its mode of display. Its suggested
identification with the blue carbon-fluting was disproved at Tulse Hill.
Metallic vapours give no certain sign of their presence in the
atmospheres of these remarkable bodies; but nebulum is stated to shine
in some.[1404] Hydrogen and helium account for a large proportion of
their spectral rays. Thirty-two Wolf-Rayet stars were investigated,
spectroscopically and spectrographically, by Professor Campbell with the
great Lick refractor in 1892-94;[1405] and several disclosed the
singularity, already noticed by him in Gamma Argûs, of giving out
mixed series, the members of which change from vivid to obscure with
increase of refrangibility. It is difficult to imagine by what
chromospheric machinery this curious result can be produced. Alcyone in
the Pleiades presents the same characteristic. Alone among the hydrogen
lines, crimson C glows in its spectrum, while all the others are dark.
Luminosity of the Wolf-Rayet kind is particularly constant, both in
quantity and quality. It seems to be incapable of developing save under
galactic conditions. All the stars marked by it lie near the central
line of the Milky Way, or in the Magellanic Clouds. They tend also to
gather into groups. Circles of four degrees radius include respectively
seven in Argo, eight in Cygnus.
The first spectroscopic star catalogue was published by Dr. Vogel at
Potsdam in 1883.[1406] It included 4,051 stars, distributed over a zone
of the heavens extending from 20° north to 20° south of the celestial
equator.[1407] More than half of these were white stars, while red stars
with banded spectra occurred in the proportion of about one-thirteenth
of the whole. To the latter genus, M. Dunér, then of Lund, now Director
of the Upsala Observatory, devoted a work of standard authority, issued
at Stockholm in 1884. This was a catalogue with descriptive particulars
of 352 stars showing banded spectra, 297 of which belong to Secchi's
third, 55 to his fourth class (Vogel's iii. _a_ and iii. _b_). Since
then discovery has progressed so rapidly, at first through the
telescopic reviews of Mr. Espin, then in the course of the photographic
survey carried on at Harvard College, that considerably over one
thousand stars are at present recognised as of the family of Betelgeux
and Mira, while about 250 have so far exhibited the spectral pattern of
19 Piscium. One fact well ascertained as regards both species is the
invariability of the type. The prismatic flutings of the one, and the
broader zones of the other, are as if stereotyped--they undergo, in
their fundamental outlines, no modification, though varying in relative
intensity from star to star. They are always accompanied by, or
superposed upon, a spectrum of dark lines, in producing which sodium and
iron have an obvious share; and certain bright rays, noticed by Secchi
with imperfect appliances as enhancing the chiaroscuro effects in
carbon-stars, came out upon plates exposed by Hale and Ellerman in 1898
with the stellar spectrograph of the Yerkes Observatory.[1408] Their
genuineness was shortly afterwards visually attested by Keeler,
Campbell, and Dunér;[1409] but no chemical interpretation has been found
for them.
A fairly complete preliminary answer to the question, What are the stars
made of? was given by Sir William Huggins in 1864.[1410] By laborious
processes of comparison between stellar dark lines and the bright rays
emitted by terrestrial substances, he sought to assure his conclusions,
regardless of cost in time and pains. He averred, indeed, that--taking
into account restrictions by weather and position--the thorough
investigation of a _single_ star-spectrum would be the work of some
years. Of two, however--those of Betelgeux and Aldebaran--he was able to
furnish detailed and accurate drawings. The dusky flutings in the
prismatic light of the first of these stars have not been identified
with the absorption of any particular substance; but associated with
them are metallic lines, of which 78 were measured, and a good many
identified by Huggins, while the wave-lengths of 97 were determined by
Vogel in 1871.[1411] A photographic research, made by Keeler at the
Alleghany Observatory in 1897, convinced him that the linear spectrum of
third-type stars of the Betelgeux pattern essentially repeats that of
the sun, but with marked differences in the comparative strength of its
components.[1412] Hydrogen rays are inconspicuously present. That an
exalted temperature reigns, at least in the lower strata of the
atmosphere, is certified by the vaporisation there of matter so
refractory to heat as iron.[1413]
Nine elements--among them iron, sodium, calcium, and magnesium--were
recognised by Huggins as having stamped their signature on the spectrum
of Aldebaran; while the existence in Sirius, and nearly all the other
stars inspected, of hydrogen, together with sundry metals, was rendered
certain or highly probable. This was admitted to be a bare gleaning of
results; nor is there reason to suppose any of his congeners inferior to
our sun in complexity of constitution. Definite knowledge on the
subject, however, made little advance beyond the point to which it was
brought by Huggins's early experiments until spectroscopic photography
became thoroughly effective as a means of research.
In this, as in so many other directions, Sir William Huggins acted as
pioneer. In March, 1863, he obtained microscopic prints of the spectra
of Sirius and Capella.[1414] But they told nothing. No lines were
visible in them. They were mere characterless streaks of light. Nine
years later Dr. Henry Draper of New York got an impression of four lines
in the spectrum of Vega. Then Huggins attacked the subject again in
1876, when the 18-inch speculum of the Royal Society had come into his
possession, using prisms of Iceland spar and lenses of rock crystal; and
this time with better success. A photograph of the spectrum of Vega
showed seven strong lines.[1415] Still he was not satisfied. He waited
and worked for three years longer. At length, on December 18, 1879, he
was able to communicate to the Royal Society[1416] results answering to
his expectations. The delicacy of eye and hand needed to obtain them may
be estimated from the single fact that the image of a star had to be
kept, by continual minute adjustments, exactly projected upon a slit
1/350 of an inch in width during nearly an hour, in order to give it
time to imprint the characters of its analyzed light upon a gelatine
plate raised to the highest pitch of sensitiveness. But by this time he
had secured in his wife a rarely qualified assistant.
The ultra-violet spectrum of the white stars--of which Vega was taken as
the type--was thus shown to be a very remarkable one. A group of broad
dark lines intersected it, arranged at intervals diminishing regularly
upward, and falling into a rhythmical succession with the visible
hydrogen lines. All belonged presumably to the same substance; and the
presumption was rendered a certainty by direct photographs of the
hydrogen spectrum taken by H. W. Vogel at Berlin a few months
earlier.[1417] In them seven of the white-star series of grouped lines
were visible; and the full complement of twelve appeared on Cornu's
plates in 1886.[1418]
In yellow stars, such as Capella and Arcturus, the same rhythmical
series was _partially_ represented, but associated with a great number
of other lines; their state, as regards ultra-violet absorption,
approximating to that of the sun; while the redder stars betrayed so
marked a deficiency in actinic rays that from Betelgeux, with an
exposure _forty times_ that required for Sirius, only a faint spectral
impression could be obtained, and from Aldebaran, in the strictly
invisible region, almost none at all.
Thus, by the means of stellar light-analysis, acquaintance was first
made with the ultra-violet spectrum of hydrogen;[1419] and its harmonic
character, as expressed by "Balmer's Law," supplies a sure test for
discriminating, among newly discovered lines, those that appertain from
those that are unrelated to it. Deslandres' five additional
prominence-rays, for instance, were at once seen to make part of the
series, because conforming to its law;[1420] while a group of six dusky
bands, photographed by Sir William and Lady Huggins, April 4,
1890,[1421] near the extreme upper end of the spectrum of Sirius, were
pronounced without hesitation, for the opposite reason, to have nothing
to do with hydrogen. Their true affinities are still a matter for
inquiry.
As regards the hydrogen spectrum, however, the stars had further
information in reserve. Until recently, it was supposed to consist of a
single harmonic series, although, by analogy, three should co-exist. In
1896, accordingly, a second, bound to the first by unmistakable
numerical relationships, was recognised by Professor Pickering in
spectrographs of the 2·5 magnitude star Zeta Puppis,[1422] and the
identification was shortly afterwards extended to prominent Wolf-Rayet
emission lines. The discovery was capped by Dr. Rydberg's indication of
the Wolf-Rayet blue band at Lambda 4,688 as the fundamental member
of the third, and principal, hydrogen series.[1423] None of the
"Pickering lines" (as they may be called to distinguish them from the
"Huggins series") can be induced to glimmer in vacuum-tubes. They seem
to characterise bodies in a primitive state,[1424] and are in many cases
associated with absorption rays of oxygen, the identification of which
by Mr. McClean in 1897[1425] was fully confirmed by Sir David
Gill.[1426] The typical "oxygen star" is Beta Crucis, one of the
brilliants of the Southern Cross; but the distinctive notes of its
spectrum occur in not a few specimens of the helium class. Thus, Sir
William and Lady Huggins photographed several ultra-violet oxygen lines
in Beta Lyræ,[1427] and found in Rigel signs of the presence of
nitrogen,[1428] which, as well as silicium, proves to be a tolerably
frequent constituent of such orbs.[1429] For some unknown reason,
metalloids tend to become effaced, as metals, in the normal course of
stellar development, exert a more and more conspicuous action.
Dr. Scheiner's spectrographic researches at Potsdam in 1890 and
subsequently, exemplify the immense advantages of self-registration. In
a restricted section of the spectrum of Capella, he was enabled to
determine nearly three hundred lines with more precision than had then
been attained in the measurement of terrestrial spectra. This star
appeared to be virtually identical with the sun in physical
constitution, although it emits, according to the best available data,
about 140 times as much light, and is hence presumably 1,600 times more
voluminous. An equally close examination of the spectrum of Betelgeux
showed the predominance in it of the linear absorption of iron;[1430]
but its characteristic flutings do not extend to the photographic
region. Spectra of the second and third orders are for this reason not
easily distinguished on the sensitive plate.
A spectrographic investigation of all the brighter northern stars was
set on foot in 1886 at the observatory of Harvard College, under the
form of a memorial to Dr. H. Draper, whose promising work in that line
was brought to a close by his premature death in 1882. No individual
exertions could, however, have realized a tithe of what has been and is
being accomplished under Professor Pickering's able direction, with the
aid of the Draper and other instruments, supplemented by Mrs. Draper's
liberal provision of funds. A novel system was adopted, or, rather, an
old one--originally used by Fraunhofer--was revived.[1431] The use of a
slit was discarded as unnecessary for objects like the stars, devoid of
sensible dimensions, and giving hence a _naturally_ pure spectrum; and a
large prism, placed in front of the object-glass, analysed at once, with
slight loss of light, the rays of all the stars in the field. Their
spectra were taken, as it were, wholesale. As many as two hundred stars
down to the eighth magnitude were occasionally printed on a single plate
with a single exposure. No cylindrical lens was employed. The movement
of the stars themselves was turned to account for giving the desirable
width to their spectra. The star was allowed--by disconnecting or
suitably regulating the clock--to travel slowly across the line of its
own dispersed light, so broadening it gradually into a band. Excellent
results were secured in this way. About fifty lines appear in the
photographed spectrum of Aldebaran, and eight in that of Vega. On
January 26, 1886, with an exposure of thirty-four minutes, a
simultaneous impression was obtained of the spectra (among many others)
of close upon forty Pleiades. With few and doubtful exceptions, they all
proved to belong to the same type. An additional argument for the common
origin of the stars forming this beautiful group was thus
provided.[1432]
The "Draper Catalogue" of stellar spectra was published in 1890.[1433]
It gives the results of a rapid analytical survey of the heavens north
of 25° of southern declination, and includes 10,351 stars, down to about
the eighth magnitude. The telescope used was of eight inches aperture
and forty-five focus, its field of view--owing to the "portrait-lens" or
"doublet" form given to it--embracing with fair definition no less than
one hundred square degrees. An objective prism eight inches square was
attached, and exposures of a few minutes were given to the most
sensitive plates that could be procured. In this way the sky was twice
covered in duplicate, each star appearing, as a rule, on four plates.
The registration of their spectra was sought to be made more distinctive
than had previously been attempted, Secchi's first type being divided
into four, his second into five subdivisions; but the differences
regarded in them could be confidently established only for stars above
the sixth magnitude. The work supplies none the less valuable materials
for general inferences as to the distribution and relations of the
spectral types. The labour of its actual preparation was borne by a
staff of ladies under the direction of Mrs. Fleming. Materials for its
completion to the southern pole have been accumulated with the identical
instrument used at Cambridge, transferred for the purpose in 1889 to
Peru, and the forthcoming "Second Draper Catalogue" will comprise 30,000
stars in both hemispheres. As supplements to this great enterprise, two
important detailed discussions of stellar spectra were issued in 1897
and 1901 respectively.[1434] The first, by Miss A. C. Maury, dealt with
681 bright stars visible in the northern hemisphere; the second, by Miss
A. J. Cannon, with 1,122 southern stars. Both authors traced, with care
and ability, the minute gradations by which the long process of stellar
evolution appears to be accomplished.
The progress of the Draper Memorial researches was marked by discoveries
of an unexampled kind.
The principle upon which "motion in the line of sight" can be detected
and measured with the spectroscope has already been explained.[1435] It
depends, as our readers will remember, upon the removal of certain
lines, dark or bright (it matters not which), from their normal places
by almost infinitesimal amounts. The whole spectrum of the moving
object, in fact, is very slightly _shoved_ hither or thither, according
as it is travelling towards or from the eye; but, for convenience of
measurement, one line is usually picked out from the rest, and attention
concentrated upon it. The application of this method to the stars,
however, is encompassed with difficulties. It needs a powerfully
dispersive spectroscope to show line-displacements of the minute order
in question; and powerful dispersion involves a strictly proportionate
enfeeblement of light. This, where the supply is already to a deplorable
extent niggardly, can ill be afforded; for which reason the operation of
determining a star's approach or recession is, even apart from
atmospheric obstacles, an excessively delicate one.
It was first executed by Sir William Huggins early in 1868.[1436]
Selecting the brightest star in the heavens as the most promising
subject of experiment, he considered the F line in the spectrum of
Sirius to be just so much displaced towards the red as to indicate (the
orbital motion of the earth being deducted) recession at the rate of
twenty-nine miles a second; and the reality and direction of the
movement were ratified by Vogel and Lohse's observation, March 22, 1871,
of a similar, but even more considerable displacement.[1437] The inquiry
was resumed by Huggins with improved apparatus in the following year,
when the velocities of thirty stars were approximately determined.[1438]
The retreat of Sirius, which proved slower than had at first been
supposed, was now announced to be shared, at rates varying from twelve
to twenty-nine miles, by Betelgeux, Rigel, Castor, Regulus, and five of
the principal stars in the Plough. Arcturus, on the contrary, gave signs
of rapid approach, as well as Pollux, Vega, Deneb in the Swan, and the
brightness of the Pointers.
Numerically, indeed, these results were encompassed with uncertainty.
Thus, Arcturus is now fully ascertained to be travelling towards the sun
at the comparatively slow pace of less than five miles a second; and
Sirius moves twice as fast in the same direction. The great difficulty
of measuring so distended a line as the Sirian F might, indeed, well
account for some apparent anomalies. The scope of Sir William Huggins's
achievement was not, however, to provide definitive data, but to
establish as practicable the method of procuring them. In this he was
thoroughly successful, and his success was of incalculable value.
Spectroscopic investigations of stellar movements may confidently be
expected to play a leading part in the unravelment of the vast and
complex relations which we can dimly detect as prevailing among the
innumerable orbs of the sidereal world; for it supplements the means
which we possess of measuring by direct observation movements transverse
to the line of sight, and thus completes our knowledge of the courses
and velocities of stars at ascertained distances, while supplying for
all a valuable index to the amount of perspective foreshortening of
apparent movement. Thus some, even if an imperfect, knowledge may at
length be gained of the revolutions of the stars--of the systems they
unite to form, of the paths they respectively pursue, and of the forces
under the compulsion of which they travel.
The applicability of the method to determining the orbital motions of
double stars was pointed out by Fox Talbot in 1871;[1439] but its use
for their discovery revealed itself spontaneously through the Harvard
College photographs. In "spectrograms" of Zeta Ursæ Majoris
(Mizar), taken in 1887, and again in 1889, the K line was seen to be
double; while on other plates it appeared single. A careful study of
Miss A. C. Maury of a series of seventy impressions indicated for the
doubling a period of fifty-two days, and showed it to affect all the
lines in the spectrum.[1440] The only available, and no doubt the true,
explanation of the phenomenon was that two similar and nearly equal
stars are here merged into one telescopically indivisible; their
combined light giving a single or double spectrum, according as their
orbital velocities are directed across or along our line of sight. The
movements of a revolving pair of stars must always be opposite in sense,
and proportionately equal in amount. That is, they at all times travel
with speeds in the inverse ratio of their masses. Hence, unless the
plane of their orbits be perpendicular to a plane passing through the
eye, there must be two opposite points where their velocities in the
line of sight reach a maximum, and two diametrically opposite points
where they touch zero. The lines in their common spectrum would thus
appear alternately double and single twice in the course of each
revolution. To that of Mizar, at first supposed to need 104 days for its
completion, a period of only twenty days fourteen hours was finally
assigned by Vogel.[1441] Anomalous spectral effects, probably due to the
very considerable eccentricity of the orbit, long impeded its
satisfactory determination. The mean distance apart of the component
stars, as now ascertained, is just twenty-two million miles, and their
joint mass quadruples that of the sun. But these are minimum estimates.
For if the orbital plane be inclined, much or little, to the line of
sight, the dimensions and mass of the system should be proportionately
increased.
An analogous discovery was made by Miss Maury in 1889. But in the
spectrum of Beta Aurigæ, the lines open out and close up on
alternate days, indicating a relative orbit[1442] with a radius of less
than eight million miles, traversed in about four days. This implies a
rate of travel for each star of sixty-five miles a second, and a
combined mass 4·7 times that of the sun. The components are
approximately equal, both in mass and light,[1443] and the system formed
by them is transported towards us with a speed of some sixteen miles a
second. The line-shiftings so singularly communicative proceed, in this
star, with perfect regularity.
This new class of "spectroscopic binaries" could never have been
visually disclosed. The distance of Beta Aurigæ from the earth, as
determined, somewhat doubtfully, by Professor Pritchard, is nearly three
and a third million times that of the earth from the sun (parallax =
0·06"); whence it has been calculated that the greatest angular
separation of the revolving stars is only five-thousandths of a second
of arc.[1444] To make this evanescent interval perceptible, a telescope
eighty feet in aperture would be required.
The zodiacal star, Spica (Alpha Virginis), was announced by Dr.
Vogel, April 24, 1890,[1445] to belong to the novel category, with the
difference, however, of possessing a nearly dark, instead of a
brilliantly lustrous companion. In this case, accordingly, the tell-tale
spectroscopic variations consist merely in a slight swinging to and fro
of single lines. No second spectrum leaves a legible trace on the plate.
Spica revolves in four days at the rate of fifty-seven miles a
second,[1446] or quicker, in proportion as its orbit is more inclined to
the line of sight, round a centre at a minimum distance of three
millions of miles. But the position of the second star being unknown,
the mass of the system remains indeterminate. The lesser component of
the splendid, slowly revolving binary, Castor, is also closely double.
Its spectral lines were found by Bélopolsky in 1896[1447] to oscillate
once in nearly three days, the secondary globe being apparently quite
obscure. Further study of the movements thus betrayed elicited the fact
that the major axis of the eclipse traversed revolves in a period of
2,100 days, as a consequence, most likely, of the flattened shape of the
stars.[1448] Still more unexpected was the simultaneous assignment, by
Campbell and Newall, of a duplex character to Capella.[1449] Here both
components shine, though with a different quality of light, one giving a
pure solar spectrum, the other claiming prismatic affinity with Procyon.
Their mutual circulation is performed in 104 days, and the radius of
their orbit cannot be less, and may be a great deal more, than
51,000,000 miles. Hence the possibility is not excluded that the
star--which has an authentic parallax of 0·08"--may be visually
resolved. Indeed, signs of "elongation" were thought to be perceptible
with the Greenwich 28-inch refractor,[1450] while only round images
could be seen at Lick.[1451] Another noteworthy case is that of Polaris,
found by Campbell to have certainly one, and probably two obscure
attendants.[1452] Through his systematic investigations of stellar
radial velocities with the Mills spectrograph, knowledge in this
department has, since 1897, progressed so rapidly that the spectroscopic
binaries of our acquaintance already number half a hundred, and ten
times as many more doubtless lie within easy range of detection.
Now it is evident that a spectroscopic binary, if the plane of its
motion made a very small angle with the line of sight, would be a
variable star. For, during a few hours of each revolution, some at least
of its light should be cut off by a transit of its dusky companion. Such
"eclipse-stars" are actually found in the heavens.
The best and longest-known member of the group is Algol in the Head of
Medusa, the "Demon-star" of the Arabs.[1453] This remarkable object,
normally above the third magnitude, loses and regains three-fifths of
its light once in 68·8 hours, the change being completed in about twelve
hours. Its definite and limited nature, and punctual recurrence,
suggested to Goodricke of York, by whom the periodicity of the star was
discovered in 1783,[1454] the interposition of a large dark satellite.
But the conditions involved by the explanation were first seriously
investigated by Pickering in 1880.[1455] He found that the phenomena
could be satisfactorily accounted for by supposing an obscure body 0·764
the bright star's diameter to revolve round it in a period identical
with that of its observed variation. This theoretical forecast was
verified with singular exactitude at Potsdam in 1889.[1456] A series of
spectral photographs taken there showed each of Algol's minima to be
preceded by a rapid recession from the earth, and succeeded by a rapid
movement of approach towards it. They take place, accordingly, when the
star is at the furthest point from ourselves of an orbit described round
an invisible companion, the transits of which across its disc betray
themselves to notice by the luminous vicissitudes they occasion. The
diameter of this orbit, traversed at the rate of twenty-six miles a
second, is just 2,000,000 miles; and it is an easy further inference
from the duration and extent of the phases exhibited that Algol itself
must be (in round numbers) one million, its attendant 830,000 miles in
diameter. Assuming both to be of the same density, Vogel found their
respective masses to be four-ninths and two-ninths that of the sun, and
their distance asunder to be 3,230,000 miles.
This singularly assorted pair of stars possibly form part of a larger
system. Their period of revolution is shorter now by six seconds than it
was in Goodricke's time; and Dr. Chandler has shown, by an exhaustive
discussion, that its inequalities are comprised in a cycle of about 130
years.[1457] They arise, in his view, from a common revolution, in that
period, of the close couple about a third distant body, emitting little
or no light, in an orbit inclined 20° to our line of vision, and of
approximately the size of that described by Uranus round the sun. The
time spent by light in crossing this orbit causes an apparent delay in
the phases of the variable, when Algol and its eclipsing satellite are
on its further side from ourselves, balanced by acceleration while they
traverse its hither side. Dr. Chandler derives confirmation for his
plausible and ingenious theory from a supposed undulation in the line
traced out by Algol's small proper motion; but the reality of this
disturbance has yet to be established.[1458] Meanwhile, M.
Tisserand,[1459] late Director of the Paris Observatory, preferred to
account for Algol's inequalities on the principle later applied by
Bélopolsky to those of Castor. That is to say, he assumed a revolving
line of apsides in an elliptical orbit traversed by a pretty strongly
compressed pair of globes. The truth of this hypothesis can be tested by
close observation of the phases of the star during the next few years.
The variable in the Head of Medusa is the exemplar of a class including
26 recognised members, all of which doubtless represent occulting
combinations of stars. But their occultations result merely from the
accident of their orbital planes passing through our line of sight;
hence the heavens must contain numerous systems similarly constituted,
though otherwise situated as regards ourselves, some of which, like
Spica Virginis, will become known through their spectroscopic changes,
while others, because revolving in planes nearly tangent to the sphere,
or at right angles to the visual line, may never disclose to us their
true nature. Among eclipsing stars should probably be reckoned the
peculiar variables, Beta Lyræ and V Puppis, each believed to
consist of a pair of bright stars revolving almost in contact.[1460]
Three stars, on the other hand, distinguished by rapid and regular
fluctuations, have been proved by Bélopolsky to be attended by
non-occulting satellites, which circulate, nevertheless, in the
identical periods of light-change.
Gore's "Catalogue of Known Variables"[1461] included, in 1884, 190
entries, and the number was augmented to 243 on its revision in
1888.[1462] Chandler's first list of 225 such objects,[1463] published
about the same time, received successive expansions in 1893 and
1896,[1464] and finally included 400 entries. A new "Catalogue of
Variable Stars," still wider in scope, will shortly be issued by the
German _Astronomische Gesellschaft_. Mr. A. W. Roberts's researches on
southern variables[1465] have greatly helped to give precision, while
adding to the extent of knowledge in this branch. Dr. Gould held the
opinion that most stars fluctuate slightly in brightness through
surface-alterations similar to, but on a larger scale than those of the
sun; and the solar analogy might be pushed somewhat further. It perhaps
affords a clue to much that is perplexing in stellar behaviour. Wolf
pointed out in 1852 the striking resemblance in character between curves
representing sun-spot frequency and curves representing the changing
luminous intensity of many variable stars. There were the same steep
ascent to maximum and more gradual decline to minimum, the same
irregularities in heights and hollows, and, it may be added, the same
tendency to a double maximum, and complexity of superposed
periods.[1466] It is impossible to compare the two sets of phenomena
thus graphically portrayed without reaching the conclusion that they are
of closely related origin. But the correspondence indicated is not, as
has often been hastily assumed, between maxima of sun-spots and minima
of stellar brightness, but just the reverse. The luminous outbursts, not
the obscurations of variable stars, obey a law analogous to that
governing the development of spots on the sun. Objects of the kind do
not, then, gain light through the closing-up of dusky chasms in their
photospheres, but by an actual increase of surface-brilliancy, together
with an immense growth of these brilliant formations--prominences and
faculæ--which, in the sun, accompany, or are appended to spots. A
comparison of light-curves with curves of spot-frequency leaves no doubt
on this point, and the strongest corroborative evidence is derived from
the emergence of bright lines in the spectra of long-period variables
rising to their recurring maxima.
Every kind and degree of variability is exemplified in the heavens. At
the bottom of the scales are stars like the sun, of which the lustre
is--tried by our instrumental means--sensibly steady. At the other
extreme are ranged the astounding apparitions of "new," or "temporary"
stars. Within the last thirty-six years eleven of these stellar guests
(as the Chinese call them) have presented themselves, and we meet with a
twelfth no farther back than April 27, 1848. But of the "new star" in
Ophiuchus found by Mr. Hind on that night, little more could be learnt
than of the brilliant objects of the same kind observed by Tycho and
Kepler. The spectroscope had not then been invented. Let us hear what it
had to tell of later arrivals.
About thirty minutes before midnight of May 12, 1866, Mr. John
Birmingham of Millbrook, near Tuam, in Ireland, saw with astonishment a
bright star of the second magnitude unfamiliarly situated in the
constellation of the Northern Crown. Four hours earlier, Schmidt of
Athens had been surveying the same part of the heavens, and was able to
testify that it was not visible there. That is to say, a few hours, or
possibly a few minutes, sufficed to bring about a conflagration, the
news of which may have occupied hundreds of years in travelling to us
across space. The rays which were its messengers, admitted within the
slit of Sir William Huggins's spectroscope, May 16, proved to be of a
composition highly significant as to the nature of the catastrophe. The
star--which had already declined below the third magnitude--showed what
was described as a double spectrum. To the dusky flutings of Secchi's
third type four brilliant rays were added.[1467] The chief of these
agreed in position with lines of hydrogen; so that the immediate cause
of the outburst was inferred to have been the eruption, or ignition, of
vast masses of that subtle kind of matter, the universal importance of
which throughout the cosmos is one of the most curious facts revealed by
the spectroscope.
T Coronæ (as the new star was called) quickly lost its adventitious
splendour. Nine days after its discovery it was again invisible to the
naked eye. It is now a pale yellow, slightly variable star near the
tenth magnitude, and finds a place as such in Argelander's charts.[1468]
It was thus obscurely known before it made its sudden leap into
notoriety.
The next "temporary," discovered by Dr. Schmidt at Athens, November 24,
1876, could lay no claim to previous recognition even in that modest
rank. It was strictly a parvenu. There was no record of its existence
until it made its appearance as a star of nearly the third magnitude, in
the constellation of the Swan. Its spectrum was examined, December 2, by
Cornu at Paris,[1469] and a few days later by Vogel and O. Lohse at
Potsdam.[1470] It proved of a closely similar character to that of T
Coronæ. A range of bright lines, including those of hydrogen, and
probably of helium, stood out from a continuous background impressed
with strong absorption. It may be presumed that in reality the gaseous
substances, which, by their sudden incandescence, had produced the
apparent conflagration, lay comparatively near the surface of the star,
while the screen of cooler materials intercepting large portions of its
light was situated at a considerable elevation in its atmosphere.
The object, meanwhile, steadily faded. By the end of the year it was of
no more than seventh magnitude. After the second week of March, 1877,
strengthening twilight combined with the decline of its radiance to
arrest further observation. It was resumed, September 2, at Dunecht,
with a strange result. Practically the whole of its scanty light (it had
then sunk below the tenth magnitude) was perceived to be gathered into a
single bright line in the green, and that the most characteristic line
of gaseous nebulæ.[1471] The star had, in fact, so far as outward
appearance was concerned, become transformed into a planetary nebula,
many of which are so minute as to be distinguishable from small stars
only by the quality of their radiations. It is now, having sunk to about
the fourteenth magnitude,[1472] entirely beyond the reach of
spectroscopic scrutiny.
Perhaps none of the marvellous changes witnessed in the heavens has
given a more significant hint as to their construction than the stellar
blaze kindled in the heart of the great Andromeda nebula some
undetermined number of years or centuries before its rays reached the
earth in the month of August, 1885. The first published discovery was by
Dr. Hartwig at Dorpat on August 31; but it was found to have been
already seen, on the 19th, by Mr. Isaac W. Ward of Belfast, and on the
17th by M. Ludovic Gully of Rouen. The _negative_ observations, on the
16th, of Tempel[1473] and Max Wolf, limited very narrowly the epoch of
the apparition. Nevertheless, it did not, like most temporaries, attain
its maximum brightness all at once. When first detected, it was of the
ninth, by September 1 it had risen to the seventh magnitude, from which
it so rapidly fell off that in March it touched the limit of visibility
(sixteenth magnitude) with the Washington 26-inch. Its light bleached
very perceptibly as it faded.[1474] During the earlier stages of its
decline, the contrast was striking between the sharply defined, ruddy
disc of the star, and the hazy, greenish-white background upon which it
was projected,[1475] and with which it was inevitably suggested to be in
some sort of physical connection.
Let us consider what evidence was really available on this point. To
begin with, the position of the star was not exactly central. It lay
sixteen seconds of arc to the south-west of the true nebular nucleus.
Its appearance did not then signify a sudden advance of the nebula
towards condensation, nor was it attended by any visible change in it
save the transient effect of partial effacement through superior
brightness.
Equally indecisive information was derived from the spectroscope. To
Vogel, Hasselberg, and Young, the light of the "Nova" seemed perfectly
continuous; but Huggins caught traces of bright lines on September 2,
confirmed on the 9th;[1476] and Copeland succeeded, on September 30, in
measuring three bright bands with an acute-angled prism specially
constructed for the purpose.[1477] A shimmer of F was suspected, and had
also been perceived by Mr. O. T. Sherman of Yale College. Still, the
effect was widely different from that of the characteristic blazing
spectrum of a temporary star, and prompted the surmise that here, too, a
variable might be under scrutiny. The star, however, was certainly so
far "new" that its rays, until their sudden accession of strength, were
too feeble to affect even our reinforced senses. Not one of the 1,283
small stars recorded in charts of the nebula could be identified with
it; and a photograph taken by Dr. Common, August 16, 1884, on which a
multitude of stars down to the fifteenth magnitude had imprinted
themselves, showed the uniform, soft gradation of nebulous light to be
absolutely unbroken by a stellar indication in the spot reserved for the
future occupation of the "Nova."[1478]
So far, then, the view that its relation to the nebula was a merely
optical one might be justified; but it became altogether untenable when
it was found that what was taken to be a chance coincidence had repeated
itself within living memory. On the 21st of May, 1860, M. Auwers
perceived at Königsberg a seventh magnitude star shining close to the
centre of a nebula in Scorpio, numbered 80 in Messier's Catalogue.[1479]
Three days earlier it certainly was not there, and three weeks later it
had vanished. The effect to Mr. Pogson (who independently discovered the
change, May 28)[1480] was as if the nebula had been _replaced_ by a
star, so entirely were its dim rays overpowered by the concentrated
blaze in their midst. Now, it is simply incredible that two outbursts of
so uncommon a character should have _accidentally_ occurred just on the
line of sight between us and the central portions of two nebulæ; we
must, then, conclude that they showed _on_ these objects because they
took place _in_ them. The most favoured explanation is that they were
what might be called effects of overcrowding--that some of the numerous
small bodies, presumably composing the nebulæ, jostled together, in
their intricate circlings, and obtained compensation in heat for their
sacrifice of motion. But this is scarcely more than a plausible
makeshift of perplexed thought. Mr. W. H. S. Monck, on the other hand,
has suggested that new stars appear when dark bodies are rendered
luminous by rushing through the gaseous fields of space,[1481] just as
meteors kindle in our atmosphere. The idea, which has been revived and
elaborated by Dr. Seeliger of Munich,[1482] is ingenious, but was not
designed to apply to our present case. Neither of the objects
distinguished by the striking variations just described is of gaseous
constitution. That in Scorpio appears under high magnifying powers as a
"compressed cluster"; that in Andromeda is perhaps, as Sir J. Herschel
suggested, "optically nebulous through the smallness of its constituent
stars"[1483]--if stars they deserve to be called.
On the 8th of December, 1891, Dr. Max Wolf took a photograph of the
region about Chi Aurigæ. No stranger so bright as the eighth
magnitude was among the stars depicted upon it. On the 10th,
nevertheless, a stellar object of the fifth magnitude, situated a couple
of degrees to the north-east of Beta Tauri and previously
unrecorded, where eleventh magnitude stars appeared, imprinted itself
upon a Harvard negative. Subsequent photographs taken at the same place
showed it to have gained about half a magnitude by the 20th; but the
plates were not then examined, and the discovery was left to be modestly
appropriated by an amateur, the Rev. Dr. Anderson of Edinburgh, by whom
it was announced, February 1, 1892, through the medium of an anonymous
postcard, to Dr. Copeland, the Astronomer Royal for Scotland.[1484] By
him and others, the engines of modern research were promptly set to
work. And to good purpose. Nova Aurigæ was the first star of its kind
studied by the universal chemical method. It is the first, accordingly,
of which authentic records can be handed down to posterity. They are of
a most remarkable character. The spectrum of the new object was
photographed at Stonyhurst and South Kensington on February 3; a few
days later, at Harvard and Lick in America, at Potsdam and Hérény on the
Continent of Europe. But by far the most complete impression was
secured, February 22, with an exposure of an hour and three-quarters, by
Sir William and Lady Huggins, through whose kindness it is reproduced in
Plate V., Fig. 1. The range of bright lines displayed in it is of
astonishing vividness and extent. It includes all the hydrogen rays dark
in the spectrum of Sirius (separately printed for comparison), besides
many others still more refrangible, as yet unidentified. Very
significant, too, is the marked character of the great prominence lines
H and K. The visual spectrum of the Nova was splendidly effective.
A quartette of brilliant green rays, two of them due to helium,
caught the eye; and they had companions too numerous to be easily
counted. The hydrogen lines were broad and bright; C blazed, as Mr.
Espin said, "like a danger-signal on a dark night"; the sodium pair were
identified at Tulse Hill, and the yellow helium ray was suspected to
lurk close beside them. Fig. 2 in the same plate shows the spectrum as
it was seen and mapped by Lady Huggins, February 2 to 6, together with
the spectra employed to test the nature of the emissions dispersed in
it. One striking feature will be at once remarked. It is that of the
pairing of bright with dark lines. Both in the visible and the
photographic regions this singular peculiarity was unmistakable; and
since the two series plainly owned the same chemical origin, their
separate visibility implied large displacement. Otherwise they would
have been superposed, not juxtaposed. Measurements of the bright rays,
accordingly, showed them to be considerably pushed down towards the red,
while their dark companions were still more pushed up towards the blue
end. Thus the spectrum of Nova Aurigæ, like that of Beta Lyræ,
with which it had many points in common, appeared to be really double.
It was supposed to combine the light of two distinct bodies, one, of a
gaseous nature, moving rapidly away from the earth, the other, giving a
more sunlike spectrum, approaching it with even higher speed. The
relative velocity determined at Potsdam for these oppositely flying
masses amounted to 550 miles a second.[1485] And this prodigious rate of
separation was fully maintained during six weeks! It did not then
represent a mere periastral rush-past.[1486] To the bodies exhibiting
its effects, and parting company for ever under its stress, it must have
belonged, with slight diminution, in perpetuity. The luminous outburst
by which they became visible was explained by Sir William Huggins, in a
lecture delivered at the Royal Institution, May 13, 1892, on the tidal
theory of Klinkerfues and Wilsing. Disturbances and deformations due to
the mutual attraction of two bulky globes at a close approach would, he
considered, "give rise to enormous eruptions of the hotter matters from
within, immensely greater, but similar in kind, to solar eruptions; and
accompanied, probably, by large electrical disturbances." The multiple
aspect and somewhat variable character of both bright and dark lines
were plausibly referred to processes of "reversal," such as are nearly
always in progress above sun-spots; but the long duration of the star's
suddenly acquired lustre did not easily fit in with the adopted
rationale. A direct collision, on the other hand, was out of the
question, since there had obviously been little, if any, sacrifice of
motion; and the substitution of a nebula for one of the "stars"[1487]
compelled recourse to scarcely conceivable modes of action for an
explanation of the perplexing peculiarities of the compound spectrum.
PLATE V.
[Illustration: Photographic and Visual Spectrum of Nova Aurigæ.
Fig. 1.--From a Photograph taken by Sir William and Lady Huggins, Feb.
22, 1892.
Fig. 2.--From a Drawing made by Lady Huggins, Feb. 2 to 6, 1892.]
An unexpected _dénouement_, however, threw all speculations off the
track. The Nova contained most of its brightness, fluctuations
notwithstanding, until March 9; after which date it ran swiftly and
uniformly down towards what was apprehended to be total extinction. No
marked change of spectrum attended its decline. When last examined at
Tulse Hill, March 24, all the more essential features of its prismatic
light were still faintly recognisable.[1488] The object was steadily
sinking on April 26, when a (supposed) final glimpse of it was caught
with the Lick 36-inch.[1489] It was then of about the sixteenth
magnitude. But on August 17 it had sprung up to the tenth, as Professors
Holden, Schaeberle, and Campbell perceived with amazement on turning the
same instrument upon its place. And to Professor Barnard it appeared,
two nights later, not only revived, but transformed into the nucleus of
a planetary nebula, 3" across.[1490] The reality of this seeming
distension, however, at once disputed, was eventually disproved. It
unquestionably arose from the imperfect focussing power of the telescope
for rays of unusual quality.[1491]
The rekindled Nova was detected in this country by Mr. H. Corder, on
whose notification Mr. Espin, on August 21, examined its nearly
monochromatic spectrum.[1492] The metamorphosis of Nova Cygni seemed
repeated.[1493] The light of the new object, like that of its
predecessor, was mainly concentrated in a vivid green band, identified
with the chief nebular line by Copeland,[1494] Von Gothard,[1495] and
Campbell.[1496] The second nebular line was also represented. Indeed,
the last-named observer recognised nearly all the eighteen lines
measured by him in the Nova as characteristic of planetary nebulæ.[1497]
Of particular interest is the emergence in the star-spectrum
photographed by Von Gothard of an ultra-violet line originally
discovered at Tulse Hill in the Orion nebula, which is also very strong
in the Lyra annular nebula.
Obviously, then, the physical constitution of Nova Aurigæ became
profoundly modified during the four months of its invisibility. The
spectrum of February was or appeared compound; that of August was
simple; it could be reasonably associated only with a single
light-source. Many of the former brilliant lines, too, had vanished, and
been replaced by others, at first inconspicuous or absent. As a result,
the solar-prominence type, to which the earlier spectrum had seemed to
conform, was completely effaced in the later. The cause of these
alterations remains mysterious, yet its effects continue. The chromatic
behaviour of the semi-extinct Nova, when scrutinised with great
refractors, shows its waning light to be distinctly nebular.[1498] Like
nearly all its congeners, the star is situated in the full stream of the
Milky Way, and we learn without surprise that micrometrical measures by
Burnham and Barnard[1499] failed to elicit from it any sign of
parallactic shifting. It is hence certain that the development of light,
of which the news reached the earth in December, 1891, must have been on
a vast scale, and of ancient date. Nova Aurigæ at its maximum assuredly
exceeded the sun many times in brightness; and its conflagration can
scarcely have occurred less, and may have occurred much more, than a
hundred years ago.
By means of the photographic surveys of the skies, carried on in both
hemispheres under Professor Pickering's superintendence, such amazing
events have been proved to be of not infrequent occurrence. Within six
years five new stars were detected from Draper Memorial, or chart-plates
by Mrs Fleming, besides the retrospective discovery of a sixth which had
rapidly burnt itself out, eight years previously, in Perseus.[1500] Nova
Normæ was the immediate successor of Nova Aurigæ; Nova Carinæ and Nova
Centauri lit up in 1895, the latter in a pre-existent nebula; Nova
Sagittarii and Nova Aquilæ attained brief maxima in 1898 and 1899
respectively. Now, three out of these five stars reproduced with
singular fidelity the spectrum of Nova Aurigæ; they displayed the same
brilliant rays shadowed, invariably on their blue sides, by dark ones.
Palpably, then, the arrangement was systematic and significant; it could
not result merely from the casually directed, opposite velocities of
bodies meeting in space. The hypothesis of stellar encounters
accordingly fell to the ground, and has been provided with no entire
satisfactory substitute. Most speculators now fully recognise that
motion-displacements cannot be made to account for the doubled spectra
of Novæ, and seek recourse instead to some kind of physical agency for
producing the observed effect.[1501] And since this is also visible in
certain permanent, though peculiar objects--notably in P Cygni,
Beta Lyræ, and Eta Carinæ--the acting cause must also evidently be
permanent and inherent.
The "new star of the new century"[1502] was a visual discovery. Dr.
Anderson duplicated, with added _éclat_, his performance of nine years
back. In the early morning of February 22, 1901, he perceived that Algol
had a neighbour of nearly its own brightness, which a photograph taken
by Mr. Stanley Williams, at Brighton, proved to have risen from below
the twelfth magnitude within the preceding 28 hours. And it was still
swiftly ascending. On the 23rd, it outshone Capella; for a brief space
it took rank as the premier star of the northern hemisphere. A decline
set in promptly, but was pursued hesitatingly. The light fluctuated
continually over a range of a couple of magnitudes, and with a close
approach, during some weeks, to a three-day periodicity. A year after
the original outburst, the star was still conspicuous with an
opera-glass. The spectrum underwent amazing changes. At first
continuous, save for fine dark lines of hydrogen and helium, it unfolded
within forty-eight hours a composite range of brilliant and dusky bands
disposed in the usual fashion of Novæ. These lasted until far on in
March, when hydrogen certainly, and probably other substances as well,
ceased to exert any appreciable absorptive action. Blue emissions of the
Wolf-Rayet type then became occasionally prominent, in remarkable
correspondence with the varying lustre of the star;[1503] finally, a
band at Lambda 3969, found by Wright at Lick to characterise nebular
spectra,[1504] assumed abnormal importance; and in July the nebular
transformation might be said to be complete. Striking alterations of
colour attended these spectral vicissitudes. White to begin with, the
star soon turned deep red, and its redness was visibly intensified at
each of its recurring minima of light. Blanching, however, ensued upon
the development of its nebulous proclivities; and its surviving rays are
of a steely hue.
All the more important investigations of Nova Persei were conducted by
photographic means. Libraries of spectral plates were collected at the
Yerkes and Lick Observatories, at South Kensington, Stonyhurst, and
Potsdam, and await the more exhaustive interpretation of the future.
Meanwhile, extraordinary revelations have been supplied by immediate
photographic delineation. On August 22 and 23, 1901, Professor Max Wolf,
by long exposures with the 16-inch Bruce twin objectives of the
Königstuhl Observatory (Heidelberg), obtained indications of a large
nebula finely ramified, extending south-east of the Nova;[1505] and the
entire formation came out in four hours with the Yerkes 2-foot
reflector, directed to it by Mr. Ritchey on September 20.[1506] It
proved to be a great spiral encircling, and apparently emanating from,
the star. But if so, tumultuously, and under stress of catastrophic
impulsions. A picture obtained by Mr. Perrine with the Crossley
refractor, in 7h. 19m., on November 7 and 8, disclosed the progress of a
startling change.[1507] Comparison with the Yerkes photograph showed
that during the intervening 48 days four clearly identifiable
condensations had become displaced, all to the same extent of about 90
seconds of arc, and in fairly concordant directions, suggesting motion
_round_ the Nova as well as away from it. The velocity implied, however,
is so prodigious as virtually to exclude the supposition of a bodily
transport of matter. It should be at the rate of no less than twenty
thousand miles a second, admitting the object to be at a distance from
us corresponding to an annual parallax of one-tenth of a second, and
actual measurements show it to be indefinitely more remote. The fact of
rapid variations in the nebula was reaffirmed, though with less
precision, from Yerkes photographs of November 9 and 13, Mr. Ritchey
inferring a general expansion of its southern portions.[1508] Much
further evidence must be at hand before a sane judgment can be formed as
to the nature of the strange events taking place in that secluded corner
of the Galaxy.[1509] And it is highly probable that the illumination of
the nebulous wreaths round the star will prove no less evanescent than
the blazing of the star itself.
We have been compelled somewhat to anticipate our narrative as regards
inquiries into the nature of nebulæ. The excursions of opinion on the
point were abruptly restricted and defined by the application to them of
the spectroscope. On August 29, 1864, Sir William Huggins sifted through
his prisms the rays of a bright planetary nebula in Draco.[1510] To his
infinite surprise, they proved to be mainly of one colour. In other
words, they avowed their origin from a mass of glowing vapour. As to
what _kind_ of vapour it might be by which Herschel's conjecture of a
"shining fluid" diffused at large throughout the cosmos was thus
unexpectedly verified, an answer only partially satisfactory could be
afforded. The conspicuous bright line of the Draco nebula seemed to
agree in position with one emitted by nitrogen, but has since proved to
be distinct from it; of its two fainter companions, one was unmistakably
the F line of hydrogen, while the other, in position intermediate
between the two, still remains unidentified.
By 1868 Huggins had satisfactorily examined the spectra of about seventy
nebulæ, of which one-third displayed a gaseous character.[1511] All of
these gave the green ray fundamental to the nebular spectrum, and
emanating from an unknown form of matter named by Sir William Huggins
"nebulum." It is associated with seven or eight hydrogen lines, with
three of "yellow" helium, and with a good many of undetermined origin.
The absence of the crimson radiation of hydrogen--perceived with
difficulty only in some highly condensed objects--is an anomaly very
imperfectly explained as a physiological effect connected with the
extreme faintness of nebular light.[1512] An approximate coincidence
between the chief nebular line and a "fluting" of magnesium having been
alleged by Lockyer in support of his meteoritic hypothesis of nebular
constitution, it became of interest to ascertain its reality. The task
was accomplished by Sir William and Lady Huggins in 1889 and 1890,[1513]
and by Professor Keeler, with the advantages of the Mount Hamilton
apparatus and atmosphere, in 1890-91.[1514] The upshot was to show a
slight but sure discrepancy as to place, and a marked diversity as to
character, between the two qualities of light. The nebular ray
(wave-length 5,007 millionths of a millimetre) is slightly more
refrangible than the magnesium fluting-edge, and it is sharp and fine,
with no trace of the unilateral haze necessarily clinging even to the
last "remnant" of a banded formation.
Planetary and annular nebulæ are, without exception, gaseous, as well as
those termed "irregular," which frequent the region of the Milky Way.
Their constitution usually betrays itself to the eye by their blue or
greenish colour; while those yielding a continuous spectrum are of a
dull white. Among the more remarkable of these are the well-known nebula
in Andromeda, and the great spiral in Canes Venatici; and, as a general
rule, the emissions of all such nebulæ as present the appearance of
star-clusters grown misty through excessive distance are of the same
kind. It would, however, be eminently rash to conclude thence that they
are really aggregations of sun-like bodies. The improbability of such an
inference has been greatly enhanced by the occurrence, at an interval of
a quarter of a century, of stellar outbursts in the midst of two of
them. For it is practically certain that the temporary stars were
equally remote with the hazy formations they illuminated; hence, if the
constituent particles of the latter be suns, the incomparably vaster
orbs by which their feeble light was well-nigh obliterated must, as was
argued by Mr. Proctor, have been on a scale of magnitude such as the
imagination recoils from contemplating. Nevertheless, Dr. Scheiner, not
without much difficulty, obtained, in January, 1899, spectrographic
prints of the Andromeda nebula, indicative, he thought, of its being a
cluster of solar stars.[1515] Sir William and Lady Huggins, on the other
hand, _saw_, in 1897, bright intermixed with dark bands in the spectrum
of the same object.[1516] And Mr. Maunder conjectures all "white" nebulæ
to be made up of sunlets in which the coronal element predominates,
while chromospheric materials assert their presence in nebulæ of the
"green" variety.[1517]
Among the ascertained analogies between the stellar and nebular systems
is that of variability of light. On October 11, 1852, Mr. Hind
discovered a small nebula in Taurus. Chacornac observed it at Marseilles
in 1854, but was confounded four years later to find it vanished.
D'Arrest missed it October 3, and redetected it December 29, 1861. It
was easily seen in 1865-66, but invisible in the most powerful
instruments from 1877 to 1880.[1518] Barnard, however, made out an
almost evanescent trace of it, October 15, 1890, with the great Lick
telescope,[1519] and saw it easily in the spring of 1895, while six
months later it evaded his most diligent search.[1520] Then again, on
September 28, 1897, the Yerkes 40-inch disclosed it to him as a mere
shimmer at the last limit of visibility; and it came out in three
diffuse patches on plates to which, on December 6 and 27, 1899, Keeler
gave prolonged exposures with the Crossley reflector.[1521] Moreover, a
fairly bright adjacent nebula, perceived by O. Struve in 1868, and
observed shortly afterwards by d'Arrest, has totally vanished, and was
most likely only a temporary apparition. These are the most authentic
instances of nebular variability. Many others have been more or less
plausibly alleged;[1522] but Professor Holden's persuasion, acquired
from an exhaustive study of the records since 1758,[1523] that the
various parts of the Orion nebula fluctuate continually in relative
lustre, has not been ratified by photographic evidence.
The case of the "trifid" nebula in Sagittarius, investigated by Holden
in 1877,[1524] is less easily disposed of. What is certain is that a
remarkable triple star, centrally situated, according to the
observations of both the Herschels, 1784-1833, in a dark space between
the three great _lobes_ of the nebula, is now, and has been since 1839,
densely involved in one of them; and since the hypothesis of relative
motion is on many grounds inadmissible, the change that has apparently
taken place must be in the distribution of light. One no less
conspicuous was adduced by Mr. H. C. Russell, director of the Sydney
Observatory.[1525] A particularly bright part of the great Argo nebula,
as drawn by Sir John Herschel, has, it would seem, almost totally
disappeared. He noticed its absence in 1871, using a 7-inch telescope,
failed equally later on to find it with an 11-1/2-inch, and his
long-exposure photographs show no vestige of it. The same structure is
missing from, or scarcely traceable in, a splendid picture of the nebula
taken by Sir David Gill in twelve hours distributed over four nights in
March, 1892.[1526] An immense gaseous expanse has, it would seem, sunk
out of sight. Materially it is no doubt there; but the radiance has left
it.
Nebulæ have no ascertained proper motions. No genuine change of place in
the heavens has yet been recorded for any one of them. All equally hold
aloof, so far as telescopic observation shows, from the busy journeyings
of the stars. This seeming immobility is partly an effect of vast
distance. Nebular parallax has, up to the present, proved evanescent,
and nebular parallactic drift, in response to the sun's advance through
space, remains likewise imperceptible.[1527] It may hence be presumed
that no nebulæ occur within the sphere occupied by the nearer stars. But
the difficulty of accurately measuring such objects must also be taken
into account. Displacements which would be conspicuous in stars might
easily escape detection in ill-defined, hazy masses. Thus the measures
executed by d'Arrest in 1857[1528] have not yet proved effective for
their designed purpose of contributing to the future detection of proper
motions. Some determinations made by Mr. Burnham with the Lick refractor
in 1891,[1529] will ultimately afford a more critical test. He found
that nearly all planetary nebulæ include a sharp stellar nucleus, the
position of which with reference to neighbouring stars could be fixed no
less precisely than if it were devoid of nebulous surroundings. Hence,
the objects located by him cannot henceforward shift, were it only to
the extent of a small fraction of a second, without the fact coming to
the knowledge of astronomers.
The spectroscope, however, here as elsewhere, can supplement the
telescope; and what it has to tell, it tells at once, without the
necessity of waiting on time to ripen results. Sir William Huggins made,
in 1874,[1530] the earliest experiments on the radial movements of
nebulæ. But with only a negative upshot. None of the six objects
examined gave signs of spectral alteration, and it was estimated that
they must have done so had they been in course of recession from or
approach towards the earth by as much as twenty-five miles a second.
With far more powerful appliances, Professor Keeler renewed the attempt
at Lick in 1890-91. His success was unequivocal. Ten planetary nebulæ
yielded perfectly satisfactory evidence of line-of-sight motion,[1531]
the swiftest traveller being the well-known greenish globe in
Draco,[1532] found to be hurrying towards the earth at the rate of forty
miles a second. For the Orion nebula, a recession of about eleven miles
was determined,[1533] the whole of which may, however, very well belong
to the solar system itself, which, by its translation towards the
constellation Lyra, is certainly leaving the great nebula pretty rapidly
behind. The anomaly of seeming nebular fixity has nevertheless been
removed; and the problem of nebular motion has begun to be solved
through the demonstrated possibility of its spectroscopic investigation.
Keeler's were the first trustworthy determinations of radial motion
obtained visually. That the similar work on the stars begun at Greenwich
in 1874, and carried on for thirteen years, remained comparatively
unfruitful, was only what might have been expected, the instruments
available there being altogether inadequate for the attainment of a high
degree of accuracy.
The various obstacles in the way of securing it were overcome by the
substitution of the sensitive plate for the eye. Air-tremors are thus
rendered comparatively innocuous; and measurements of stellar lines
displaced by motion with reference to fiducial lines from terrestrial
sources, photographed on the same plates, can be depended upon within
vastly reduced limits of error. Studies for the realisation of the
"spectrographic" method were begun by Dr. Vogel and his able assistant,
Dr. Scheiner, at Potsdam in 1887. Their preliminary results,
communicated to the Berlin Academy of Sciences, March 15, 1888, already
showed that the requirements for effective research in this important
branch were at last about to be complied with. An improved instrument
was erected in the autumn of the same year, and the fifty-one stars,
bright enough for determination with a refractor of 11 inches aperture,
were promptly taken in hand. A list of their motions in the line of
sight, published in 1892,[1534] was of high value, both in itself and
for what it promised. One noteworthy inference from the data it
collected was that the eye tends, under unfavourable circumstances, to
exaggerate the line-displacements it attempts to estimate. The
velocities photographically arrived at were of much smaller amounts than
those visually assigned. The average speed of the Potsdam stars came out
only 10·4 miles a second, the quickest among them being Aldebaran, with
a recession of thirty miles a second. More lately, however, Deslandres
and Campbell have determined for Zeta Herculis and Eta Cephei
respectively approaching rates of forty-four and fifty-four miles
a second.
The installation, in 1900, of a photographic refractor 31-1/2 inches in
aperture, coupled with a 20-inch guiding telescope, will enable Dr.
Vogel to investigate spectrographically some hundreds of stars fainter
than the second magnitude; and the materials thus accumulated should
largely help to provide means for a definite and complete solution of
the more than secular problem of the sun's advance through space. The
solution should be complete, because including a genuine determination
of the sun's velocity, apart from assumptions of any kind. M. Homann's
attempt, in 1885,[1535] to extract some provisional information on the
subject from the radial movements of visually determined stars gave a
fair earnest of what might be done with materials of a better quality.
He arrived at a goal for the sun's way shifted eastward to the
constellation Cygnus--a result congruous with the marked tendency of
recently determined apexes to collect in or near Lyra; and the most
probable corresponding velocity seemed to be about nineteen miles a
second, or just that of the earth in its orbit. A more elaborate
investigation of the same kind, based by Professor Campbell in
1900[1536] upon the motions of 280 stars, determined with extreme
precision, suffered in completeness through lack of available data from
the southern hemisphere. The outcome, accordingly, was an apex most
likely correctly placed as regards right ascension, but displaced
southward by some fifteen degrees. The speed of twelve miles a second,
assigned to the solar translation, approximates doubtless very closely
to the truth.
A successful beginning was made in nebular spectrography by Sir William
Huggins, March 7, 1882.[1537] Five lines in all stamped themselves upon
the plate during forty-five minutes of exposure to the rays of the
strange object in Orion. Of these, four were the known visible lines,
and a fifth, high up in the ultra-violet, at wave-length 3,727, has
evidently peculiar relationships, as yet imperfectly apprehended. It is
strong in the spectra of many planetaries; it helped to characterise the
nebular metamorphosis of Nova Aurigæ, yet failed to appear in Nova
Persei. Two additional hydrogen lines, making six in all, were
photographed at Tulse Hill, from the Orion nebula, in 1890;[1538] and
Dr. Copeland's detection in 1886[1539] of the yellow ray D_3 gave the
first hint of the presence of helium in this prodigious formation. Nor
are there wanting spectroscopic indications of its physical connection
with the stars visually involved in it. Sir William and Lady Huggins
found a plate exposed February 5, 1888, impressed with four groups of
fine bright lines, originating in the continuous light of two of the
trapezium-stars, but extending some way into the surrounding
nebula.[1540] And Dr. Scheiner[1541] argued a wider relationship from
the common possession, by the nebula and the chief stars in the
constellation Orion, of a blue line, bright in the one case, dark in the
others, since identified as a member of one of the helium series.
The structural unity of the stellar and nebular orders in this extensive
region of the sky has also, by direct photographic means, been
unmistakably affirmed.
The first promising autographic picture of the Orion nebula was obtained
by Draper, September 30, 1880.[1542] The marked approach towards a still
more perfectly satisfactory result shown by his plates of March, 1881
and 1882, was unhappily cut short by his death. Meanwhile, M. Janssen
was at work in the same field from 1881, with his accustomed
success.[1543] But Dr. A. Ainslie Common left all competitors far behind
with a splendid picture, taken January 30, 1883, by means of an exposure
of thirty-seven minutes in the focus of his 3-foot silver-on-glass
mirror.[1544] Photography may thereby be said to have definitely assumed
the office of historiographer to the nebulæ, since this one impression
embodies a mass of facts hardly to be compassed by months of labour with
the pencil, and affords a record of shape and relative brightness in the
various parts of the stupendous object it delineates which must prove
invaluable to the students of its future condition. Its beauty and merit
were officially recognised by the award of the Astronomical Society's
Gold Medal in 1884.
A second picture of equal merit, obtained by the same means, February
28, 1883, with an exposure of one hour, is reproduced in the
frontispiece. The vignette includes two specimens of planetary
photography. The Jupiter, with the great red spot conspicuous in the
southern hemisphere, is by Dr. Common. It dates from September 3, 1879,
and was accordingly one of the earliest results with his 36-inch, the
direct image in which imprinted itself in a fraction of a second, and
was subsequently enlarged on paper about twelve times. The exquisite
little picture of Saturn was taken at Paris by MM. Paul and Prosper
Henry, December 21, 1885, with their 13-inch photographic refractor. The
telescopic image was in this case magnified eleven times previous to
being photographed, an exposure of about five seconds being allowed; and
the total enlargement, as it now appears, is nineteen times. A trace of
the dusky ring perceptible on the original negative is lost in the
print.
A photograph of the Orion nebula taken by Dr. Roberts in 67 minutes,
November 30, 1886, made a striking disclosure of the extent of that
prodigious object. More than six times the nebulous area depicted on Dr.
Common's plates is covered by it, and it plainly shows an adjacent
nebula, separately catalogued by Messier, to belong to the same vast
formation.
This disposition to annex and appropriate has come out more strongly
with every increase of photographic power. Plates exposed at Harvard
College in March, 1888, with an 8-inch portrait-lens (the same used in
the preparation of the Draper Catalogue) showed the old-established
"Fish-mouth" nebula not only to involve the stars of the sword-handle,
but to be in tolerably evident connection with the most easterly of the
three belt-stars, from which a remarkable nebulous appendage was found
to proceed.[1545] A still more curious discovery was made by W. H.
Pickering in 1889.[1546] Photographs taken in three hours from the
summit of Wilson's Peak in California revealed the existence of an
enormous, though faint spiral structure, enclosing in its span of nearly
seventeen degrees the entire stellar and nebulous group of the Belt and
Sword, from which it most likely, although not quite traceably, issues
as if from a nucleus. A startling glimpse is thus afforded of the
cosmical importance of that strange "hiatus" in the heavens which
excited the wonder of Huygens in 1656. The inconceivable attenuation of
the gaseous stuff composing it was virtually demonstrated by Mr.
Ranyard.[1547]
In March, 1885, Sir Howard Grubb mounted for Dr. Isaac Roberts, at
Maghull, near Liverpool (his observatory has since been transferred to
Crowborough in Sussex), a silver-on-glass reflector of twenty inches
aperture, constructed expressly for use in celestial photography. A
series of nebula-pictures, obtained with this fine instrument, have
proved highly instructive both as to the structure and extent of these
wonderful objects; above all, one of the great Andromeda nebula, to
which an exposure of three hours was given on October 1, 1888.[1548] In
it a convoluted structure replaced and rendered intelligible the
anomalously rifted mass seen by Bond in 1847.[1549] The effects of
annular condensation appeared to have stamped themselves upon the plate,
and two attendant nebulæ presented the aspect of satellites already
separated from the parent body, and presumably revolving round it. The
ring-nebula in Lyra was photographed at Paris in 1886, and shortly
afterwards by Von Gothard with a 10-inch reflector,[1550] and he
similarly depicted in 1888 the two chief spiral and other nebulæ.[1551]
Photographs of the Lyra nebula taken at Algiers in 1890,[1552] and at
the Vatican observatory in 1892,[1553] were remarkable for the strong
development of a central star, difficult of telescopic discernment, but
evidently of primary importance to the annular structure around.
The uses of photography in celestial investigations become every year
more manifold and more apparent. The earliest chemical star-pictures
were those of Castor and Vega, obtained with the Cambridge refractor in
1850 by Whipple of Boston under the direction of W. C. Bond. Double-star
photography was inaugurated under the auspices of G. P. Bond, April 27,
1857, with an impression, obtained in eight seconds, of Mizar, the
middle star in the handle of the Plough. A series of measures from
sixty-two similar images gave the distance and position-angle of its
companion with about the same accuracy attainable by ordinary
micrometrical operations; and the method and upshot of these novel
experiments were described in three papers remarkably forecasting the
purposes to be served by stellar photography.[1554] The matter next fell
into the able hands of Rutherfurd, who completed in 1864 a fine object
glass (of 11-1/2 inches) corrected for the ultra-violet rays,
consequently useless for visual purposes. The sacrifice was recompensed
by conspicuous success. A set of measurements from his photographs of
nearly fifty stars in the Pleiades, and their comparison with Bessel's
places, enabled Dr. Gould to announce, in 1866, that during the
intervening third of a century no changes of importance had occurred in
their relative positions.[1555] And Mr. Harold Jacoby[1556] similarly
ascertained the fixity of seventy-five of Rutherfurd's Atlantids,
between the epoch 1873 and that of Dr. Elkin's heliometric triangulation
of the cluster in 1886,[1557] extending the interval to twenty-seven
years by subsequent comparisons with plates taken at Lick, September 27,
1900.[1558] Positive, however, as well as negative results have ensued
from the application of modern methods to that antique group.
On October 19, 1859, Wilhelm Tempel, a Saxon peasant by origin, later a
skilled engraver, discovered with a small telescope, bought out of his
scanty savings, an elliptical nebulosity, stretching far to the
southward from the star Merope. It attracted the attention of many
observers, but was so often missed, owing to its extreme susceptibility
to adverse atmospheric influences, as to rouse unfounded suspicions of
its variability. The detection of this evasive object gave a hint,
barely intelligible at the time, of further revelations of the same kind
by more cogent means.
A splendid photograph of 1,421 stars in the Pleiades, taken by the MM.
Henry with three hours' exposure, November 16, 1885, showed one of the
brightest of them to have a small spiral nebula, somewhat resembling a
strongly-curved comet's tail, attached to it. The reappearance of this
strange appurtenance on three subsequent plates left no doubt of its
real existence, visually attested at Pulkowa, February 5, 1886, by one
of the first observations made with the 30-inch equatoreal.[1559] Much
smaller apertures, however, sufficed to disclose the "Maia nebula,"
_once it was known to be there_. Not only did it appear greatly extended
in the Vienna 27-inch,[1560] but MM. Perrotin and Thollon saw it with
the Nice 15-inch, and M. Kammermann of Geneva, employing special
precautions, with a refractor of only ten inches aperture.[1561] The
advantage derived by him for bringing it into view, from the insertion
into the eye-piece of a uranium film, gives, with its photographic
intensity, valid proof that a large proportion of the light of this
remarkable object is of the ultra-violet kind.
The beginning thus made was quickly followed up. A picture of the
Pleiades procured at Maghull in eighty-nine minutes, October 23, 1886,
revealed nebulous surroundings to no less than four leading stars of the
group, namely, Alcyone, Electra, Merope, and Maia; and a second
impression, taken in three hours on the following night, showed further
"that the nebulosity extends in streamers and fleecy masses till it
seems almost to fill the spaces between the stars, and to extend far
beyond them."[1562] The coherence of the entire mixed structure was,
moreover, placed beyond doubt by the visibly close relationship of the
stars to the nebulous formations surrounding them in Dr. Roberts's
striking pictures. Thus Goldschmidt's notion that all the clustered
Pleiades constitute, as it were, a second Orion trapezium in the midst
of a huge formation of which Tempel's nebula is but a fragment,[1563]
has been to some extent verified. Yet it seemed fantastic enough in
1863.
Then in 1888 the MM. Henry gave exposures of four hours each to several
plates, which exhibited on development some new features of the
entangled nebulæ. The most curious of these was the linking together of
stars by nebulous chains. In one case seven aligned stars appeared
strung on a silvery filament, "like beads on a rosary."[1564] The "rows
of stars," so often noticed in the sky, may, then, be concluded to have
more than an imaginary existence. Of the 2,326 stars recorded in these
pictures, a couple of hundred among the brightest can, at the outside,
be reckoned as genuine Pleiades. The great majority were relegated, by
Pickering's[1565] and Stratonoff's[1566] counts of the stellar populace
_in_ and _near_ the cluster, to the position of outsiders from it. They
are undistinguished denizens of the abysmal background upon which it is
projected.
Investigations of its condition were carried a stage further by Barnard.
On November 14, 1890,[1567] he discovered visually with the Lick
refractor a close nebulous satellite to Merope, photographs of which
were obtained by Keeler in 1898.[1568] It appears in them of a rudely
pentagonal shape, a prominent angle being directed towards the adjacent
star. Finally, an exposure of ten hours made by Barnard with the Willard
lens indicated the singular fact that the entire group is embedded in a
nebulous matrix, streaky outliers of which blur a wide surface of the
celestial vault.[1569] The artist's conviction of the reality of what
his picture showed was confirmed by negatives obtained by Bailey at
Arequipa in 1897, and by H. C. Wilson at Northfield (Minnesota) in
1898.[1570]
With the Ealing 3-foot reflector, sold by Dr. Common to Mr. Crossley,
and by him presented to the Lick Observatory, Professor Keeler took in
1899 a series of beautiful and instructive nebula[1571] photographs; One
of the Trifid may be singled out as of particular excellence. An
astonishing multitude of new nebulæ were revealed by trial-exposures
with this instrument. A "conservative estimate" gave 120,000 as the
number coming within its scope. Moreover, the majority of those actually
recorded were of an unmistakable spiral character, and they included
most of Sir John Herschel's "double nebulæ," previously supposed to
exemplify the primitive history of binary stellar systems.[1572] Dr. Max
Wolf's explorations with a 6-inch Voigtländer lens in 1901 emphatically
reaffirmed the inexhaustible wealth of the nebular heavens. In one
restricted region, midway between Præsepe and the Milky Way, he located
135 nebulæ, where only three had until then been catalogued; and he
counted 108 such objects clustering round the star 31 Comæ
Berenices,[1573] and so closely that all might be occulted together by
the moon. The general photographic Catalogue of Nebulæ which Dr. Wolf
has begun to prepare[1574] will thus be a most voluminous work.
The history of celestial photography at the Cape of Good Hope began with
the appearance of the great comet of 1882. No special apparatus was at
hand; so Sir David Gill called in the services of a local artist, Mr.
Allis of Mowbray, with whose camera, strapped to the Observatory
equatoreal, pictures of conspicuous merit were obtained. But their
particular distinction lay in the multitude of stars begemming the
background. (See Plate III.) The sight of them at once opened to the
Royal Astronomer a new prospect. He had already formed the project of
extending Argelander's "Durchmusterung" from the point where it was left
by Schönfeld to the southern pole; and his ideas regarding the means of
carrying it into execution crystallised at the needle touch of the
cometary experiments. He resolved to employ photography for the purpose.
The exposure of plates was accordingly begun, under the care of Mr. Ray
Woods, in 1885; and in less than six years, the sky, from 19° of south
latitude to the pole, had been covered in duplicate. Their measurement,
and the preparation of a catalogue of the stars imprinted upon them,
were generously undertaken by Professor Kapteyn, and his laborious task
has at length been successfully completed. The publication, in 1900, of
the third and concluding volume of the "Cape Photographic
Durchmusterung"[1575] placed at the disposal of astronomers a
photographic census of the heavens fuller and surer than the
corresponding visual enumeration executed at Bonn. It includes 454,875
stars, nearly to the tenth magnitude, and their positions are reliable
to about one second of arc.
The production of this important work was thus a result of the Cape
comet-pictures; yet not the most momentous one. They turned the scale in
favour of recourse to the camera when the MM. Henry encountered, in
their continuation of Chacornac's half-finished enterprise of ecliptical
charting, sections of the Milky Way defying the enumerating efforts of
eye and hand. The perfect success of some preliminary experiments made
with an instrument constructed by them expressly for the purpose was
announced to the Academy of Sciences at Paris, May 2, 1885. By its means
stars estimated as of the sixteenth magnitude clearly recorded their
presence and their places; and the enormous increase of knowledge
involved may be judged of from the fact that, in a space of the Milky
Way in Cygnus 2° 15' by 3°, where 170 stars had been mapped by the old
laborious method, about five thousand stamped their images on a single
Henry plate.
These results suggested the grand undertaking of a general photographic
survey of the heavens, and Gill's proposal, June 4, 1886, of an
International Congress for the purpose of setting it on foot was
received with acclamation, and promptly acted upon. Fifty-six delegates
of seventeen different nationalities met in Paris, April 16, 1887, under
the presidentship of Admiral Mouchez, to discuss measures and organise
action. They resolved upon the construction of a Photographic Chart of
the whole heavens, comprising stars of a fourteenth magnitude, to the
surmised number of twenty millions; to be supplemented by a Catalogue,
framed from plates of comparatively short exposure, giving start to the
eleventh magnitude. These will probably amount to about one million and
a quarter. For procuring both sets of plates, instruments were
constructed precisely similar to that of the MM. Henry, which is a
photographic refractor, thirteen inches in aperture, and eleven feet
focus, attached to a guiding telescope of eleven inches aperture,
corrected, of course, for the visual rays. Each place covers an area of
four square degrees, and since the series must be duplicated to prevent
mistakes, about 22,000 plates will be needed for the Chart alone. The
task of preparing them was apportioned among eighteen observatories
scattered over the globe, from Mexico to Melbourne; but three in South
America having become disabled or inert, were replaced in 1900 by those
at Cordoba, Montevideo, and Perth, Western Australia. Meanwhile, the
publication of results has begun, and is likely to continue for at least
a quarter of a century. The first volume of measures from the Potsdam
Catalogue-plates was issued in 1899, and its successors, if on the same
scale, must number nearly 400. Moreover, ninety-six heliogravure
enlargements from the Paris Chart-plates, distributed in the same year,
supplied a basis for the calculation that the entire Atlas of the sky,
composed of similar sheets, will form a pile thirty feet high and two
tons in weight![1576] It will, however, possess an incalculable
scientific value. For millions of stars can be determined by its means,
from their imprinted images, with an accuracy comparable to that
attainable by direct meridian observations.
One of the most ardent promoters of the scheme it may be expected to
realise was Admiral Mouchez, the successor of Leverrier in the direction
of the Paris Observatory. But it was not granted to him to see the
fruition of his efforts. He died suddenly June 25, 1892.[1577] Although
not an astronomer by profession, he had been singularly successful in
pushing forward the cause of the science he loved, while his genial and
open nature won for him wide personal regard. He was replaced by M.
Tisserand, whose mathematical eminence fitted him to continue the
traditions of Delaunay and Leverrier. But his career, too, was unhappily
cut short by an unforeseen death on October 20, 1896; and the more
eminent among the many qualifications of his successor, M. Maurice
Loewy, are of the practical kind.
The sublime problem of the construction of the heavens has not been
neglected amid the multiplicity of tasks imposed upon the cultivators of
astronomy by its rapid development. But data of a far higher order of
precision, and indefinitely greater in amount, than those at the
disposal of Herschel or Struve must be accumulated before any definite
conclusions on the subject are possible. The first organised effort
towards realising this desideratum was made by the German Astronomical
Society in 1865, two years after its foundation at Heidelberg. The
original programme consisted in the _exact_ determination of the places
of all Argelander's stars to the ninth magnitude (exclusive of the polar
zone), from the reobservation of which, say, in the year 1950,
astronomers of two generations hence may gather a vast store of
knowledge--directly of the apparent motions, indirectly of the mutual
relations binding together the suns and systems of space. Thirteen
observatories in Europe and America joined in the work, now virtually
terminated. Its scope was, after its inception, widened to include
southern zones as far as the Tropic of Capricorn; this having been
rendered feasible by Schönfeld's extension (1875-1885) of Argelander's
survey. Thirty thousand additional stars thus taken in were allotted in
zones to five observatories. Another important undertaking of the same
class is the reobservation of the 47,300 stars in Lalande's _Histoire
Céleste_. Begun under Arago in 1855, its upshot has been the publication
of the great Paris Catalogue, issued in eight volumes, between 1887 and
1902. From a careful study of their secular changes in position, M.
Bossert has already derived the proper motions of a couple of thousand
out of nearly fifty thousand stars enumerated in it.
Through Dr. Gould's unceasing labours during his fifteen years'
residence at Cordoba, a detailed acquaintance with southern stars was
brought about. His _Uranometria Argentina_ (1879) enumerates the
magnitudes of 8,198 out of 10,649 stars visible to the naked eye under
those transparent skies; 33,160 down to 9-1/2 magnitude are embraced in
his "zones"; and the Argentine General Catalogue of 32,468 southern
stars was published in 1886. Valuable work of the same kind has been
done at the Leander McCormick Observatory, Virginia, by Professor O.
Stone; while the late Redcliffe observer's "Cape Catalogue for 1880"
affords inestimable aid to the practical astronomer south of the line,
which has been reinforced with several publications issued by the
present Astronomer Royal at the Cape. Moreover, the gigantic task
entered upon in 1860 by Dr. C. H. F. Peters, director of the Litchfield
Observatory, Clinton (N.Y.), and of which a large instalment was
finished in 1882, deserves honourable mention. It was nothing less than
to map all stars down to, and even below, the fourteenth magnitude,
situated within 30° on either side of the ecliptic, and so to afford "a
sure basis for drawing conclusions with respect to the changes going on
in the starry heavens."[1578]
It is tolerably safe to predict that no work of its kind and for its
purpose will ever again be undertaken. In a small part of one night
stars can now be got to register themselves more numerously and more
accurately than by the eye and hand of the most skilled observer in the
course of a year. Fundamental catalogues, constructed by the old,
time-honoured method, will continue to furnish indispensable
starting-points for measurement; and one of especial excellence was
published by Professor Newcomb in 1899;[1579] but the relative places of
the small crowded stars--the sidereal [Greek: hoi pholloi]--will
henceforth be derived from their autographic statements on the sensitive
plate. Even the secondary purpose--that of asteroidal discovery--served
by detailed stellar enumeration, is more surely attained by photography
than by laborious visual comparison. For planetary movement betrays
itself in a comparatively short time by turning the imprinted image of
the object affected by it from a dot into a trail.
In the arduous matter of determining star distances progress has been
steady, and bids fair to become rapidly accelerated. Together, yet
independently, Gill and Elkin carried out, at the Cape Observatory in
1882-83, an investigation of remarkable accuracy into the parallaxes of
nine southern stars. One of these was the famous Alpha Centauri,
the distance of which from the earth was ascertained to be just
one-third greater than Henderson had made it. The parallax of Sirius, on
the other hand, was doubled, or its distance halved; while Canopus
proved to be quite immeasurably remote--a circumstance which,
considering that, among all the stellar multitude, it is outshone only
by the radiant Dog-star, gives a stupendous idea of its real splendour
and dimensions.
Inquiries of this kind were, for some years, successfully pursued at the
observatory of Dunsink, near Dublin. Annual perspective displacements
were by Dr. Brünnow detected in several stars, and in others remeasured
with a care which inspired just confidence. His parallax for Alpha
Lyræ (0·13") was authentic, though slightly too large (Elkin's final
results gave Pi = 0·082"); and the received value for the
parallax of the swiftly travelling star "Groombridge 1,830" scarcely
differs from that arrived at by him in 1871 (Pi = 0·09"). His
successor as Astronomer-Royal for Ireland, Sir Robert Stawell Ball (now
Lowndean Professor of Astronomy in the University of Cambridge), has
done good service in the same department. For besides verifying
approximately Struve's parallax of half a second of arc for 61 Cygni, he
refuted, in 1811, by a sweeping search for (so-called) "large"
parallaxes, certain baseless conjectures of comparative nearness to the
earth, in the case of red and temporary stars.[1580] Of 450 objects thus
cursorily examined, only one star of the seventh magnitude, numbered
1,618 in Groombridge's Circumpolar Catalogue, gave signs of measurable
vicinity. Similarly, a reconnaissance among rapidly moving stars lately
made by Dr. Chase with the Yale heliometer[1581] yielded no really
large, and only eight appreciable parallaxes among the 92 subjects of
his experiments.
A second campaign in stellar parallax was undertaken by Gill and Elkin
in 1887. But this time the two observers were in opposite hemispheres.
Both used heliometers. Dr. Elkin had charge of the fine instrument then
recently erected in Yale College Observatory; Sir David Gill employed
one of seven inches, just constructed under his directions, in
first-rate style, by the Repsolds of Hamburg. Dr. Elkin completed in
1888 his share of the more immediate joint programme, which consisted in
the determination, by direct measurement, of the average parallax of
stars of the first magnitude. It came out, for the ten northern
luminaries, after several revisions, 0·098", equivalent to a
light-journey of thirty-three years. The deviations from this average
were, indeed, exceedingly wide. Two of the stars, Betelgeux and
Alpha Cygni, gave no certain sign of any perspective shifting; of the rest,
Procyon, with a parallax of 0·334", proved the nearest to our system. At
the mean distance concluded for these ten brilliant stars, the sun would
show as of only fifth magnitude; hence it claims a very subordinate rank
among the suns of space. Sir David Gill's definitive results were
published in 1900.[1582] As the average parallax of the eleven brightest
stars in the southern hemisphere, they gave 0·13", a value enhanced by
the exceptional proximity of Alpha Centauri. Yet four of these
conspicuous objects--Canopus, Rigel, Spica, and Beta Crucis--gave
no sign of perspective response to the annual change in our point of
view. The list included eleven fainter stars with notable proper
motions, and most of these proved to have fairly large parallaxes. Among
other valuable contributions to this difficult branch may be instanced
Bruno Peter's measurements of eleven stars with the Leipzig heliometer,
1887-92;[1583] Kapteyn's application of the method by differences in
right ascension to fifteen stars observed on the meridian 1885-89;[1584]
and Flint's more recent similar determinations at Madison,
Wisconsin.[1585]
The great merit of having rendered photography available for the
sounding of the celestial depths belongs to Professor Pritchard. The
subject of his initial experiment was 61 Cygni. From measurements of 200
negatives taken in 1886, he derived for that classic star a parallax of
0·438", in satisfactory agreement with Ball's of 0·468". A detailed
examination convinced the Astronomer-Royal of its superior accuracy to
Bessel's result with the heliometer. The Savilian Professor carried out
his project of determining all second magnitude stars to the number of
about thirty,[1586] conveniently observable at Oxford, obtaining as the
general outcome of the research an average parallax of 0·056", for
objects of that rank. But this value, though in itself probable, cannot
be accepted as authoritative, in view of certain inaccuracies in the
work adverted to by Jacoby,[1587] Hermann Davis, and Gill. The method
has, nevertheless, very large capabilities. Professor Kapteyn showed, in
1889,[1588] the practicability of deriving parallaxes wholesale from
plates exposed at due intervals, and applied his system, in 1900, with
encouraging success, to a group of 248 stars.[1589] The apparent absence
of spurious shiftings justified the proposal to follow up the completion
of the Astrographic Chart with the initiation of a photographic
"Parallax Durchmusterung."
Observers of double stars are among the most meritorious, and need to be
among the most patient and painstaking workers in sidereal astronomy.
They are scarcely as numerous as could be wished. Dr. Doberck,
distinguished as a computer of stellar orbits, complained in 1882[1590]
that data sufficient for the purpose had not been collected for above 30
or 40 binaries out of between five and six hundred certainly or probably
within reach. The progress since made is illustrated by Mr. Gore's
useful Catalogue of Computed Binaries, including fifty-nine entries,
presented to the Royal Irish Academy, June 9, 1890.[1591] Few have done
more towards supplying the deficiency of materials than the late Baron
Ercole Dembowski of Milan. He devoted the last thirty years of his life,
which came to an end January 19, 1881, to the revision of the Dorpat
Catalogue, and left behind him a store of micrometrical measures as
numerous as they are precise.
Of living observers in this branch, Mr. S. W. Burnham is beyond question
the foremost. While pursuing legal avocations at Chicago, he diverted
his scanty leisure by exploring the skies with a 6-inch telescope
mounted in his back-yard; and had discovered, in May, 1882, one thousand
close and mostly very difficult double stars.[1592] Summoned as chief
assistant to the new Lick Observatory in 1888, he resumed the work of
his predilection with the 36-inch and 12-inch refractors of that
establishment. But although devoting most of his attention to
much-needed remeasurements of known pairs, he incidentally divided no
less than 274 stars, the majority of which lay beyond the resolving
power of less keen and effectually aided eyesight. One of his many
interesting discoveries was that of a minute companion to Alpha
Ursæ Majoris (the first Pointer), which already gives unmistakable signs
of orbital movement round the shining orb it is attached to. Another
pair, Kappa Pegasi, detected in 1880, was found in 1892 to have
more than completed a circuit in the interim.[1593] Its period of a
little over eleven years is the shortest attributable to a _visible_
binary system, except that of Delta Equulei, provisionally
determined by Professor Hussey in 1900 at 5·7 years,[1594] and indicated
by spectroscopic evidence to be of uncommon brevity.[1595] Burnham's
Catalogue of 1,290 Double Stars, discovered by him from 1871 to
1899,[1596] is a record of unprecedented interest. Nearly all the 690
pairs included in it, 2" or less than 2" apart, must be physically
connected; and they offer a practically unlimited field for
investigation; while the notes, diagrams, and orbits appended profusely
to the various entries, are eminently helpful to students and computers.
The author is continuing his researches at the Yerkes Observatory,
having quitted the Lick establishment in 1892. The first complete
enrolment of southern double stars was made by Mr. R. T. A. Innes in
1899.[1597] The couples enumerated, twenty-one per cent. of which are
separated by less than one second of arc, are 2,140 in number. They
include 305 discovered by himself. Dr. See gathered a rich harvest of
nearly 500 new southern pairs with the Lowell 24-inch refractor in
1897.[1598] Professor Hough's discoveries in more northerly zones amount
to 623;[1599] Hussey's at Lick to 350; and Aitken's already to over 300.
There is as yet no certainty that the stars of 61 Cygni form a true
binary combination. Mr. Burnham, indeed, holds them to be in course of
definitive separation; and Professor Hall's observations at Washington,
1879 to 1891, although favouring their physical connection, are far from
decisive on the point.[1600] Dr. Wilsing, from certain anomalous
displacements of their photographed images, concluded in 1893[1601] the
presence of an invisible third member of the system, revolving in a
period of twenty-two months; but the effects noticed by him were
probably illusory.
Important series of double-star observations were made by Perrotin at
Nice in 1883-4;[1602] by Hall, with the 26-inch Washington equatoreal,
1874 to 1891;[1603] by Schiaparelli from 1875 onward; by Glasenapp, O.
Stone, Leavenworth, Seabroke, and many besides. Finally, Professor
Hussey's revision of the Pulkowa Catalogue[1604] is a work of the _teres
atque rotundus_ kind, which leaves little or nothing to be desired. The
methods employed in double-star determinations remain, at the beginning
of the twentieth century, essentially unchanged. The camera has scarcely
encroached upon this part of the micrometer's domain.[1605]
A research of striking merit into the origin of binary stars was
published in 1892 by Dr. T. J. J. See, in the form of an Inaugural
Dissertation for his doctor's degree in the University of Berlin. The
main result was to show the powerful effects of tidal friction in
prescribing the course of their development from double nebulæ,
revolving almost in contact, to double suns, far apart, yet inseparable.
The high eccentricities of their eventual orbits were shown to result
necessarily from this mode of action, which must operate with enormous
strength on closely conjoined, nearly equal masses, such as the rapidly
revolving pairs disclosed by the spectroscope. That these are still in
an early stage of their life-history is probable in itself, and is
re-affirmed by the exceedingly small density indicated for eclipsing
stars by the ratio of phase-duration to period.
Stellar photometry, initiated by the elder Herschel, and provided with
exact methods by his son at the Cape, by Steinheil and Seidel at Munich,
has of late years assumed the importance of a separate department of
astronomical research. Two monumental works on the subject, compiled on
opposite sides of the Atlantic, were thus appropriately coupled in the
bestowal of the Royal Astronomical Society's Gold Medal in 1886. Harvard
College Observatory led the way under the able direction of Professor E.
C. Pickering. His photometric catalogue of 4,260 stars,[1606]
constructed from nearly 95,000 observations of light-intensity during
the years 1879-82, constitutes a record of incalculable value for the
detection and estimation of stellar variability. It was succeeded in
1885 by Professor Pritchard's "Uranometria Nova Oxoniensis," including
photometric determinations of the magnitude of all naked-eye stars, from
the pole to ten degrees south of the equator to the number of 2,784. The
instrument employed was the "wedge photometer," which measures
brightness by resistance to extinction. A wedge of neutral-tint glass,
accurately divided to scale, is placed in the path of the stellar rays,
when the thickness of it they have power to traverse furnishes a
criterion of their intensity. Professor Pickering's "meridian
photometer," on the other hand, is based upon Zöllner's principle of
equalization effected by a polarising apparatus. After all, however, as
Professor Pritchard observed, "the eye is the real photometer," and its
judgment can only be valid over a limited range.[1607] Absolute
uniformity, then, in estimates made by various means, under varying
conditions, and by different observers, is not to be looked for; and it
is satisfactory to find substantial agreement attainable and attained.
Only in an insignificant fraction of the stars common to the Harvard and
Oxford catalogues discordances are found exceeding one-third of a
magnitude; a large proportion (71 per cent.) agree within one-fourth, a
considerable minority (31 per cent.) within one-tenth of a
magnitude.[1608] The Harvard photometry was extended, on the same scale,
to the opposite pole in a catalogue of the magnitudes of 7,922 southern
stars,[1609] founded on Professor Bailey's observations in Peru,
1889-91. Measurements still more comprehensive were subsequently
executed at the primary establishment. With a meridian photometer of
augmented power, the surprising number of 473,216 settings were made
during the years 1891-98, nearly all by the indefatigable director
himself, and they afforded materials for a "Photometric Durchmusterung,"
published in 1901, including all stars to 7·5 magnitude north of
declination -40°.[1610] A photometric zone, 20° wide, has for some time
been in course of observation at Potsdam by MM. Müller and Kempf. The
instrument employed by them is constructed on the polarising principle
as adapted by Zöllner.
Photographic photometry has meanwhile risen to an importance if anything
exceeding that of visual photometry. For the usefulness of the great
international star-chart now being prepared would be gravely compromised
by systematic mistakes regarding the magnitudes of the stars registered
upon it. No entirely trustworthy means of determining them have,
however, yet been found. There is no certainty as to the relative times
of exposure needed to get images of stars representative of successive
photometric ranks. All that can be done is to measure the proportionate
diameters of such images, and to infer, by the application of a law
learned from experience, the varied intensities of light to which they
correspond. The law is, indeed, neither simple nor constant. Different
investigators have arrived at different formulæ, which, being purely
empirical, vary their nature with the conditions of experiment. Probably
the best expedient for overcoming the difficulty is that devised by
Pickering, of simultaneously photographing a star and its secondary
image, reduced in brightness by a known amount.[1611] The results of its
use will be exhibited in a catalogue of 40,000 stars to the tenth
magnitude, one for each square degree of the heavens. A photographic
photometry of all the lucid stars, modelled on the visual photometry of
1884, is promised from the same copious source of novelties. The
magnitudes of the stars in the Draper Catalogue were determined, so to
speak, spectrographically. The quantity measured in all cases was the
intensity of the hydrogen line near G. By the employment of this
definite and uniform test, results were obtained, of special value
indeed, but in strong disaccord with those given by less exclusive
determinations.
Thought, meantime, cannot be held aloof from the great subject upon the
future illustration of which so much patient industry is being expended.
Nor are partial glimpses denied to us of relations fully discoverable,
perhaps, only through centuries of toil. Some important points in
cosmical economy have, indeed, become quite clear within the last fifty
years, and scarcely any longer admit of a difference of opinion. One of
these is that of the true status of nebulæ.
This was virtually settled by Sir J. Herschel's description in 1847 of
the structure of the Magellanic clouds; but it was not until Whewell, in
1853, and Herbert Spencer, in 1858,[1612] enforced the conclusions
necessarily to be derived therefrom that the conception of the nebulæ as
remote galaxies, which Lord Rosse's resolution of many into stellar
points had appeared to support, began to withdraw into the region of
discarded and half-forgotten speculations. In the Nubeculæ, as Whewell
insisted,[1613] "there coexist, in a limited compass and in
indiscriminate position, stars, clusters of stars, nebulæ, regular and
irregular, and nebulous streaks and patches. These, then, are different
kinds of things in themselves, not merely different to us. There are
such things as nebulæ side by side with stars and with clusters of
stars. Nebulous matter resolvable occurs close to nebulous matter
irresolvable."
This argument from coexistence in nearly the same region of space,
reiterated and reinforced with others by Mr. Spencer, was urged with his
accustomed force and freshness by Mr. Proctor. It is unanswerable. There
is no maintaining nebulæ to be simply remote worlds of stars in the face
of an agglomeration like the Nubecula Major, containing in its
(certainly capacious) bosom _both_ stars and nebulæ. Add the facts that
a considerable proportion of these perplexing objects are gaseous, and
that an intimate relation obviously subsists between the mode of their
scattering and the lie of the Milky Way, and it becomes impossible to
resist the conclusion that both nebular and stellar systems are parts of
a single scheme.[1614]
As to the stars themselves, the presumption of their approximate
uniformity in size and brightness has been effectually dissipated.
Differences of distance can no longer be invoked to account for
dissimilarity in lustre. Minute orbs, altogether invisible without
optical aid, are found to be indefinitely nearer to us than such radiant
objects as Canopus, Betelgeux, or Rigel. Moreover, intensity of light is
perceived to be a very imperfect index to real magnitude. Brilliant suns
are swayed from their course by the attractive power of massive yet
faintly luminous companions, and suffer eclipse from obscure
interpositions. Besides, effective lustre is now known to depend no less
upon the qualities of the investing atmosphere than upon the extent and
radiative power of the stellar surface. Red stars must be far larger in
proportion to the light diffused by them than white or yellow
stars.[1615] There can be no doubt that our sun would at least double
its brightness were the absorption suffered by its rays to be reduced to
the Sirian standard; and, on the other hand, that it would lose half its
present efficiency as a light-source if the atmosphere partially veiling
its splendours were rendered as dense as that of Aldebaran.
Thus, variety of all kinds is seen to abound in the heavens; and it must
be admitted that the consequent insecurity of all hypotheses as to the
relative distances of individual stars singularly complicates the
question of their allocation in space. Nevertheless, something has been
learnt even on that point; and the tendency of modern research is, on
the whole, strongly confirmatory of the views expressed by Herschel in
1802. He then no longer regarded the Milky Way as the mere visual effect
of an enormously extended stratum of stars, but as an actual
aggregation, highly irregular in structure, made up of stellar clouds
and groups and nodosities. All the facts since ascertained fit in with
this conception, to which Proctor added arguments favouring the view,
since adopted by Barnard[1616] and Easton,[1617] that the stars forming
the galactic stream are not only situated more closely together, but are
also really, as well as apparently, of smaller dimensions than the lucid
orbs studding our skies. By the laborious process of isographically
charting the whole of Argelander's 324,000 stars, he brought out in
1871[1618] signs of relationship between the distribution of the
brighter stars and the complex branchings of the Milky Way, which has
been stamped as authentic by Newcomb's recent statistical
inquiries.[1619] There is, besides, a marked condensation of stars,
especially in the southern hemisphere, towards a great circle inclined
some twenty degrees to the galactic plane; and these were supposed by
Gould to form with the sun a subordinate cluster, of which the
components are seen projected upon the sky as a zone of stellar
brilliants.[1620] The zone has, however, galactic rather than solar
affinities, and represents, perhaps, not a group, but a stream.
The idea is gaining ground that the Milky Way is designed, in its main
outlines, on a spiral pattern, and that its various branches and
sections are consequently situated at very different distances from
ourselves. Proctor gave a preliminarily interpretation of their
complexities on this principle, and Easton of Rotterdam[1621] has
renewed the attempt with better success.
A most suggestive delineation of the Milky Way, completed in 1889, after
five years of labour, by Dr. Otto Boedicker, Lord Rosse's astronomer at
Parsonstown, was published by lithography in 1892. It showed a curiously
intricate structure, composed of dimly luminous streams, and shreds, and
patches, intermixed with dark gaps and channels. Ramifications from the
main trunk ran out towards the Andromeda nebula and the "Bee-hive"
cluster in Cancer, involved the Pleiades and Hyades, and, winding round
the constellation of Orion, just attained the Sword-handle nebula. The
last delicate touches had scarcely been put to the picture, when the
laborious eye-and-hand method was, in this quarter, as already in so
many others, superseded by a more expeditious process. Professor Barnard
took the first photographs ever secured of the true Milky Way, July 28,
August 1 and 2, 1889, at the Lick Observatory. Special conditions were
required for success; above all, a wide field and a strong light-grasp,
both complied with through the use of a 6-inch portrait-lens. Even thus,
the sensitive plate needed some hours to pick out the exceedingly faint
stars collected in the galactic clouds. These cannot be photographed
under the nebulous aspect they wear to the eye; the camera takes note of
their real nature, and registers their constituent stars rank by rank.
Hence the difficulty of disclosing them. "In the photographs made with
the 6-inch portrait-lens," Professor Barnard wrote, "besides myriads of
stars, there are shown, for the first time, the vast and wonderful
cloud-forms, with all their remarkable structure of lanes, holes, and
black gaps, and sprays of stars. They present to us these forms in all
their delicacy and beauty, as no eye or telescope can ever hope to see
them."[1622] In Plate VI. one of these strange galactic landscapes is
reproduced. It occurs in the Bow of Sagittarius, not far from the Trifid
nebula, where the aggregations of the Milky Way are more than usually
varied and characteristic. One of their distinctive features comes out
with particular prominence. It will be noticed that the bright mass near
the centre of the plate is tunnelled with dark holes and furrowed by
dusky lanes. Such interruptions recur perpetually in the Milky Way. They
are exemplified on the largest scale in the great rift dividing it into
two branches all the way from Cygnus to Crux; and they are reproduced in
miniature in many clusters.
PLATE VI.
[Illustration: Region of the Milky Way in Sagittarius--showing a double
black aperture.
Photographed by Professor E. E. Barnard.]
Mr. H. C. Russell, at Sydney in 1890, successfully imitated Professor
Barnard's example.[1623] His photographs of the southern Milky Way have
many points of interest. They show the great rift, black to the eye, yet
densely star-strewn to the perception of the chemical retina; while the
"Coal-sack" appears absolutely dark only in its northern portion. His
most remarkable discovery, however, was that of the spiral character of
the two Nubeculæ. With an effective exposure of four and a half hours,
the Greater Cloud came out as "a complex spiral, with two centres";
while the similar conformation of its minor companion developed only
after eight hours of persistent actinic action. The revelation is full
of significance.
Scarcely less so, although after a different fashion, is the disclosure
on plates exposed by Dr. Max Wolf, with a 5-inch lens, in June, 1891, of
a vastly extended nebula, bringing some of the leading stars in Cygnus
into apparently organic connection with the piles of galactic star-dust
likewise involved by it.[1624] Barnard has similarly found great tracts
of the Milky Way to be photographically nebulous, and the conclusion
seems inevitable that we see in it a prodigious mixed system, resembling
that of the Pleiades in point of composition, though differing widely
from it in plan of structure. Of corroborative testimony, moreover, is
the discovery independently resulting from Gill's and Pickering's
photographic reviews, that stars of the first type of spectrum largely
prevail in the galactic zone of the heavens.[1625] With approach to that
zone, Kapteyn noticed a steady growth of actinic intensity relative to
visual brightness in the stars depicted on the Cape Durchmusterung
plates.[1626] In other words, stellar light is, in the Milky Way,
_bluer_ than elsewhere. And the reality of the primitive character hence
to be inferred for the entire structure was, in a manner, certified by
Mr. McClean's observation that Helium stars--the supposed immediate
products of nebulous matter--crowd towards its medial plane.
The first step towards the unravelment of the tangled web of stellar
movements was taken when Herschel established the reality, and indicated
the direction of the sun's journey. But the gradual shifting backward of
the whole of the celestial scenery amid which we advance accounts for
only a part of the observed displacements. The stars have motions of
their own besides those reflected upon them from ours. All attempts,
however, to grasp the general scheme of these motions have hitherto
failed. Yet they have not remained wholly fruitless. The community of
slow movement in Taurus, upon which Mädler based his famous theory, has
proved to be a fact, and one of very extended significance.
In 1870 Mr. Proctor undertook to chart down the directions and
proportionate amounts of about 1,600 proper motions, as determined by
Messrs. Stone and Main, with the result of bringing to light the
remarkable phenomenon termed by him "star-drift."[1627] Quite
unmistakably, large groups of stars, otherwise apparently disconnected,
were seen to be in progress together, in the same direction and at the
same rate, across the sky. An example of this kind of unanimity was
alleged by him in the five intermediate stars of the Plough; and that
the agreement in thwartwise motion is no casual one is practically
demonstrated by the concordant radial velocities determined at Potsdam
for four out of the five objects in question. All of these approach the
earth at the rate of about eighteen miles a second; and the fifth and
faintest, Delta Ursæ, though not yet measured, may be held to share
their advance. One of them, moreover, Zeta Ursæ, alias Mizar,
carries with it three other stars--Alcor, the Arab "Rider" of the horse,
visible to the naked eye, besides a telescopic and a spectroscopic
attendant. So that the group may be regarded as octuple. It is of vast
compass. Dr. Höffler assigned to it in 1897[1628]--although on grounds
more or less hypothetical--a mean parallax corresponding to a
light-journey of 192 years, which would give to the marching squadron a
total extent of at least fourteen times the distance from the sun to
Alpha Centauri, while implying for its brightest member--Eta
Ursæ Majoris--the lustre of six hundred suns. The organising principle
of this grand scheme must long remain mysterious.
It is no solitary example. Particular association, indeed--as was
surmised by Michell far back in the eighteenth century--appears to be
the rule rather than an exception in the sidereal system. Stars are
bound together by twos, by threes, by dozens, by hundreds. Our own sun
is, perhaps, not exempt from this gregarious tendency. Yet the search
for its companions has, up to the present, been unavailing. Gould's
cluster[1629] seems remote and intangible; Kapteyn's collection of solar
stars proved to have been a creation of erroneous data, and was
abolished by his unrelenting industry. Rather, we appear to have secured
a compartment to ourselves for our long journey through space. A
practical certainty has, at any rate, been gained that whatever
aggregation holds the sun as a constituent is of a far looser build than
the Pleiades or Præsepe. Of all such majestic communities the laws and
revolutions remain, as yet, inaccessible to inquiry; centuries may
elapse before even a rudimentary acquaintance with them begins to
develop; while the economy of the higher order of association, which we
must reasonably believe that they unite to compose, will possibly
continue to stimulate and baffle human curiosity to the end of time.
FOOTNOTES:
[Footnote 1369: _Report Brit. Assoc._, 1868, p. 166. Rutherfurd gave a
rudimentary sketch of a classification of the kind in December, 1862,
but based on imperfect observation. See _Am. Jour. of Sc._, vol. xxxv.,
p. 77.]
[Footnote 1370: _Publicationem_, Potsdam, No. 14, 1884, p. 31.]
[Footnote 1371: Von Konkoly _once_ derived from a slow-moving meteor a
hydro-carbon spectrum. A. S. Herschel, _Nature_, vol. xxiv., p. 507.]
[Footnote 1372: _Phil. Trans._, vol. cliv., p. 429.]
[Footnote 1373: _Am. Jour. of Sc._, vol. xix., p. 467.]
[Footnote 1374: _Photom. Unters._, p. 243.]
[Footnote 1375: _Spectre Solaire_, p. 38.]
[Footnote 1376: Mr. J. Birmingham, in the Introduction to his Catalogue
of Red Stars, adduced sundry instances of colour-change in a direction
the opposite to that assumed by Zöllner to be the inevitable result of
time. _Trans. R. Irish Acad._, vol. xxvi., p. 251. A learned discussion
by Dr. T. J. J. See, moreover, enforces the belief that Sirius was
absolutely _red_ eighteen hundred years ago. _Astr. and Astroph._, vol.
xi., p. 269.]
[Footnote 1377: _Phil. Trans.,_ vol. clxiv., p. 492.]
[Footnote 1378: _Astr. Nach._, No. 2,000.]
[Footnote 1379: _Proc. Roy. Soc._, vols. xvi., p. 31; xvii., p. 48.]
[Footnote 1380: _Annalen der Physik_, Bd. xx., p. 155.]
[Footnote 1381: _Ibid._, p. 153.]
[Footnote 1382: _Knowledge_, vol. xiv., p. 101.]
[Footnote 1383: _Meteoritic Hypothesis_, p. 380.]
[Footnote 1384: _Phil. Trans._, vol. cxci. A., p. 128; _Spectra of
Southern Stars_, p. 3.]
[Footnote 1385: See the author's _System of the Stars_, p. 84.]
[Footnote 1386: A designation applied by Sir Norman Lockyer to
third-type stars.]
[Footnote 1387: See _ante_, p. 198.]
[Footnote 1388: _Bothkamp Beobachtungen_, Heft ii., p. 146.]
[Footnote 1389: _Astr. Nach._, No. 2,539.]
[Footnote 1390: _Ibid._, No. 2,548; _Observatory_, vol. vi., p. 332.]
[Footnote 1391: _Month. Not._, vol. xlvii., p. 92.]
[Footnote 1392: _Publ. Astr. Pac. Soc._, vol. i., p. 80; _Observatory_,
vol. xiii., p. 46.]
[Footnote 1393: _Lockyer, Proc. Roy. Soc._, vol. lvii., p. 173.]
[Footnote 1394: _Astr. Nach._, No. 3,129.]
[Footnote 1395: _Month. Not._, vol. lix., p. 505.]
[Footnote 1396: _Astr. Nach._, No. 2,581.]
[Footnote 1397: _Ibid._, Nos. 2,651-2.]
[Footnote 1398: _Ibid._, No. 3,051; _Astr. and Astrophysics_, vol. xi.,
p. 25; Bélopolsky, _Astr. Nach._, No. 3,129.]
[Footnote 1399: _Comptes Rendus_, t. lxv., p. 292.]
[Footnote 1400: _Copernicus_, vol. iii., p. 207.]
[Footnote 1401: _System of the Stars_, p. 70; _Harvard Annals_, vol.
xxviii., pt. ii., p. 243 (Miss Cannon).]
[Footnote 1402: _Potsdam Publ._, No. 14, p. 17.]
[Footnote 1403: _Proc. Roy. Soc._, vol. xlix., p. 33.]
[Footnote 1404: Miss A. J. Cannon, _Harvard Annals_, vol. xxviii., pt.
ii., p. 141.]
[Footnote 1405: _Astr. and Astroph._, vol. xiii., p. 448.]
[Footnote 1406: _Potsdam Publ._, No. 2.]
[Footnote 1407: The results of Von Konkoly's extension of Vogel's work
to 15° of south declination were published in _O Gyalla Beobachtungen_,
Bd. viii., Th. ii., 1887.]
[Footnote 1408: _Astroph. Jour._, vols. viii., p. 237; ix., p. 271.]
[Footnote 1409: _Ibid._, vol. ix., p. 119.]
[Footnote 1410: _Phil. Trans._, vol. cliv., p. 413. Some preliminary
results were embodied in a "note" communicated to the Royal Society,
February 19, 1863 (_Proc. Roy. Soc._, vol. xii., p. 444).]
[Footnote 1411: _Bothkamp Beob._, Heft i., p. 25.]
[Footnote 1412: _Astroph. Jour._, vol. vi., p. 423.]
[Footnote 1413: _Phil. Trans._, vol. cliv., p. 429, _note_.]
[Footnote 1414: _Month. Not._, vol. xxiii., p. 180.]
[Footnote 1415: _Proc. Roy. Soc._, vol. xxv., p. 446.]
[Footnote 1416: _Phil. Trans._, vol. clxxi., p. 669; _Atlas of Stellar
Spectra_, p. 22.]
[Footnote 1417: _Astr. Nach._, No. 2,301; _Monatsb._, Berlin, 1879, p.
119; 1880, p. 192.]
[Footnote 1418: _Jour. de Physique_, t. v., p. 98.]
[Footnote 1419: _System of the Stars_, p. 39.]
[Footnote 1420: See _ante_, p. 198.]
[Footnote 1421: _Proc. Roy. Soc._, vol. xlviii., p. 314.]
[Footnote 1422: _Harvard Circulars_, Nos. 12, 18; _Astroph. Jour._, vol.
v., p. 92.]
[Footnote 1423: _Astroph. Jour._, vol. vi., p. 233.]
[Footnote 1424: McClean, _Phil. Trans._, vol. cxci. A., p. 129.]
[Footnote 1425: _Proc. Roy. Soc._, vol. lxii., p. 417.]
[Footnote 1426: _Ibid._, April 27, 1899; _Astroph. Jour._, vol. x., p.
272.]
[Footnote 1427: _Astr. Nach._, No. 3,565.]
[Footnote 1428: _Ibid._, No. 3,583.]
[Footnote 1429: Lunt, _Astroph. Jour._, vol. xi., p. 262; _Proc. Roy.
Soc._, vol. lxvi., p. 44; Lockyer, _ibid._, November 23, 1899; _Nature_,
vol. lxi., p. 263.]
[Footnote 1430: _Die Spectralanalyse_, p. 314.]
[Footnote 1431: _Henry Draper Memorial, First Ann. Report_, 1887.]
[Footnote 1432: _Mem. Amer. Acad._, vol. xi., p. 215.]
[Footnote 1433: _Harvard Annals_, vol. xxvii.]
[Footnote 1434: _Harvard Annals_, vol. xxviii., parts i. and ii.]
[Footnote 1435: See _ante_, p. 201.]
[Footnote 1436: _Phil. Trans._, vol. clviii., p. 529.]
[Footnote 1437: Schellen, _Die Spectralanalyse_, Bd. ii., p. 326 (ed.
1883).]
[Footnote 1438: _Proc. Roy. Soc._, vol. xx., p. 386.]
[Footnote 1439: _System of the Stars_, p. 199.]
[Footnote 1440: Pickering, _Am. Jour. of Sc._, vol. xxxix., p. 46;
Vogel, _Astr. Nach._ No. 3,017.]
[Footnote 1441: _Sitzungsberichte_, Berlin, May 2, 1901; _Astroph.
Jour._, vol. xiii., p. 324.]
[Footnote 1442: The "relative orbit" of a double star is that described
by one round the other as a fixed point. Micrometrical measures are
always thus executed. But in reality both stars move in opposite
directions, and at rates inversely as their masses round their common
centre of gravity.]
[Footnote 1443: Vogel, _Astr. Nach._, Nos. 3,017, 3,039.]
[Footnote 1444: Huggins, _Pres. Address_, 1891; Cornu, _Sur la Méthode
Doppler-Fizeau_ p. D. 38.]
[Footnote 1445: _Sitzungsb._, Berlin, 1890, p. 401; _Astr. Nach._, No.
2,995.]
[Footnote 1446: _Ibid._]
[Footnote 1447: _Astroph. Jour._, vol. v., p. 1; Newall, _Month. Not._,
vol. lvii., p. 575.]
[Footnote 1448: _Bull. de l'Acad. de St. Pétersb._, tt. vi., viii.]
[Footnote 1449: _Astroph. Jour._, vol. x., p. 177; _Month. Not._, vol.
lx., p. 418; Vogel, _Sitzungsb._, Berlin, April 19, 1900.]
[Footnote 1450: _Month. Not._, vol. lx., p. 595.]
[Footnote 1451: Hussey, _Astr. Jour._, No. 484.]
[Footnote 1452: _Astroph. Jour._, vols. x, p. 180; xiv., p. 140; _Lick
Bulletin_, No. 4; Bélopolsky, _Astr. Nach._, No. 3,637.]
[Footnote 1453: The significance of the name "El Ghoul" leaves little
doubt that the Arab astronomers took note of this star's variability. E.
M. Clerke, _Observatory_, vol. xv., p. 271.]
[Footnote 1454: _Phil. Trans._, vol. lxxiii., p. 484.]
[Footnote 1455: _Proc. Amer. Acad._, vol. xvi., p. 17; _Observatory_,
vol. iv., p. 116. For a preliminary essay by T. S. Aldis, see _Phil.
Mag._, vol. xxxix., p. 363, 1870.]
[Footnote 1456: _Astr. Nach._, No. 2,947.]
[Footnote 1457: _Astr. Jour._, Nos. 165-6, 255-6, 509. See also
_Knowledge_, vol. xv., p. 186.]
[Footnote 1458: Bauschinger, _V. J. S. Astr. Ges._, Jahrg. xxix.; but
_cf._ Searle, _Harvard Annals_, vol. xxix., p. 223; Boss, _Astr. Jour._,
No. 343.]
[Footnote 1459: _Comptes Rendus_, t. cxx., p. 125.]
[Footnote 1460: Myers, _Astroph. Jour._, vol. vii., p. 1; A. W. Roberts,
_Ibid._, vol. xiii., p. 181.]
[Footnote 1461: _Proc. R. Irish Ac._, July, 1884.]
[Footnote 1462: _Ibid._, vol. i., p. 97.]
[Footnote 1463: _Astr. Jour._, Nos. 179, 180.]
[Footnote 1464: _Ibid._, Nos. 300, 379.]
[Footnote 1465: _Astr. Jour._, Nos. 491-2.]
[Footnote 1466: _System of the Stars_, p. 125.]
[Footnote 1467: _Proc. Roy. Soc._, vol. xv., p. 146.]
[Footnote 1468: Weiss, _Astr. Nach._, No. 1,590; Espin, _Ibid._, No.
3,200.]
[Footnote 1469: _Comptes Rendus_, t. lxxxiii., p. 1172.]
[Footnote 1470: _Monatsb._, Berlin, 1877, pp. 241, 826.]
[Footnote 1471: _Copernicus_, vol. ii., p. 101.]
[Footnote 1472: Burnham, _Month. Not._, vol. lii., p. 457.]
[Footnote 1473: _Astr. Nach._, No. 2,682.]
[Footnote 1474: A. Hall, _Am. Jour. of Sc._, vol. xxxi., p. 301.]
[Footnote 1475: Young, _Sid. Messenger_, vol. iv., p. 282; Hasselberg,
_Astr. Nach._, No. 2,690.]
[Footnote 1476: _Report Brit. Assoc._, 1885, p. 935.]
[Footnote 1477: _Month. Not._, vol. xlvii., p. 54.]
[Footnote 1478: _Nature_, vol. xxxii., p. 522.]
[Footnote 1479: _Astr. Nach._, Nos. 1,267, 2,715.]
[Footnote 1480: _Month. Not._, vol. xxi., p. 32.]
[Footnote 1481: _Observatory_, vol. viii., p. 335.]
[Footnote 1482: _Astr. Nach._, No. 3,118; _Astr. and Astroph._, vol.
xi., p. 907.]
[Footnote 1483: _Cape Results_, p. 137.]
[Footnote 1484: _Trans. R. Soc. of Edinburgh_, vol. xxvii., p. 51;
_Astr. and Astroph._, August, 1892, p. 593.]
[Footnote 1485: Vogel, _Astr. Nach._, No. 3,079.]
[Footnote 1486: _Observatory_, vol. xv., p. 287; Seeliger, _Astr.
Nach._, No. 3,118; _Astr. and Astroph._, vol. xi., p. 906.]
[Footnote 1487: Ranyard, _Knowledge_, vol. xv., p. 110.]
[Footnote 1488: _Proc. Roy. Soc._, vol. li., p. 492.]
[Footnote 1489: Burnham, _Month. Not._, vol. liii., p. 58.]
[Footnote 1490: _Astr. Nach._, Nos. 3,118, 3,143.]
[Footnote 1491: Renz, _Ibid._, Nos. 3,119, 3,238; Huggins, _Astr. and
Astroph._, vol. xiii., p. 314.]
[Footnote 1492: _Astr. Nach._, No. 3,111.]
[Footnote 1493: Bélopolsky, _Astr. Nach._, No. 3,120.]
[Footnote 1494: _Nature_, September 15, 1892.]
[Footnote 1495: _Astr. Nach._, Nos. 3,122, 3,129.]
[Footnote 1496: _Ibid._, No. 3,133; _Astr. and Astroph._, vol. xi., p.
715.]
[Footnote 1497: _Publ. Astr. Pac. Soc._, vol. iv., p. 244.]
[Footnote 1498: Barnard, _Astroph. Jour._, vol. xiv., p. 152; Campbell,
_Observatory_, vol. xxiv., p. 360.]
[Footnote 1499: _Pop. Astr._, March, 1895, p. 307.]
[Footnote 1500: _Harvard Circular_, No. 4, December 20, 1895. The first
Nova Persei was spectrographically recorded in 1887.]
[Footnote 1501: Vogel, _Sitzungsb._, Berlin, April 19, 1900, p. 389.]
[Footnote 1502: Sidgreaves, _Observatory_, vol. xxiv., p. 191.]
[Footnote 1503: _Ibid._, _Knowledge_, vol. xxv., p. 10.]
[Footnote 1504: _Lick Bulletin_, No. 8.]
[Footnote 1505: _Astr. Nach._, No. 3,736.]
[Footnote 1506: _Astroph. Jour._, vol. xiv., p. 167.]
[Footnote 1507: _Lick Bulletin_, No. 10.]
[Footnote 1508: _Astroph. Jour._, vols. xiv., p. 293; xv., p. 129.]
[Footnote 1509: _Cf._ the theories on the subject of M. Wolf, _Astr.
Nach._, Nos. 3,752, 3,753; Kapteyn, _Ibid._, No. 3,756; F. W. Very,
_Ibid._, No. 3,771; and W. E. Wilson, _Proc. Roy. Dublin Soc._, No. 45,
p. 556.]
[Footnote 1510: _Phil. Trans._, vol. cliv., p. 437.]
[Footnote 1511: _Phil. Trans._, vol. clviii., p. 540. The true
proportion seems to be about one-tenth (_Harvard Annals_, vol. xxvi.,
pt. ii., p. 205), the Tulse Hill working-list having been formed of
specially selected objects.]
[Footnote 1512: Scheiner, _Astr. Nach._, No. 3,476; _Astroph. Jour._,
vol. vii., p. 231; Campbell, _Ibid._, vols. ix., p. 312; x., p. 22.]
[Footnote 1513: _Proc. Roy. Soc._, vols. xlvi., p. 40; xlviii., p. 202.]
[Footnote 1514: _Publ. Astr. Pac. Soc._, vol. ii., p. 265; _Proc. Roy.
Soc._, vol. xlix., p. 399.]
[Footnote 1515: _Astr. Nach._, No 3,549.]
[Footnote 1516: _Atlas of Stellar Spectra_, p. 125.]
[Footnote 1517: _Knowledge_, vol. xix., p. 39.]
[Footnote 1518: _Astr. Nach._, Nos. 1,366, 1,391, 1,689; Chambers,
_Descriptive Astr._ (3rd ed.), p. 543; Flammarion, _L'Univers Sidéral_,
p. 818.]
[Footnote 1519: _Month. Not._, vol. li., p. 94.]
[Footnote 1520: _Ibid._, vol. lix., p. 372.]
[Footnote 1521: _Ibid._, vol. lx., p. 424.]
[Footnote 1522: Dreyer, _Ibid._, vol. lii., p. 100.]
[Footnote 1523: _Wash. Obs._, vol. xxv., App. 1.]
[Footnote 1524: _Am. Jour. of Sc._, vol. xiv., p. 433; C. Dreyer,
_Month. Not._, vol. xlvii., p. 419.]
[Footnote 1525: _Ibid._, vol. li., p. 496.]
[Footnote 1526: Reproduced in _Knowledge_, April, 1893.]
[Footnote 1527: Unless an exception be found in the Pleiades nebulæ,
which may be assumed to share the small apparent movement of the stars
they adhere to.]
[Footnote 1528: _Abhandl. Akad. der Wiss._, Leipzig, 1857, Bd. iii., p.
295.]
[Footnote 1529: _Month. Not._, vol. lii., p. 31.]
[Footnote 1530: _Proc. Roy. Soc._, 1874, p. 251.]
[Footnote 1531: _Publ. Astr. Pac. Soc._, vol. ii., p. 278.]
[Footnote 1532: _System of the Stars_, p. 257.]
[Footnote 1533: _Proc. Roy. Soc._, vol. xlix., p. 399.]
[Footnote 1534: _Potsdam Publ._, Bd. vii., Th. i.]
[Footnote 1535: _Astr. Nach._, No. 2,714; Schönfeld, _V. J. S. Astr.
Ges._, Jahrg. xxi., p. 58.]
[Footnote 1536: _Astroph. Journ._, vol. xiii., p. 80.]
[Footnote 1537: _Proc. Roy. Soc._, vol. xxxiii., p. 425; _Report Brit.
Assoc._, 1882, p. 444. An impression of the four lower lines in the same
spectrum was almost simultaneously obtained by Dr. Draper. _Comptes
Rendus_, t. xciv., p. 1243.]
[Footnote 1538: _Proc. Roy. Soc._, vol. xlviii., p. 213.]
[Footnote 1539: _Month. Not._, vol. xlviii., p. 360.]
[Footnote 1540: _Proc. Roy. Soc._, vol. xlvi., p. 40; _System of the
Stars_, p. 79.]
[Footnote 1541: _Sitzungsb._, Berlin, February 13, 1890.]
[Footnote 1542: _Wash. Obs._, vol. xxv., App. i., p. 226.]
[Footnote 1543: _Comptes Rendus_, t. xcii., p. 261.]
[Footnote 1544: _Month. Not._, vol. xliii., p. 255.]
[Footnote 1545: _Harvard Annals_, vol. xviii., p. 116.]
[Footnote 1546: _Sid. Mess._, vol. ix., p. 1.]
[Footnote 1547: _Knowledge_, vol. xv., p. 191.]
[Footnote 1548: _Month. Not._, vol. xlix., p. 65.]
[Footnote 1549: _System of the Stars_, p. 269.]
[Footnote 1550: _Astr. Nach._, Nos. 2,749, 2,754.]
[Footnote 1551: Vogel, _Astr. Nach._, 2,854.]
[Footnote 1552: _Nature_, vol. xliii., p. 419.]
[Footnote 1553: _L'Astronomie_, t. xl., p. 171.]
[Footnote 1554: _Astr. Nach._, Bände xlvii., p. 1; xlviii., p. 1; xlix.,
p. 81. Pickering, _Mem. Am. Ac._, vol. xi., p. 180.]
[Footnote 1555: Gould on Celestial Photography, _Observatory_, vol. ii.,
p. 16.]
[Footnote 1556: _Annals N. Y. Acad. of Sciences_, vol. vi., p. 239,
1892; Elkin, _Publ. Astr. Pac. Soc._, vol. iv., p. 134.]
[Footnote 1557: _Trans. Yale Observatory_, vol. i., pt. i.]
[Footnote 1558: _Astroph. Jour._, vol. xiii., p. 56.]
[Footnote 1559: _Astr. Nach._, No. 2,719.]
[Footnote 1560: _Ibid._, No. 2,726.]
[Footnote 1561: _Ibid._, No. 2,730.]
[Footnote 1562: _Month. Not._, vol. xlvii., p. 24.]
[Footnote 1563: _Les Mondes_, t. iii., p. 529.]
[Footnote 1564: Mouchez, _Comptes Rendus_, t. cvi., p. 912.]
[Footnote 1565: _Astr. Nach._, No. 3,422.]
[Footnote 1566: _Ibid._, No. 3,441.]
[Footnote 1567: _Ibid._, Nos. 3,018, 3,032.]
[Footnote 1568: _Journ. Brit. Astr. Assoc._, vol. ix., p. 133.]
[Footnote 1569: _Astr. Nach._, No. 3,253.]
[Footnote 1570: _Observatory_, vol. xxi., pp. 351, 386.]
[Footnote 1571: Reproduced in _Astroph. Journ._, vol. xi., p. 324.]
[Footnote 1572: _Ibid._, p. 347.]
[Footnote 1573: _Astr. Nach._, No. 3,704.]
[Footnote 1574: _Sitzungsb. Bayer. Akad._, March 23, 1901.]
[Footnote 1575: _Annals of the Cape Observatory_, vols. iii., iv., v.]
[Footnote 1576: _Month. Not._, vol. lx., p. 381.]
[Footnote 1577: D. Klumpke, _Observatory_, vol. xv., p. 305.]
[Footnote 1578: Gilbert, _Sid. Mess._, vol. i., p. 288.]
[Footnote 1579: _Astr. Papers for the Amer. Ephemeris_, vol. viii., pt.
ii.]
[Footnote 1580: _Nature_, vol. xxiv., p. 91; _Dunsink Observations_, pt.
v., 1884.]
[Footnote 1581: Elkin, _Report for 1891-92_, p. 25; Newcomb, _The
Stars_, p. 151.]
[Footnote 1582: _Annals of the Cape Observatory_, vol. viii., pt. ii.
Some of the measures were made by Messrs. Finlay and de Sitter.]
[Footnote 1583: _Astr. Nach._, No. 3,483; _Observatory_, vol. xxi., p.
180.]
[Footnote 1584: _Annalen der Sternwarte in Leiden_, Bd. vii.]
[Footnote 1585: _Report of Harvard Conference in 1898_ (Snyder).]
[Footnote 1586: _Researches in Stellar Parallax_, pt. ii., 1892.]
[Footnote 1587: _V. J. S. Astr. Ges._, Jahrg., xxviii., p. 117.]
[Footnote 1588: _Bulletin de la Carte du Ciel_, No. 1, p. 262.]
[Footnote 1589: _Publ. of the Astr. Laboratory at Groningen_, No. 1.]
[Footnote 1590: _Nature_, vol. xxvi., p. 177.]
[Footnote 1591: _Proc. R. Irish Acad._, vol. i., p. 571, ser. iii.]
[Footnote 1592: _Mem. R. A. S._, vol. xlvii., p. 178.]
[Footnote 1593: _Astr. Nach._, No. 3,142.]
[Footnote 1594: _Publ. Astr. Pac. Soc._, No. 76.]
[Footnote 1595: Campbell, _Lick Bulletin_, No. 4.]
[Footnote 1596: _Publ. Yerkes Observatory_, vol. i., 1900.]
[Footnote 1597: _Annals Cape Observatory_, vol. ii., pt. ii.]
[Footnote 1598: _Astr. Jour._, Nos. 431-2.]
[Footnote 1599: W. J. Hussey, _Publ. Astr. Pac. Soc._, No. 74.]
[Footnote 1600: _Astr. Jour._, No. 258.]
[Footnote 1601: _Sitzungsberichte_, Berlin, October 26, 1893.]
[Footnote 1602: _Annales de l'Obs. de Nice_, t. ii.]
[Footnote 1603: _Washington Observations_, 1888, App. i.]
[Footnote 1604: _Publ. Lick Observatory_, vol. v., 1901.]
[Footnote 1605: T. Lewis, _Observatory_, vol. xvi., p. 312.]
[Footnote 1606: _Harvard Annals_, vol. xiv., pt. i., 1884.]
[Footnote 1607: _Observatory_, vol. viii., p. 309.]
[Footnote 1608: _Month. Not._, vol. xlvi., p. 277.]
[Footnote 1609: _Harvard Annals_, vol. xxxiv.]
[Footnote 1610: _Ibid._, vol. xlv.]
[Footnote 1611: _Carte Phot. du Ciel. Réunion du Comité Permanent_,
Paris, 1891, p. 100.]
[Footnote 1612: _Essays_ (2nd ser.), _The Nebular Hypothesis_.]
[Footnote 1613: _On the Plurality of Worlds_, p. 214 (2nd ed.).]
[Footnote 1614: Proctor, _Month. Not._, vol. xxix., p. 342.]
[Footnote 1615: This remark was first made by J. Michell, _Phil.
Trans._, vol. lvii., p. 25 (1767).]
[Footnote 1616: _Pop. Astr._, No. 45.]
[Footnote 1617: _Astroph. Jour._, vol. i., p. 220.]
[Footnote 1618: _Month. Not._, vols. xxxi., p. 175; xxxii., p. 1.]
[Footnote 1619: _The Stars_, p. 273.]
[Footnote 1620: _System of the Stars_, p. 384; _Old and New Astronomy_,
p. 749 (Ranyard).]
[Footnote 1621: _Astroph. Jour._, vol. xii., p. 156.]
[Footnote 1622: _Publ. Astr. Pac. Soc._, vol. ii., p. 242.]
[Footnote 1623: _Month. Not._, vol. li., pp. 40, 97. For reproductions
of some of the photographs in question, see _Knowledge_, vol. xiv., p.
50.]
[Footnote 1624: _Astr. Nach._, No. 3,048; _Observatory_, vol. xiv., p.
301.]
[Footnote 1625: _Proc. Roy. Inst._, May 29, 1891 (Gill).]
[Footnote 1626: _Annals Cape Obs._, iii., Introduction, p. 22.]
[Footnote 1627: _Proc. Roy. Soc._, vol. xviii., p. 169.]
[Footnote 1628: _Astr. Nach._, No. 3,456; _Observatory_, vol. xxi., p.
65; Newcomb, _The Stars_, p. 80.]
[Footnote 1629: _Month. Not._, vol. xl., p. 249.]
CHAPTER XIII
_METHODS OF RESEARCH_
Comparing the methods now available for astronomical inquiries with
those in use forty years ago, we are at once struck with the fact that
they have multiplied. The telescope has been supplemented by the
spectroscope and the photographic camera. Now, this really involves a
whole world of change. It means that astronomy has left the place where
she dwelt apart in rapt union with mathematics, indifferent to all
things on earth save only to those mechanical improvements which should
aid her to penetrate further into the heavens, and has descended into
the forum of human knowledge, at once a suppliant and a patron,
alternately invoking help from and promising it to each of the sciences,
and patiently waiting upon the advances of all. The science of the
heavenly bodies has, in a word, become a branch of terrestrial physics,
or rather a higher kind of integration of all their results. It has,
however, this leading peculiarity, that the materials for the whole of
its inquiries are telescopically furnished. They are such as come very
imperfectly, or not at all, within the cognisance of the unarmed eye.
Spectroscopic and photographic apparatus are simply additions to the
telescope. They do not supersede or render it of less importance. On the
contrary, the efficacy of their action depends primarily upon the
optical qualities of the instruments they are attached to. Hence the
development, to their fullest extent, of the powers of the telescope is
of vital moment to the progress of modern physical astronomy, while the
older mathematical astronomy could afford to remain comparatively
indifferent to it.
The colossal Rosse reflector still marks, as to size, the _ne plus
ultra_ of performance in that line. A mirror four feet in diameter was,
however, sent out to Melbourne by the late Thomas Grubb of Dublin in
1870. This is mounted in the Cassegrainian manner, so that the observer
looks straight through it towards the object viewed, of which he really
sees a twice-reflected image. The dust-laden atmosphere of Melbourne is
said to impede very seriously the usefulness of this originally fine
instrument.
It may be doubted whether so large a spectrum will ever again be
constructed. A new material for the mirrors of reflecting telescopes,
proposed by Steinheil in 1856, and independently by Foucault in
1857,[1630] has in a great measure superseded the use of a metallic
alloy. This is glass upon which a thin film of silver has been deposited
by a chemical process originally invented by Liebig. It gives a
peculiarly brilliant reflective surface, throwing back more light than a
metallic mirror of the same area, in the proportion of about sixteen to
nine. Resilvering, too, involves much less risk and trouble than
repolishing a speculum. The first use of this plan on a large scale was
in an instrument of thirty-six inches aperture, finished by Calver for
Dr. Common in 1879. To its excellent qualities turned to account with
rare skill, his triumphs in celestial photography are mainly due. A more
daring experiment was the construction and mounting, by Dr. Common
himself, of a 5-foot reflector. But the first glass disc ordered from
France for the purpose proved radically defective. When figured,
polished, and silvered, towards the close of 1888, it gave elliptical
instead of circular star-images.[1631] A new one had to be procured, and
was ready for astronomical use in 1891. The satisfactory nature of its
performance is vouched for by the observations made with it upon
Jupiter's new satellite in December, 1892. This instrument, to which a
Newtonian form has been given, had no rival in respect of
light-concentration at the time when it was built. It has now two--the
Paris 50-inch refractor and the Yerkes 5-foot reflector.
It is, however, in the construction of refracting telescopes that the
most conspicuous advances have recently been made. The Harvard College
15-inch achromatic was mounted and ready for work in June, 1847. A
similar instrument had already for some years been in its place at
Pulkowa. It was long before the possibility of surpassing these
masterpieces of German skill presented itself to any optician. For
fifteen years it seemed as if a line had been drawn just there. It was
first transgressed in America. A portrait-painter of Cambridgeport,
Massachusetts, named Alvan Clark, had for some time amused his leisure
with grinding lenses, the singular excellence of which was discovered in
England by Mr. Dawes in 1853.[1632] Seven years passed, and then an
order came from the University of Mississippi for an object-glass of the
unexampled size of eighteen inches. An experimental glance through it to
test its definition resulted, as we have seen, in the detection of the
companion of Sirius, January 31, 1862. It never reached its destination
in the South. War troubles supervened, and it was eventually sent to
Chicago, where it served Professor Hough in his investigations of
Jupiter, and Mr. Burnham in his scrutiny of double stars.
The next step was an even longer one, and it was again taken by a
self-taught optician, Thomas Cooke, the son of a shoemaker at
Allerthorpe, in the East Riding of Yorkshire. Mr. Newall of Gateshead
ordered from him in 1863 a 25-inch object-glass. It was finished early
in 1868, but at the cost of shortening the life of its maker, who died
October 19, 1868, before the giant refractor he had toiled at for five
years was completely mounted. This instrument, the fine qualities of
which had long been neutralized by an unfavourable situation, was
presented by Mr. Newall to the University of Cambridge, a few weeks
before his death, April 21, 1889. Under the care of his son, Mr. Frank
Newall, it has proved highly efficient in the delicate work of measuring
stellar radial motions.
Close upon its construction followed that of the Washington 26-inch, for
which twenty thousand dollars were paid to Alvan Clark. The most
illustrious point in its career, entered upon in 1873, has been the
discovery of the satellites of Mars. Once known to be there, these were,
indeed, found to be perceptible with very moderate optical means (Mr.
Wentworth Erck saw Deimos with a 7-inch Clark); but the first detection
of such minute objects is a feat of a very different order from their
subsequent observation. Dr. See's perception with this instrument, in
1899, of Neptune's cloud-belts, and his refined series of micrometrical
measures of the various planets, attest the unimpaired excellence of its
optical qualities.
It held the primacy for more than eight years. Then, in December, 1880,
the place of honour had to be yielded to a 27-inch achromatic, built by
Howard Grubb (son and successor of Thomas Grubb) for the Vienna
Observatory. This, in its turn, was surpassed by two of respectively
29-1/2 and 30 inches, sent by Gautier of Paris to Nice, and by Alvan
Clark to Pulkowa; and an object-glass, three feet in diameter, was in
1886 successfully turned out by the latter firm for the Lick Observatory
in California. The difficulties, however, encountered in procuring discs
of glass of the size and purity required for this last venture seemed to
indicate that a term to progress in this direction was not far off. The
flint was, indeed, cast with comparative ease in the workshops of M.
Feil at Paris. The flawless mass weighed 170 kilogrammes, was over 38
inches across, and cost 10,000 dollars. But with the crown part of the
designed achromatic combination things went less smoothly. The
production of a perfect disc was only achieved after _nineteen_
failures, involving a delay of more than two years; and the glass for a
third lens, designed to render the telescope available at pleasure for
photographic purposes, proved to be strained, and consequently went to
pieces in the process of grinding. It has been replaced by one of 33
inches, with which a series of admirable lunar and other photographs
have been taken.
Nor is the difficulty in obtaining suitable material the only obstacle
to increasing the size of refractors. The "secondary spectrum," as it is
called, also interposes a barrier troublesome to surmount. True
achromatism cannot be obtained with ordinary flint and crown-glass; and
although in lenses of "Jena glass," outstanding colour is reduced to
about one-sixth its usual amount, their term of service is fatally
abridged by rapid deterioration. Nevertheless, a 13-inch objective of
the new variety was mounted at Königsberg in 1898; and discs of Jena
crown and flint, 23 inches across, were purchased by Brashear at the
Chicago Exhibition of 1893. An achromatic combination of three kinds of
glass, devised by Mr. A. Taylor[1633] for Messrs. Cooke of York, has
less serious drawbacks, but has not yet come into extensive use.
Meanwhile, in giant telescopes affected to the full extent by chromatic
aberration, such as the Lick and Yerkes refractors, the differences of
focal length for the various colours are counted by inches,[1634] and
this not through any lack of skill in the makers, but by the necessity
of the case. Embarrassing consequences follow. Only a small part of the
spectrum of a heavenly body, for instance, can be distinctly seen at one
time; and a focal adjustment of half an inch is required in passing from
the observation of a planetary nebula to that of its stellar nucleus. A
refracting telescope loses, besides, one of its chief advantages over a
reflector when its size is increased beyond a certain limit. That
advantage is the greater luminosity of the images given by it.
Considerably more light is transmitted through a glass lens than is
reflected from an equal metallic surface. But only so long as both are
of moderate dimensions. For the glass necessarily grows in thickness as
its area augments, and consequently stops a larger percentage of the
rays it refracts. So that a point at length arrives--fixed by the late
Dr. Robinson at a diameter a little short of 3 feet[1635]--where the
glass and the metal are, in this respect, on an equality; while above it
the metal has the advantage. And since silvered glass gives back
considerably more light than speculum metal, the stage of equalisation
with lenses is reached proportionately sooner where this material is
employed.[1636]
The most distinctive faculty of reflectors, however, is that of bringing
rays of all refrangibilities to a focus together. They are naturally
achromatic. None of the beams they collect are thrown away in
colour-fringes, obnoxious both in themselves and as a waste of the chief
object of astrophysicists' greed--light. Reflectors, then, are in this
respect specially adapted to photographic and spectrographic use. But
they have a countervailing drawback. The penalties imposed by bigness
are for them peculiarly heavy. Perfect definition becomes with
increasing size, more and more difficult to attain; once attained, it
becomes more and more difficult to keep. For the huge masses of material
employed to form great object-glasses or mirrors tend with every
movement to become deformed by their own weight. Now, the slightest
bending of a mirror is fatal to its performance, the effect being
doubled by reflection; while in a lens alteration of figure is
compensated by the equal and contrary flexures of the opposing surfaces,
so that the emergent beams pursue much the same paths as if the curves
of the refracting medium had remained theoretically perfect. For this
reason work of precision must remain the province of refracting
telescopes, although great reflectors retain the primacy in the
portraiture of the heavenly bodies, as well as in certain branches of
spectroscopy. Professor Hale, accordingly, summarised a valuable
discussion on the subject by asserting[1637] "that the astrophysicist
may properly consider the reflector to be an even more important part of
his instrumental equipment than the refractor." A new era in its
employment west of the Atlantic opened with the transfer from Halifax to
Mount Hamilton of the Crossley reflector. Its prerogatives in nebular
photography were splendidly indicated in 1899 by Professor Keeler's
exquisite and searching portrayals of the cloud-worlds of space, and
those obtained two years later, with a similar, though smaller,
instrument, by Professor Ritchey of the Yerkes Observatory, were fully
comparable with them. The performances of the Yerkes 5-foot reflector
still belong to the future.
Ambition as regards telescopic power is by no means yet satisfied. Nor
ought it to be. The advance of astrophysical researches of all kinds
depends largely upon light-grasp. For the spectroscopic examination of
stars, for the measurement of their motions in the line of sight, for
the discovery and study of nebulæ, for stellar and nebular photography,
the cry continually is "more light." There is no enterprising head of an
observatory but must feel cramped in his designs if he can command no
more than 14 or 15 inches of aperture, and he aspires to greater
instrumental capacity, not merely with a view to the chances of
discovery, but for the steady prosecution of some legitimate line of
inquiry. Thus projects of telescope-building on a large scale are rife,
and some obtain realisation year by year. Sir Howard Grubb finished in
1893 a 28-inch achromatic for the Royal Observatory, Greenwich; the
Thompson equatoreal, mounted at the same establishment in 1897, carries
on a single axis a 26-inch photographic refractor and a 30-inch
silvered-glass reflector; the Victoria telescope, inaugurated at the
Cape in 1901, comprises a powerful spectrographic apparatus, together
with a chemically corrected 24-inch refractor, the whole being the
munificent gift of Mr. Frank McClean; at Potsdam, at Meudon, at Paris,
at Alleghany, engines for light-concentration have been, or shortly will
be, erected of dimensions which, two generations back, would have seemed
extravagant and impossible.
Perhaps the finest, though not absolutely the greatest, among them,
marked the summit and end of the performances of Alvan G. Clark, the
last survivor of the Cambridgeport firm.
In October, 1892, Mr. Yerkes of Chicago offered an unlimited sum for the
provision of the University of that city with a "superlative" telescope.
And it happened, fortunately, that a pair of glass discs, nearly 42
inches in diameter, and of perfect quality, were ready at hand. They had
been cast by Mantois for the University of Southern California, when the
erection of a great observatory on Wilson's Peak was under
consideration. In the Clark workshop they were combined into a superb
objective, brought to perfection by trials and delicate touches
extending over nearly five years. Then the maker accompanied it to its
destination, by the shore of a far Western Lake Geneva, and died
immediately after his return, June 9, 1897. Nor has the implement of
celestial research he just lived to complete been allowed to "rust
unburnished." Manipulated by Hale, Burnham, and Barnard, it has done
work that would have been impracticable with less efficient optical aid.
Its construction thus marks a noticeable enlargement of astronomical
possibilities, exemplified--to cite one among many performances--by
Barnard's success in keeping track of cluster-variables when below the
common limit of visual perception.
With the Lick telescope results have also been achieved testifying to
its unsurpassed excellence. Holden's and Schaeberle's views of planetary
nebulæ, Burnham's and Hussey's hair's-breadth star-splitting operations,
Keeler's measurements of nebular radial motion, Barnard's detections and
prolonged pursuit of faint comets, his discovery of Jupiter's tiny moon,
Campbell's spectroscopic determinations--all this could only have been
accomplished, even by an exceptionally able and energetic staff, with
the aid of an instrument of high power and quality. But there was
another condition which should not be overlooked.
The best telescope may be crippled by a bad situation. The larger it is,
indeed, the more helpless is it to cope with atmospheric troubles. These
are the worst plagues of all those that afflict the astronomer. No
mechanical skill avails to neutralise or alleviate them. They augment
with each increase of aperture; they grow with the magnifying powers
applied. The rays from the heavenly bodies, when they can penetrate the
cloud-veils that too often bar their path, reach us in an enfeebled,
scattered, and disturbed condition. Hence the twinkling of stars, the
"boiling" effects at the edges of sun, moon, and planets; hence
distortions of bright, effacements of feeble telescopic images; hence,
too, the paucity of the results obtained with many powerful
light-gathering machines.
No sooner had the Parsonstown telescope been built than it became
obvious that the limit of profitable augmentation of size had, under
climatic conditions at all nearly resembling those prevailing there,
been reached, if not overpassed; and Lord Rosse himself was foremost to
discern the need of pausing to look round the world for a clearer and
stiller air than was to be found within the bounds of the United
Kingdom. With this express object Mr. Lassell transported his 2-foot
Newtonian to Malta in 1852, and mounted there, in 1860, a similar
instrument of fourfold capacity, with which, in the course of about two
years, 600 new nebulæ were discovered. Professor Piazzi Smyth's
experiences during a trip to the Peak of Teneriffe in 1856 in search of
astronomical opportunities[1638] gave countenance to the most sanguine
hopes of deliverance, at suitable elevated stations, from some of the
oppressive conditions of low-level star-gazing; yet for a number of
years nothing effectual was done for their realisation. Now, at last,
however, mountain observatories are not only an admitted necessity but
an accomplished fact; and Newton's long forecast of a time when
astronomers would be compelled, by the developed powers of their
telescopes, to mount high above the "grosser clouds" in order to use
them,[1639] had been justified by the event.
James Lick, the millionaire of San Francisco, had already chosen, when
he died, October 1, 1876, a site for the new observatory, to the
building and endowment of which he had devoted a part of his large
fortune. The situation of the establishment is exceptional and splendid.
Planted on one of the three peaks of Mount Hamilton, a crowning summit
of the Californian Coast Range, at an elevation of 4,200 feet above the
sea, in a climate scarce rivalled throughout the world, it commands
views both celestial and terrestrial which the lover of nature and
astronomy may alike rejoice in. Impediments to observation are there
found to be most materially reduced. Professor Holden, who was
appointed, in 1885, president of the University of California and
director of the new observatory affiliated to it, stated that during six
or seven months of the year an unbroken serenity prevails, and that half
the remaining nights are clear.[1640] The power of continuous work thus
afforded is of itself an inestimable advantage; and the high visual
excellences testified to by Mr. Burnham's discovery, during a two
months' trip to Mount Hamilton in the autumn of 1879, of forty-two new
double stars with a 6-inch achromatic, gave hopes since fully realised
of a brilliant future for the Lick establishment. Its advantages are
shared, as Professor Holden desired them to be, by the whole
astronomical world.[1641] A sort of appellate jurisdiction was at once
accorded to the great equatoreal, and more than one disputed point has
been satisfactorily settled by recourse to it.
Its performances, considered both as to quality and kind, are unlikely
to be improved upon by merely outbidding it in size, unless the care
expended upon the selection of its site be imitated. Professor Pickering
thus showed his customary prudence in reserving his efforts to procure a
great telescope until Harvard College owned a dependent observatory
where it could be employed to advantage. This was found by Mr. W. H.
Pickering, after many experiments in Colorado, California, and Peru, at
Arequipa, on a slope of the Andes, 8,000 feet above the sea-level. Here
the post provided for by the "Boyden Fund" was established in 1891,
under ideal meteorological conditions. Temperature preserves a "golden
mean"; the barometer is almost absolutely steady; the yearly rainfall
amounts to no more than three or four inches. No wonder, then, that the
"seeing" there is of the extraordinary excellence attested by Mr.
Pickering's observations. In the absence of bright moonlight, he tells
us,[1642] eleven Pleiades can always be counted; the Andromeda nebula
appears to the naked eye conspicuously bright, and larger than the full
moon; third magnitude stars have been followed to their disappearance at
the true horizon; the zodiacal light spans the heavens as a complete
arch, the "Gegenschein" forming a regular part of the scenery of the
heavens. Corresponding telescopic facilities are enjoyed. The chief
instrument at the station, a 13-inch equatoreal by Clark, shows the
fainter parts of the Orion nebula, photographed at Harvard College in
1887, by which the dimensions given to it in Bond's drawing are doubled;
stars are at times seen encircled by half a dozen immovable diffraction
rings, up to twelve of which have been counted round Alpha Centauri;
while on many occasions no available increase of magnifying power
availed to bring out any wavering in the limbs of the planets. Moreover,
the series of fine nights is nearly unbroken from March to November.
The facilities thus offered for continuous photographic research
rendered feasible Professor Bailey's amazing discovery of variable
star-clusters. They belong exclusively to the "globular" class, and the
peculiarity is most strikingly apparent in the groups known as
Omega Centauri, and Messier 3, 5, and 15. A large number of their minute
components run through perfectly definite cycles of change in periods
usually of a few hours.[1643] Altogether, about 500 "cluster-variables"
have been recorded since 1895. It should be mentioned that Mr. David
Packer and Dr. Common discerned, about 1890, some premonitory symptoms
of light-fluctuation among the crowded stars of Messier 5.[1644] With
the Bruce telescope, a photographic doublet 24 inches in diameter, a
store of 5,686 negatives was collected at Arequipa between 1896 and
1901. Some were exposed directly, others with the intervention of a
prism; and all are available for important purposes of detection or
investigation.
Vapours and air-currents do not alone embarrass the use of giant
telescopes. Mechanical difficulties also oppose a formidable barrier to
much further growth in size. But what seems a barrier often proves to be
only a fresh starting-point; and signs are not wanting that it may be
found so in this case. It is possible that the monumental domes and huge
movable tubes of our present observatories will, in a few decades, be as
much things of the past as Huygens's "aerial" telescopes. It is certain
that the thin edge of the wedge of innovation has been driven into the
old plan of equatoreal mounting.
M. Loewy, the present director of the Paris Observatory, proposed to
Delaunay in 1871 the direction of a telescope on a novel system. The
design seemed feasible, and was adopted; but the death of Delaunay and
the other untoward circumstances of the time interrupted its execution.
Its resumption, after some years, was rendered possible by M.
Bischoffsheim's gift of 25,000 francs for expenses, and the _coudé_ or
"bent" equatoreal has been, since 1882, one of the leading instruments
at the Paris establishment.
Its principle is briefly this: The telescope is, as it were, its own
polar axis. The anterior part of the tube is supported at both ends, and
is thus fixed in a direction pointing towards the pole, with only the
power of twisting axially. The posterior section is joined on to it at
right angles, and presents the object-glass, accordingly, to the
celestial equator, in the plane of which it revolves. Stars in any other
part of the heavens have their beams reflected upon the object-glass by
means of a plane rotating mirror placed in front of it. The observer,
meanwhile, is looking steadfastly down the bent tube towards the
invisible _southern_ pole. He would naturally see nothing whatever were
it not that a second plane mirror is fixed at the "elbow" of the
instrument, so as to send the rays which have traversed the object-glass
to his eye. He never needs to move from his place. He watches the stars,
seated in an arm-chair in a warm room, with as perfect convenience as if
he were examining the seeds of a fungus with a microscope. Nor is this a
mere gain of personal ease. The abolition of hardship includes a vast
accession of power.[1645]
Among other advantages of this method of construction are, first, that
of added stability, the motion given to the ordinary equatoreal being
transferred, in part, to an auxiliary mirror. Next, that of increased
focal length. The fixed part of the tube can be made almost indefinitely
long without inconvenience, and with enormous advantage to the optical
qualities of a large instrument. Finally, the costly and unmanageable
cupola is got rid of, a mere shed serving all purposes of protection
required for the "coudé."
The desirability of some such change as that which M. Loewy has realised
has been felt by others. Professor Pickering sketched, in 1881, a plan
for fixing large refractors in a permanently horizontal position, and
reflecting into them, by means of a shifting mirror, the objects desired
to be observed.[1646] The observations for his photometric catalogues
are, in fact, made with a "broken transit," in which the line of sight
remains permanently horizontal, whatever the altitude of the star
examined. Sir Howard Grubb, moreover, set up, in 1882, a kind of
siderostat at the Crawford Observatory, Cork. In a paper read before the
Royal Society, January 21, 1884, he proposed to carry out the principle
on a more extended scale;[1647] and shortly afterwards undertook its
application to a telescope 18 inches in aperture for the Armagh
Observatory.[1648] The chief honours, however, remain to the Paris
inventor. None of the prognosticated causes of failure have proved
effective. The loss of light from the double reflection is
insignificant. The menaced deformation of images is, through the
exquisite skill of the MM. Henry in producing plane mirrors of all but
absolute perfection, quite imperceptible. The definition was admitted to
be singularly good. Sir David Gill stated in 1884 that he had never
measured a double star so easily as he did Gamma Leonis by its
means.[1649] Sir Norman Lockyer pronounced it to be "one of the
instruments of the future"; and the principle of its construction was
immediately adopted by the directors of the Besançon and Algiers
Observatories, as well as for a 17-inch telescope destined for a new
observatory at Buenos Ayres. At Paris, it has since been carried out on
a larger scale. A "coudé," of 23-1/2 inches aperture and 62 feet focal
length was in 1890 installed at the National Observatory, and has served
M. Loewy for his ingenious studies on refraction and aberration--above
all, for taking the magnificent plates of his lunar atlas. The "bent"
form is capable of being, but has not yet been, adapted to
reflectors.[1650]
The "coelostat," in the form given to it by Professor Turner, has
proved an invaluable adjunct to eclipse-equipments. It consists
essentially of a mirror rotating in forty-eight hours on an axis in its
own plane, and parallel to the earth's axis. In the field of a telescope
kept rigidly pointed towards such a mirror, stars appear immovably
fixed. The employment of long-focus lenses for coronal photography is
thus facilitated, and the size of the image is proportional to the
length of the focus. Professor Barnard, accordingly, depicted the
totality of 1900 with a horizontal telescope 61-1/2 feet long, fed by a
mirror 18 inches across, the diameter of the moon on his plates being 7
inches. The largest siderostat in the world is the Paris 50-inch
refractor, which formed the chief attraction of the Palais d'Optique at
the Exhibition of 1900. It has a focal length of nearly 200 feet, and
can be used either for photographic or for visual purposes.
Celestial photography has not reached its grand climacteric; yet its
earliest beginnings already seem centuries behind its present
performances. The details of its gradual yet rapid improvement are of
too technical a nature to find a place in these pages. Suffice it to say
that the "dry-plate" process, with which such wonderful results have
been obtained, appears to have been first made available by Sir William
Huggins in photographing the spectrum of Vega in 1876, and was then
successively adopted by Common, Draper, and Janssen. Nor should Captain
Abney's remarkable extension of the powers of the camera be left
unnoticed. He began his experiments on the chemical action of red and
infra-red rays in 1874, and at length succeeded in obtaining a
substance--the "blue" bromide of silver--highly sensitive to these
slower vibrations of light. With its aid he explored a vast, unknown,
and for ever invisible region of the solar spectrum, presenting to the
Royal Society, December 5, 1879,[1651] a detailed map of its infra-red
portion (wave-lengths 7,600 to 10,750), from which valuable inferences
may yet be derived as to the condition of the various kinds of matter
ignited in the solar atmosphere. Upon plates rendered "orthochromatic"
by staining with alizarine, or other dye-stuffs, the whole visible
spectrum can now be photographed; but those with their maximum of
sensitiveness near G are found preferable, except where the results of
light-analysis are sought to be completely recorded. And since
photographic refractors are corrected for the blue rays, exposures with
them of orthochromatic surfaces would be entirely futile.
The chemical plate has two advantages over the human retina:[1652]
First, it is sensitive to rays which are utterly powerless to produce
any visual effect; next, it can accumulate impression almost
indefinitely, while from the retina they fade after one-tenth part of a
second, leaving it a continually renewed _tabula rasa_.
It is, accordingly, quite possible to photograph objects so faint as to
be altogether beyond the power of any telescope to reveal--witness the
chemical disclosure of the invisible nebula encircling Nova Persei--and
we may thus eventually learn whether a blank space in the sky truly
represents the end of the stellar universe in that direction, or whether
farther and farther worlds roll and shine beyond, veiled in the
obscurity of immeasurable distance.
Of many ingenious improvements in spectroscopic appliances the most
fundamentally important relate to what are known as "gratings." These
are very finely striated surfaces, by which light-waves are brought to
interfere, and are thus sifted out, strictly according to their
different lengths, into "normal" spectra. Since no universally valid
measures can be made in any others, their production is quite
indispensable to spectroscopic science. Fraunhofer, who initiated the
study of the diffraction spectrum, used a real grating of very fine
wires: but rulings on glass were adopted by his successors, and were by
Nobert executed with such consummate skill that a single square inch of
surface was made to contain 100,000 hand-drawn lines. Such rare and
costly triumphs of art, however, found their way into very few hands,
and practical availability was first given to this kind of instrument by
the inventiveness and mechanical dexterity of two American
investigators. Both Rutherfurd's and Rowland's gratings are
machine-ruled, and reflect instead of transmitting the rays they
analyse; but Rowland's present to them a very much larger diffractive
surface, and consequently possess a higher resolving power. The first
preliminary to his improvements was the production, in 1882, of a
faultless screw, those previously in use having been the inevitable
source of periodical errors in striation, giving, in their turn,
ghost-lines as subjects of spectroscopic study.[1653] Their abolition
was not one of Rowland's least achievements. With his perfected machine
a metallic area of 6-1/4 by 4-1/4 inches can be ruled with exquisite
accuracy to almost any degree of fineness; he considered, however,
43,000 lines to the inch to be the limit of usefulness.[1654] The ruled
surface is, moreover, concave, and hence brings the spectrum to a focus
without a telescope. A slit and an eye-piece are alone needed to view
it, and absorption of light by glass lenses is obviated--an advantage
especially sensible in dealing with the ultra- or infra-visible rays.
The high qualities of Rowland's great photographic map of the solar
spectrum were thus based upon his previous improvement of the
instrumental means used in its execution. The amount of detail shown in
it is illustrated by the appearance on the negatives of 150 lines
between H and K; and many lines depict themselves as double which, until
examined with a concave grating, had passed for one and indivisible. A
corresponding hand-drawing, for which M. Thollon received in 1886 the
Lalande Prize, exhibits, not the diffractive, but the prismatic spectrum
as obtained with bisulphide of carbon prisms of large dispersive power.
About one-third of the visible gamut of the solar radiations (A to _b_)
is covered by it; it includes 3,200 lines, and is over ten metres
long.[1655] The grating is an expensive tool in the way of light. Where
there is none to spare, its advantages must be foregone. They could not,
accordingly, be turned to account in stellar spectroscopy until the Lick
telescope was at hand to supply more abundant material for research. By
the use thus made possible of Rowland's gratings, Professor Keeler was
able to apply enormous dispersion to the rays of stars and nebulæ, and
so to attain a previously unheard-of degree of accuracy in their
measurement. His memorable detection of nebular movement in line of
sight ensued as a consequence. Professor Campbell, his successor, has
since obtained, by the same means, the first satisfactory photographs of
stellar diffraction-spectra.
The means at the disposal of astronomers have not multiplied faster than
the tasks imposed upon them. Looking back to the year 1800, we cannot
fail to be astonished at the change. The comparatively simple and serene
science of the heavenly bodies known to our predecessors, almost perfect
so far as it went, incurious of what lay beyond its grasp, has developed
into a body of manifold powers and parts, each with its separate mode
and means of growth, full of strong vitality, but animated by a restless
and unsatisfied spirit, haunted by the sense of problems unsolved, and
tormented by conscious impotence to sound the immensities it perpetually
confronts.
Knowledge might be said, when the _Mécanique Céleste_ issued from the
press, to be bounded by the solar system; but even the solar system
presented itself under an aspect strangely different from what it now
wears. It consisted of the sun, seven planets, and twice as many
satellites, all circling harmoniously in obedience to a universal law,
by the compensating action of which the indefinite stability of their
mutual relations was secured. The occasional incursion of a comet, or
the periodical presence of a single such wanderer chained down from
escape to outer space by planetary attraction, availed nothing to impair
the symmetry of the majestic spectacle.
Now, not alone the ascertained limits of the system have been widened by
a thousand millions of miles, with the addition of one more giant planet
and seven satellites to the ancient classes of its members, but a
complexity has been given to its constitution baffling description or
thought. Five hundred circulating planetary bodies bridge the gap
between Jupiter and Mars, the complete investigation of the movements of
any one of which would overtask the energies of a lifetime. Meteorites,
strangers, apparently, to the fundamental ordering of the solar
household, swarm, nevertheless, by millions in every cranny of its
space, returning at regular intervals like the comets so singularly
associated with them, or sweeping across it with hyperbolic velocities,
brought, perhaps, from some distant star. And each of these cosmical
grains of dust has a theory far more complex than that of Jupiter; it
bears within it the secret of its origin, and fulfils a function in the
universe. The sun itself is no longer a semi-fabulous, fire-girt globe,
but the vast scene of the play of forces as yet imperfectly known to us,
offering a boundless field for the most arduous and inspiring
researches. Among the planets the widest variety in physical habitudes
is seen to prevail, and each is recognised as a world apart, inviting
inquiries which, to be effective, must necessarily be special and
detailed. Even our own moon threatens to break loose from the trammels
of calculation, and commits "errors" which sap the very foundations of
the lunar theory, and suggest the formidable necessity for its complete
revision. Nay, the steadfast earth has forfeited the implicit confidence
placed in it as a time-keeper, and questions relating to the stability
of the earth's axis and the constancy of the earth's rate of rotation
are among those which it behoves the future to answer. Everywhere there
is multiformity and change, stimulating a curiosity which the rapid
development of methods of research offers the possibility of at least
partially gratifying.
Outside the solar system, the problems which demand a practical solution
are virtually infinite in number and extent. And these have all arisen
and crowded upon our thoughts within less than a hundred years. For
sidereal science became a recognised branch of astronomy only through
Herschel's discovery of the revolutions of double stars in 1802. Yet
already it may be, and has been called, "the astronomy of the future,"
so rapidly has the development of a keen and universal interest attended
and stimulated the growth of power to investigate this sublime subject.
What has been done is little--is scarcely a beginning; yet it is much in
comparison with the total blank of a century past. And our knowledge
will, we are easily persuaded, appear in turn the merest ignorance to
those who come after us. Yet it is not to be despised, since by it we
reach up groping fingers to touch the hem of the garment of the Most
High.
FOOTNOTES:
[Footnote 1630: _Comptes Rendus_, t. xliv., p. 339.]
[Footnote 1631: A. A. Common, _Memoirs R. Astr. Soc._, vol. i., p. 118.]
[Footnote 1632: Newcomb, _Pop. Astr._, p. 137.]
[Footnote 1633: _Month. Not._, vol. liv., p. 67.]
[Footnote 1634: Keeler, _Publ. Astr. Pac. Soc._, vol. ii., p. 160.]
[Footnote 1635: H. Grubb, _Trans. Roy. Dub. Soc._, vol. i. (new ser.),
p. 2.]
[Footnote 1636: Hale, nevertheless (_Astroph. Jour._, vol. v., p. 128),
considers that refractors preserve their superiority of visual
light-grasp over Newtonian reflectors up to an aperture of 52-1/2, while
equalisation is reached for the photographic rays at 34 inches.]
[Footnote 1637: _Astroph. Jour._, vol. v., p. 130.]
[Footnote 1638: _Phil. Trans._, vol. cxlviii., p. 465.]
[Footnote 1639: _Optics_, p. 107 (2nd ed., 1719).]
[Footnote 1640: _Observatory_, vol. viii., p. 85.]
[Footnote 1641: Holden on Celestial Photography, _Overland Monthly_,
Nov., 1886.]
[Footnote 1642: _Observatory_, vol. xv., p. 283.]
[Footnote 1643: Bailey, _Astroph. Jour._, vol. x., p. 255.]
[Footnote 1644: _Harvard Circulars_, Nos. 2, 18, 24, 33;]
[Footnote 1645: Loewy, _Bull. Astr._, t. i., p. 286; _Nature_, vol.
xxix., p. 36.]
[Footnote 1646: _Nature_, vol. xxiv., p. 389.]
[Footnote 1647: _Ibid._, vol. xxix., p. 470.]
[Footnote 1648: _Trans. R. Dublin Soc._, vol. iii., p. 61.]
[Footnote 1649: _Observatory_, vol. vii., p. 167.]
[Footnote 1650: Loewy, _Bull. Astr._, t. i., p. 265.]
[Footnote 1651: _Phil. Trans._, vol. clxxi., p. 653.]
[Footnote 1652: Janssen, _L'Astronomie_, t. ii., p. 121.]
[Footnote 1653: Rev. A. L. Cortie, _Astr. and Astrophysics_, vol. xi.,
p. 400.]
[Footnote 1654: _Phil. Mag._, vol. xiii., 1882, p. 469.]
[Footnote 1655: _Bull. Astr._, t. iii., p. 331.]
APPENDIX
TABLE I
CHRONOLOGY, 1774-1893
1774, March 4 Herschel's first observation. Subject, the Orion Nebula.
1774 Sun-spots geometrically proved to be depressions by
Wilson.
1774 First experimental determination of the earth's mean
density by Maskelyne.
1781, March 13 Discovery of Uranus.
1782 Herschel's first Catalogue of Double Stars.
1783 Herschel's first investigation of the sun's movement
in space.
1783 Goodricke's discovery of Algol's law of variation.
1784 Analogy between Mars and the Earth pointed out by
Herschel.
1784 Construction of the Heavens investigated by Herschel's
method of star-gauging. "Cloven-disc" plan of the
Milky Way.
1784 Discovery of binary stars anticipated by Michell.
1786 Herschel's first Catalogue of Nebulæ.
1787, Jan. 11 Discovery by Herschel of two Uranian moons
(Oberon and Titania).
1787, Nov. 19 Acceleration of the moon explained by Laplace.
1789 Herschel's second Catalogue of Nebulæ, and
classification by age of these objects.
1789 Completion of Herschel's forty-foot reflector.
1789, Aug. 28 His discovery with it of the two inner Saturnian
and Sept. 17 satellites.
1789 Repeating-circle invented by Borda.
1789 Five-foot circle constructed by Ramsden for Piazzi.
1790 Maskelyne's Catalogue of thirty-six fundamental stars.
1791 Herschel propounds the hypothesis of a fluid
constitution for nebulæ.
1792 Atmospheric refraction in Venus announced by Schröter.
1794 Rotation-period of Saturn fixed by Herschel at 10h. 16m.
1795 Herschel's theory of the solar constitution.
1796 Herschel's first measures of comparative stellar
brightness.
1796 Laplace's Nebular Hypothesis published in _Exposition
du Système du Monde_.
1797 Publication of Olbers's method of computing
cometary orbits.
1798 Retrograde motions of Uranian satellites detected
by Herschel.
1799 Publication of first two volumes of _Mécanique Céleste_.
1799, May 7 Transit of Mercury observed by Schröter.
1799, Nov. 12 Star-shower observed by Humboldt at Cumana.
1800 _Monatliche Correspondenz_ started by Von Zach.
1800 Invisible heat-rays detected in the solar spectrum
by Herschel.
1801, Jan. 1 Discovery of Ceres by Piazzi.
1801 Publication of Lalande's _Histoire Céleste_.
1801 Investigation by Herschel of solar emissive variability
in connection with spot-development.
1802, March 28 Discovery of Pallas by Olbers.
1802 Herschel's third Catalogue of Nebulæ.
1802 Herschel's discovery of binary stars.
1802 Marks of clustering in the Milky Way noted by Herschel.
1802 Wollaston records seven dark lines in the solar
spectrum.
1802, Nov. 9 Transit of Mercury observed by Herschel.
1804, Sept. 2 Discovery of Juno by Harding.
1804 Foundation of Optical Institute at Munich.
1805 Herschel's second determination of the solar apex.
1807, March 29 Discovery of Vesta by Olbers.
1811 Herschel's theory of the development of stars from
nebulæ.
1811, Feb. 9 Death of Maskelyne. Pond appointed to succeed him
as Astronomer-Royal.
1811, Sept. 12 Perihelion passage of great comet.
1812 Theory of electrical repulsion in comets originated
by Olbers.
1812, Sept. 15 Perihelion passage of Pons's comet.
1814 Herschel demonstrates the irregular distribution of
stars in space.
1815 Fraunhofer maps 324 dark lines in the solar spectrum.
1818 Publication of Bessel's _Fundamenta Astronomiæ_.
1819 Recognition by Encke of the first short-period comet.
1819, June 26 Passage of the earth through the tail of a comet.
1820 Foundation of the Royal Astronomical Society.
1821 Foundation of Paramatta Observatory.
1821, September First number of _Astronomische Nachrichten_.
1822, May 24 First calculated return of Encke's comet.
1822, August 25 Death of Herschel.
1823 Bessel introduces the correction of observations for
personal equation.
1823 Fraunhofer examines the spectra of fixed stars.
1824 Distance of the sun concluded by Encke to be
95-1/4 million miles.
1824 Publication of Lohrmann's Lunar Chart.
1824 Dorpat refractor mounted equatoreally.
1826 Commencement of Schwabe's observations of sun-spots.
1826, Feb. 27 Biela's discovery of a comet.
1827 Orbit of a binary star calculated by Savary.
1829 Completion of the Royal Observatory at the Cape of
Good Hope.
1829 The Königsberg heliometer mounted.
1830 Publication of Bessel's _Tabulæ Regiomontanæ_.
1832 Discovery by Brewster of "atmospheric lines" in the
solar spectrum.
1833 Magnetic observatory established at Göttingen.
1833, Nov. 12,13 Star-shower visible in North America.
1833 Completion of Sir J. Herschel's survey of the northern
heavens.
1834, Jan. 16 Sir J. Herschel's landing at the Cape.
1835, September Airy appointed Astronomer-Royal in succession to Pond.
1835, Nov. 16 Perihelion passage of Halley's comet.
1837 Solar movement determined by Argelander.
1837 Bessel's application of the heliometer to measurements
of stellar parallax.
1837 Publication of Beer and Mädler's _Der Mond_.
1837 Publication of Struve's _Mensuræ Micrometricæ_.
1837, Dec. 16 Outburst of Eta Carinæ observed by Sir J. Herschel.
1837 Thermal power of the sun measured by Herschel and
Pouillet.
1838 Parallax of 61 Cygni determined by Bessel.
1839, Jan. 9 Parallax of Alpha Centauri announced by Henderson.
1839 Completion of Pulkowa Observatory.
1839 Solidity of the earth concluded by Hopkins.
1840, March 2 Death of Olbers.
1840 First attempt to photograph the moon by J. W. Draper.
1842 Doppler enounces principle of colour-change by motion.
1842 Conclusion of Baily's experiments in weighing the Earth.
1842, July 8 Total solar eclipse. Corona and prominences observed
by Airy, Baily, Arago, and Struve.
1843, Feb. 27 Perihelion-passage of great comet.
1845, February Completion of Parsonstown reflector.
1845, April Discovery with it of spiral nebulæ.
1845, April 2 Daguerreotype of the sun taken by Foucault and Fizeau.
1845, Oct. 21 Place of Neptune assigned by Adams.
1845, Dec. 8 Discovery of Astræa by Hencke.
1845, Dec. 29 Duplication of Biela's comet observed at Yale College.
1846 Melloni's detection of heating effects from moonlight.
1846, March 17 Death of Bessel.
1846, Sept. 23 Discovery of Neptune by Galle.
1846, Oct. 10 Neptune's satellite discovered by Lassell.
1847 Publication of Sir J. Herschel's _Results of
Observations at the Cape of Good Hope_.
1847 Cyclonic theory of sun-spots stated by him.
1848 J. R. Mayer's meteoric hypothesis of solar conservation.
1848 Motion-displacements of Fraunhofer lines adverted to
by Fizeau.
1848, April 27 New Star in Ophiuchus observed by Hind.
1848, Sept. 19 Simultaneous discovery of Hyperion by Bond and Lassell.
1849 First experimental determination of the velocity of
light (Fizeau).
1850, July 17 Vega photographed at Harvard College.
1850, Nov. 15 Discovery by Bond of Saturn's dusky ring.
1851 O. Struve's first measurements of Saturn's ring-system
1851, July 28 Total solar eclipse observed in Sweden.
1851, Oct. 24 Discovery by Lassell of two inner Uranian satellites.
1851 Schwabe's discovery of sun-spot periodicity published by
Humboldt.
1852, May 6 Coincidence of magnetic and sun-spot periods announced
by Sabine.
1852, Oct. 11 Variable nebula in Taurus discovered by Hind.
1852 Lassell's two-foot reflector transported to Malta.
1853 Adams shows Laplace's explanation of the moon's
acceleration to be incomplete.
1854 Hansen infers from lunar theory a reduced value for
the distance of the sun.
1854 Helmholtz's "gravitation theory" of solar energy.
1856 Piazzi Smyth's observations on the Peak of Teneriffe.
1857 Saturn's rings shown by Clerk Maxwell to be of meteoric
formation.
1857, April 27 Double-star photography initiated at Harvard College.
1858 Solar photography begun at Kew.
1858, Sept. 30 Perihelion of Donati's comet.
1859 Spectrum analysis established by Kirchhoff and Bunsen.
1859 Carrington's discovery of the compound nature of the
sun's rotation.
1859, Sept. 1 Luminous solar outburst and magnetic storm.
1859, Oct. 19 Merope nebula discovered by Tempel.
1859, Dec. 15 Chemical constitution of the sun described by Kirchhoff.
1860, Feb. 27 Discovery by Liais of a "double comet."
1860, May 21 New star in Scorpio detected by Auwers.
1860, July 18 Total solar eclipse observed in Spain. Prominences
shown by photography to be solar appendages.
1861, June 30 The earth involved in the tail of a great comet.
1861-1862 Kirchhoff's map of the solar spectrum.
1862 Solar hydrogen-absorption recognised by Ångström.
1862, Jan. 31 Discovery by Alvan G. Clark of the companion of Sirius.
1862 Foucault determines the sun's distance by the velocity
of light.
1862 Opposition of Mars. Determination of solar parallax.
1862 Completion of _Bonner Durchmusterung_.
1863 Secchi's classification of stellar spectra.
1863 Foundation of the German Astronomical Society.
1864, March 5 Rotation period of Mars determined by Kaiser.
1864 Huggins's first results in stellar spectrum analysis.
1864, Aug. 5 Spectroscopic examination of Tempel's comet by
Donati shows it to be composed of glowing gas.
1864, Aug. 29 Discovery by Huggins of gaseous nebulæ.
1864 Value of 91,000,000 miles adopted for the sun's
distance.
1864 Croll's explanation of glacial epochs.
1864, Nov. 23 Death of Struve.
1865, Jan. 4 Spectroscopic observation by Huggins of the occultation
of Eta Piscium.
1865, Jan. 16 Faye's theory of the solar constitution.
1865 Kew results published.
1865 Zöllner argues for a high temperature in the great
planets.
1866 Identity of the orbits of the August meteors and
of comet 1862 iii. demonstrated by Schiaparelli.
1866 Delaunay explains lunar acceleration by a lengthening
of the day through tidal friction.
1866, March 4 Spectroscopic study of the sun's surface by Lockyer.
1866, March 12 New star in Corona Borealis detected by Birmingham.
1866, October Schmidt announces the disappearance of the lunar crater
Linné.
1866, Nov. 13 Meteoric shower visible in Europe.
1867 Period of November meteors determined by Adams.
1867, Aug. 29 Total solar eclipse. Minimum sun-spot type of corona
observed by Grosch at Santiago.
1867 Discovery of gaseous stars in Cygnus by Wolf and Rayet.
1868, February Principle of daylight spectroscopic visibility of
prominences started by Huggins.
1868, Aug. 18 Great Indian eclipse. Spectrum of prominences observed.
1868, Aug. 19 Janssen's first daylight view of a prominence.
1868, Oct. 26 Lockyer and Janssen independently announce their
discovery of the spectroscopic method.
1868 Doppler's principle applied by Huggins to measure
stellar radial movements.
1868 Publication of Ångström's map of the normal solar
spectrum.
1868 Spectrum of Winnecke's comet found by Huggins to agree
with that of olefiant gas.
1869, Feb. 11 Tenuity of chromospheric gases inferred by Lockyer and
Frankland.
1869, Feb. 13 Huggins observes a prominence with an "open slit."
1869, Aug. 7 American eclipse. Detection of bright-line coronal
spectrum.
1870 Mounting of Newall's 25-inch achromatic at Gateshead.
1870 Proctor indicates the prevalence of drifting movements
among the stars.
1870 A solar prominence photographed by Young.
1870, Dec. 22 Sicilian eclipse. Young discovers reversing layer.
1871, May 11 Death of Sir J. Herschel.
1871, June 9 Line-displacements due to solar rotation detected by
Vogel.
1871, Dec. 12 Total eclipse visible in India. Janssen observes
reflected Fraunhofer lines in spectrum of corona.
1872 Conclusion of a three years' series of observations on
lunar heat by Lord Rosse.
1872 Spectrum of Vega photographed by H. Draper.
1872 Faye's cyclonic hypothesis of sun-spots.
1872 Young's solar-spectroscopic observations at Mount
Sherman.
1872 Cornu's experiments on the velocity of light.
1872, Nov. 27 Meteoric shower connected with Biela's comet.
1873 Determination of mean density of the earth by
Cornu and Baille.
1873 Solar photographic work begun at Greenwich.
1873 Erection of 26-inch Washington refractor.
1874 Light-equation redetermined by Glasenapp.
1874 Vogel's classification of stellar spectra.
1874, Dec. 8 Transit of Venus.
1876 Publication of Neison's _The Moon_.
1876, Nov. 24 New star in Cygnus discovered by Schmidt.
1876 Spectrum of Vega photographed by Huggins. First
use of dry gelatine plates in celestial photography.
1877, May 19 Klein observes a supposed new lunar crater (Hyginus N.).
1877 Measurement by Vogel of selective absorption in solar
atmosphere.
1877, Aug. 16-17 Discovery of two satellites of Mars by Hall at
Washington.
1877, Sept. 23 Death of Leverrier.
1877 Canals of Mars discovered by Schiaparelli.
1877 Opposition of Mars observed by Gill at Ascension.
Solar parallax deduced = 8.78".
1878, January Stationary meteor-radiants described by Denning.
1878 Publication of Schmidt's
_Charte der Gebirge des Mondes_.
1878 First observations of Great Red Spot on Jupiter.
1878 Conclusion of Newcomb's researches on the lunar theory.
1878, May 6 Transit of Mercury.
1878 Foundation of Selenographical Society.
1878, July 29 Total eclipse visible in America. Vast equatoreal
extension of the corona.
1878, October Completion of Potsdam Astrophysical Observatory.
1878, Dec. 12 Lockyer's theory of celestial dissociation communicated
to the Royal Society.
1879 Michelson's experiments on the velocity of light.
1879 Publication of Gould's _Uranometria Argentina_.
1879, November Observations of the spectra of sun-spots begun at
South Kensington.
1879, Dec. 5 Abney's map of the infra-red solar spectrum presented
to the Royal Society.
1879, Dec. 18 Ultra-violet spectra of white stars described by
Huggins.
1879, Dec. 18 Communication of G. H. Darwin's researches into
the early history of the moon.
1880, Jan. 31 Discovery at Cordoba of a great southern comet.
1880 Conditions of Algol's eclipses determined by Pickering.
1880 Pickering computes mass-brightness of binary stars.
1880, Sept. 30 Draper's photograph of the Orion nebula.
1880 The bolometer invented by Langley.
1881, Jan. 20 Communication of G. H. Darwin's researches into the
effects of tidal friction on the evolution of the
solar system.
1881 Langley's observations of atmospheric absorption on
Mount Whitney.
1881, June 16 Perihelion of Tebbutt's comet.
1881, June 24 Its spectrum photographed by Huggins.
1881, June Photographs of Tebbutt's comet by Janssen and Draper.
1881, Aug. 15 Retirement of Sir George Airy. Succeeded by Christie.
1881, Aug. 22 Perihelion of Schaeberle's comet.
1881 Publication of Stone's Cape Catalogue for 1880.
1882 Struve's second measures of Saturn's ring-system.
1882 Newcomb's determination of the velocity of light.
Resulting solar parallax = 8·79".
1882 Correction by Nyrén of Struve's constant of aberration.
1882, March 7 Spectrum of Orion nebula photographed by Huggins.
1882, May 17 Total solar eclipse observed at Sohag in Egypt.
1882, May 27 Sodium-rays observed at Dunecht in spectrum of
Comet Wells.
1882, June 10 Perihelion of Comet Wells.
1882, Sept. 17 Perihelion of Great Comet. Daylight detection by Common.
Transit observed at the Cape.
1882, Sept. 18 Iron lines identified in spectrum by Copeland and
J. G. Lohse.
1882, September Photographs of comet taken at the Cape Observatory,
showing a background crowded with stars.
1882, Dec. 6 Transit of Venus.
1882 Duplication of Martian canals observed by Schiaparelli.
1882 Completion by Loewy at Paris of first equatoreal Coudé.
1882 Rigidity of the earth concluded from tidal observations
by G. H. Darwin.
1882 Experiments by Huggins on photographing the corona
without an eclipse.
1882 Publication of Holden's _Monograph of the Orion Nebula_.
1883, Jan. 30 Orion Nebula photographed by Common.
1883, May 6 Caroline Island eclipse.
1883, June 1 Great comet of 1882 observed from Cordoba at a distance
from the earth of 470 million miles.
1883 Parallaxes of nine southern stars measured by Gill
and Elkin.
1883 Catalogue of the spectra of 4,051 stars by Vogel.
1884, Jan. 25 Return to perihelion of Pons's comet.
1884 Photometric Catalogue by Pickering of 4,260 stars.
1884 Publication of Gore's Catalogue of Variable Stars.
1884 Publication of Faye's _Origine du Monde_.
1884, Oct. 4 Eclipse of the moon. Heat-phases measured by Boeddicker
at Parsonstown.
1884 Dunér's Catalogue of Stars with Banded Spectra.
1884 Backlund's researches into the movements of
Encke's comet.
1885, February Langley measures the lunar heat-spectrum.
1885 Publication of _Uranometria Nova Oxoniensis_.
1885, Aug. 17 New star in Andromeda nebula discerned by Gully.
1885, Sept. 5 Thollon's drawing of the solar spectrum presented to the
Paris Academy.
1885, Sept. 9 Solar eclipse visible in New Zealand.
1885, Nov. 16 Photographic discovery by Paul and Prosper Henry of
a nebula in the Pleiades.
1885, Nov. 27 Shower of Biela meteors.
1885 Thirty-inch achromatic mounted at Pulkowa.
1885 Publication of Rowland's photographic map of the normal
solar spectrum.
1885 Bakhuyzen's determination of the rotation period of
Mars.
1885 Stellar photographs by Paul and Prosper Henry.
1886, Jan. 26 Spectra of forty Pleiades simultaneously photographed
at Harvard College.
1886, Feb. 5 First visual observation of the Maia nebula with the
Pulkowa 30-inch refractor.
1886, March Photographs by the Henrys of the Pleiades, showing
2,326 stars with nebulæ intermixed.
1886, May Photographic investigations of stellar parallax
undertaken by Pritchard.
1886, May 6 Periodical changes in spectra of sun-spots announced
by Lockyer.
1886, June 4 An international Photographic Congress proposed by Gill.
1886, Aug. 29 Total eclipse of the sun observed at Grenada.
1886, Oct. 1 Roberts's photograph showing annular structure of
the Andromeda nebula.
1886, Dec. 8 Roberts's photograph of the Pleiades nebulosities.
1886 Solar heat-spectrum extended by Langley to below
five microns.
1886, Dec. 28 Detection by Copeland of helium-ray in spectrum of
the Orion nebula.
1886 Thirty-inch refractor mounted at Nice.
1886 Publication of Argentine General Catalogue.
1886 Completion of Auwers's reduction of Bradley's
observations.
1886 Draper Memorial photographic work begun at
Harvard College.
1886 Photographic detection at Harvard College of bright
hydrogen lines in spectra of variables (Mira Ceti
and U Orionis).
1887, Jan. 18 Discovery by Thome at Cordoba of a great comet
belonging to the same group as the comet of 1882.
1887 Publication of Lockyer's _Chemistry of the Sun_.
1887, April 16 Meeting at Paris of the International Astrophotographic
Congress.
1887 Heliometric triangulation of the Pleiades by Elkin.
1887 L. Struve's investigation of the sun's motion, and
redetermination of the constant of precession.
1887 Von Konkoly's extension to 15° S. Dec. of Vogel's
spectroscopic Catalogue.
1887 Auwers's investigation of the solar diameter.
1887 Publication of Schiaparelli's Measures of Double Stars
(1875-85).
1887, April 8 Death of Thollon at Nice.
1887, Aug. 19 Total eclipse of the sun. Shadow-path crossed Russia.
Observations marred by bad weather.
1887, November Langley's researches on the temperature of the moon.
1887, Nov. 17 Lockyer's _Researches on Meteorites_ communicated to
the Royal Society.
1887 Completion of 36-inch Lick refractor.
1888 Küstner's detection of variations in the latitude of
Berlin brought before the International Geodetic
Association.
1888 Chandler's first Catalogue of Variable Stars.
1888 Mean parallax of northern first magnitude stars
determined by Elkin.
1888 Publication of Dreyer's _New General Catalogue_ of
7,844 nebulæ.
1888 Vogel's first spectrographic determinations of stellar
radial motion.
1888 Carbon absorption recognised in solar spectrum by
Trowbridge and Hutchins.
1888, Jan. 28 Total eclipse of the moon. Heat-phases measured at
Parsonstown.
1888, Feb. 5 Remarkable photograph of the Orion nebula spectrum
taken at Tulse Hill.
1888, June 1 Activity of the Lick Observatory begun.
1888 Completion of Dr. Common's 5-foot reflector.
1888 Heliometric measures of Iris for solar parallax at the
Cape, Newhaven (U.S.A.), and Leipsic.
1888 Loewy describes a comparative method of determining
constant of aberration.
1888 Presentation of the Dunecht instrumental outfit to
the nation by Lord Crawford. Copeland succeeds
Piazzi Smyth as Astronomer-Royal for Scotland.
1888, Sept. 12 Death of R. A. Proctor.
1889 Photograph of the Orion nebula taken by W. H. Pickering,
showing it to be the nucleus of a vast spiral.
1889 Discovery at a Harvard College of the first-known
spectroscopic doubles, Zeta Ursæ Majoris and
Beta Aurigæ.
1889 Eclipses of Algol demonstrated spectrographically by
Vogel.
1889 Completion of photographic work for the Southern
Durchmusterung.
1889 Boeddicker's drawing of the Milky Way.
1889 Draper Memorial photographs of southern star-spectra
taken in Peru.
1889 Pernter's experiments on scintillation from the
Sonnblick.
1889 H. Struve's researches on Saturn's satellites.
1889 Harkness's investigation of the masses of Mercury,
Venus, and the Earth.
1889 Heliometric measures of Victoria and Sappho at the Cape.
1889, Jan. 1 Total solar eclipse visible in California.
1889, Feb. 7 Foundation of the Astronomical Society of the Pacific.
1889, March Investigation by Sir William and Lady Huggins of
the spectrum of the Orion nebula.
1889, July-Aug. First photographs of the Milky Way taken by Barnard.
1889, August 2 Observation by Barnard of four companions to Brooks's
comet.
1889, Nov. 1 Passage of Japetus behind Saturn's dusky ring observed
by Barnard.
1889, December Schiaparelli announces synchronous rotation and
revolution of Mercury.
1889, Dec. 22 Total eclipse of the sun visible in Guiana. Death of
Father Perry, December 27.
1889 Spectrum of Uranus investigated visually by Keeler,
photographically by Huggins.
1890 Long-exposure photographs of ring-nebula in Lyra.
1890 Determinations of the solar translation by L. Boss
and O. Stumpe.
1890 Schiaparelli finds for Venus an identical period of
rotation and revolution.
1890 Publication of Thollon's map of the solar spectrum.
1890 Bigelow's mathematical theory of coronal structures.
1890 Foundation of the British Astronomical Association.
1890 Measurements by Keeler at Lick of nebular radial
movements.
1890 Janssen's ascent of Mont Blanc, by which he ascertained
the purely terrestrial origin of the oxygen-absorption
in the solar spectrum.
1890 Newcomb's discussion of the transits of Venus of
1761 and 1769.
1890 Spiral structure of Magellanic Clouds displayed in
photographs taken by H. C. Russell of Sydney.
1890 Publication of the Draper Catalogue of Stellar Spectra.
1890, April 24 Spica announced by Vogel to be a spectroscopic binary.
1890, June Gore's Catalogue of computed Binaries.
1890, November Study by Sir William and Lady Huggins of the
spectra of Wolf and Rayet's stars in Cygnus.
1890, November Discovery by Barnard of a close nebulous companion
to Merope in the Pleiades.
1890, November McClean Spectrographs of the High and Low Sun.
1891 Capture-theory of comets developed by Callandreau,
Tisserand, and Newton.
1891 Dunér's spectroscopic researches on the sun's rotation.
1891 Preponderance of Sirian stars in the Milky Way
concluded by Pickering, Gill, and Kapteyn.
1891 Detection by Mrs. Fleming of spectral variations
corresponding to light-changes in Beta Lyræ.
1891 Establishment of the Harvard College Station at
Arequipa in Peru (height 8,000 feet).
1891 Variations of latitude investigated by Chandler.
1891 Prominence-photography set on foot by Hale at
Chicago and Deslandres at Paris.
1891 Schmidt's Theory of Refraction in the Sun.
1891, April Meeting at Paris of the Permanent Committee for the
Photographic Charting of the Heavens.
1891, May 9 Transit of Mercury.
1891, Aug. 19 Presidential Address by Huggins at the Cardiff Meeting
of the British Association.
1891, Dec. 10 Nova Aurigæ photographed at Harvard College.
1891, Dec. 20 Photographic maximum of Nova Aurigæ.
1891, Dec. 22 First photographic discovery of a minor planet by
Max Wolf at Heidelberg.
1892 Commencement of international photographic charting
work.
1892 Photographic determination by Scheiner of 833 stars
in the Hercules Cluster (M 13).
1892 Publication of Vogel's spectrographic determinations
for fifty-one stars.
1892 Publication of Pritchard's photographic parallaxes.
1892, Jan. 2 Death of Sir George Airy.
1892, Jan. 21 Death of Professor Adams.
1892, Feb. 1 Announcement by Anderson of the outburst of a new
star in Auriga.
1892, Feb. 5 Appearance of the largest sun-spot ever photographed
at Greenwich.
1892, March Photograph of Argo nebula taken by Gill in twelve hours.
1892, March 6 Discovery of a bright comet by Swift.
1892, June 29 Death of Admiral Mouchez. Succeeded by Tisserand
as director of the National Observatory, Paris.
1892, Aug. 4 Favourable Opposition of Mars.
1892, Aug. 17 Rediscovery at Lick of Nova Aurigæ.
1892, Sept. 9 Discovery by Barnard of Jupiter's inner satellite.
1892, Oct. 12 First photographic discovery of a comet by Barnard.
1892, Nov. 6 Discovery of Holmes's comet.
1892, Nov. 23 Shower of Andromede meteors visible in America.
1892 Poynting's Determination of the Earth's Mean Density.
1892 Dunér's Investigation of the System of upsilon Cygni.
1892 Photographic investigation by Deslandres of the
spectra of prominences.
1892 Photographs of the sun with faculæ and chromospheric
surroundings taken by Hale with a single exposure.
1892 Investigation by T. J. J. See of the ancient colour of
Sirius.
1892 Publication of T. J. J. See's Thesis on the Evolution
of Binary Systems.
1892 Chandler's theory of Algol's inequalities.
1892 Nebula in Cygnus photographically discovered by
Max Wolf.
1893, Jan. 28 Kapteyn's investigation of the structure of the stellar
universe.
1893, March 10 Gill announces his results from the Opposition of
Victoria, among them a solar parallax = 8.809".
1893, April 16 Total solar eclipse observed in South America and
West Africa.
1893 Publication of Kruger's _Catalog der Farbigen Sterne_.
1893 Conclusion of Boys's series of Experiments on the
Density of the Earth.
1893 Publication of _Cordoba Durchmusterung_, vol. i.
1893 Fabry shows comets to be dependents of the Solar System.
1893 Publication of Easton's _Voie Lactée_.
1893 Campbell detects bright H Alpha in Gamma Argûs and
Alcyone.
1893 Nova Normæ photographed July 10; discovered on
plates, October 26.
1893, May 28 Death of Professor Pritchard.
1893, July 27 Installation of 28-inch refractor at the Royal
Observatory, Greenwich.
1893, December Exterior nebulosities of Pleiades photographed by
Barnard.
1893, Dec. 6 Death of Rudolf Wolf.
1894, January Sun-spot maximum.
1894 Publication of Potsdam _Photometric Durchmusterung_,
part i.
1894 Publication of Roberts's _Celestial Photographs_,
vol. i.
1894 Wilson and Gray's determination of the sun's
temperature.
1894 Barnard's micrometric measures of asteroids.
1894 McClean's gift of an astrophysical outfit to the Cape
Observatory.
1894 Establishment of the Lowell Observatory at Flagstaff,
Arizona.
1894 Taylor's triple achromatic objective described.
1894, April 3 Discovery of Gale's Comet.
1894 Sampson's investigation of the sun's rotation.
1894, Oct. 20 Favourable opposition of Mars.
1894, Nov. 11 Transit of Mercury.
1894, December Howlett impugns the Wilsonian theory of sun-spots.
1894, Dec. 14 Death of A. Cowper Ranyard.
1895 Publication of Newcomb's _Astronomical Constants_.
1895 Bailey's Photometric Catalogue of 7,922 Southern Stars.
1895 Bailey's photographic discovery of variable star
clusters.
1895 Publication of E. W. Brown's _Lunar Theory_.
1895 Tisserand's theory of the inequalities of Algol.
1895 Stratonoff's determination of the sun's rotation from
photographs of faculæ.
1895 Binary character of Eta Aquilæ spectroscopically
recognised by Bélopolsky.
1895 Presentation of the Crossley reflector to the Lick
Observatory.
1895, March 23 Great nebula in Ophiuchus discovered photographically
by Barnard.
1895, March 25 Ramsay's capture of Helium.
1895, April Constitution of Saturn's rings spectrographically
demonstrated by Keeler.
1895 Binary character of Delta Cephei spectroscopically
detected by Bélopolsky.
1895, June 11 Death of Daniel Kirkwood.
1895, July 7 Death of F. W. G. Spörer.
1895, October Nova Carinæ spectrographically discovered by
Mrs. Fleming.
1895, Dec. 12 Nova Centauri spectrographically discovered by
Mrs. Fleming.
1895, Dec. 28 Death of John Russell Hind.
1896 Gill's Report on the Geodetic Survey of South Africa.
1896 Appearance of Loewy's Photographic Atlas of the Moon,
part i.
1896, January Fessenden's electrostatic theory of comets.
1896 Chandler's Third Catalogue of Variable Stars.
1896 Publication of Lick Observatory Photographic Atlas
of the Moon, part i.
1896, February Effects of pressure on wave-length described by
Humphreys and Mohler.
1896, April 5 Opening of new Scottish Royal Observatory on Blackford
Hill, Edinburgh.
1896, April Pickering's photometric determinations of light curves
of variable stars.
1896 One of the stars of Castor spectroscopically resolved
into two by Bélopolsky.
1896, May Third Astrographic Chart Conference at Paris.
1896, Aug. 9 Total eclipse of the sun visible in Novaya Zemlya.
Reversing layer photographed by Shackleton.
1896, Aug. 30 Death of Hubert A. Newton.
1896, Sept. 18 Death of Hippolyte Fizeau.
1896, Oct. 20 Death of F. Tisserand. Succeeded by Maurice Loewy.
1896, Nov. 13 Detection by Schaeberle of Procyon's missing satellite.
1896, Nov. 26 Death of Benjamin Apthorp Gould.
1896, November Second series of hydrogen-lines discovered by Pickering
in stellar spectra.
1896, December Zeeman's discovery of spectral modifications through
magnetic influence.
1896, December Oxygen-absorption identified in the sun by Runge and
Paschen.
1896 Study of lunar formations by Loewy and Puiseux.
1896 Mounting of the Mills spectrograph at the
Lick Observatory.
1897 Installation at Greenwich of the Thompson 26-inch
photographic refractor.
1897 Publication of Miss Maury's Discussion of the
Photographed Spectra of 681 Stars.
1897 Callandreau's researches on cometary disaggregation.
1897 Braun's determination of the earth's mean density.
1897 Tenuity of calcium vapour in chromosphere demonstrated
spectroscopically by Sir William and Lady Huggins.
1897 Completion at the Cape Observatory of McClean's
spectrographic survey of the heavens.
1897 Twenty-one Wolf-Rayet stars found by Mrs. Fleming
in Magellanic Cloud.
1897 Percival Lowell's _New Observations on the Planet
Mercury_ presented to the American Academy.
1897, April 8 McClean recognises oxygen-absorption in helium stars.
1897, May 9 Death of E. J. Stone, Radcliffe Observer.
1897, June 10 Death of Alvan G. Clark.
1897, June 18 Spectrum of a meteor photographed at Arequipa.
1897, Oct. 21 Inauguration of the Yerkes Observatory.
1897 Rabourdin's photographs of nebulæ with the Meudon
reflector.
1897 Dr. See's discoveries of Southern double stars with the
Lowell 24-inch refractor.
1898, Jan. 22 Total eclipse of the sun visible in India.
1898, February Binary character of Zeta Geminorum ascertained
spectroscopically by Bélopolsky.
1898 Star with proper motion of nearly 9" discovered by
Innes and Kapteyn from the Cape Durchmusterung plates.
1898, March 8 Nova Sagittarii photographed on Draper Memorial plates.
1898, June 20 Opening of Grand-ducal Observatory at Königsstuhl,
Heidelberg.
1898 Keeler succeeds Holden as Director of the Lick
Observatory.
1898 Bruno Peter's results in stellar parallax.
1898 Lewis Swift's discoveries of nebulæ at Echo Mountain,
California.
1898 Hale's photographic investigation of carbon stars.
1898, Aug. 14 Discovery of Eros by Witt.
1898 Flint's investigations of stellar parallax by meridian
differences.
1898 Easton's spiral theory of the Milky Way.
1898 Seeliger's research on star distribution.
1898, October Multiple hydrogen-bands observed by Campbell in
Mira Ceti.
1898, November Orbit of a Leonid meteor photographically determined
by Elkin.
1899 Publication of Potsdam _Photometric Durchmusterung_,
part ii.
1899 Innes's _Reference Catalogue of Southern Double Stars_.
1899 Keeler's photographs of nebulæ with the Crossley
reflector and generalization of their spiral
character.
1899, January Spectrum of Andromeda nebula photographed by Scheiner.
1899, April Photographic discovery of Nova Aquilæ by Mrs. Fleming.
1899, Aug. 26 Installation of 31-inch photographic refractor at
Potsdam.
1899 Campbell's detection of Polaris as spectroscopically
triple.
1899, October Duplicate discovery by Campbell and Newall of
Capella as a spectroscopic binary.
1899, Nov. 15 Failure of the Leonids. Deflection of the stream
predicted by Johnstone Stoney and Downing.
1899, December Publication of Sir William and Lady Huggins's _Atlas
of Representative Stellar Spectra_.
1899 Thirty-two-inch photographic refractor mounted at
Meudon.
1899 Issue of first volume of Potsdam measures of
international catalogue plates.
1900, Jan. 27 Kapteyn's determination of the apex of solar motion.
1900 Chase's measures for parallax of swiftly-moving stars.
1900 Publication of Gill's _Researches on Stellar Parallax_.
1900 Kapteyn proposes a method for a stellar parallax
Durchmusterung, and gives specimen results for
248 stars.
1900 Burnham's general catalogue of 1,290 double stars.
1900 Publication of the concluding volume of the _Cape
Photographic Durchmusterung_.
1900, May 28 Spanish-American total eclipse of the sun.
1900, July International Conference at Paris. Co-operation
arranged of fifty-eight observatories in measures of
Eros for solar parallax.
1900 Horizontal refractor, of 50 inches aperture, 197 feet
focus, installed in Paris Exhibition.
1900, Aug. 12 Death of Professor Keeler. Succeeded by Campbell
in direction of Lick Observatory.
1900, November Opposition of Eros.
1900 Publication of Roberts's _Celestial Photographs_,
vol. ii.
1900 Complete publication of Langley's researches on the
infra-red spectrum.
1900 Printing begun of Paris section of International
Photographic Catalogue.
1901, Feb. 22 Nova Persei discovered by Anderson.
1901, February Variability of Eros announced by Oppolzer.
1901, April 23 Apparition of a great comet at the Cape.
1901 Publication of Pickering's _Photometric Durchmusterung_.
1901 Miss Cannon's discussion of the spectra of 1,122
Southern stars.
1901 Kapteyn's investigation of mean stellar parallax.
1901 Campbell's determination of the sun's velocity.
1901 Porter's research on the solar motion in space.
1901 Bigelow's magnetic theory of the solar corona.
1901 Hussey's measurements of the Pulkowa double stars.
1901 Radial velocities of the components of Delta Equulei
measured at Lick.
1901, April 16 Death of Henry A. Rowland.
1901, June Nebular spectrum derived from Nova Persei.
1901, Aug. 23 Nebula near Nova Persei photographed by Max Wolf.
1901, Sept. 20 The same exhibited in spiral form on a plate taken by
Ritchey at the Yerkes Observatory.
1901, Nov. 8 Photograph taken by Perrine with the Crossley
reflector showed nebula in course of rapid change.
1901, Sept. 19 Unveiling of the McClean "Victoria" telescope at the
Royal Observatory, Cape of Good Hope.
1901 Sun-spot minimum.
TABLE II.
CHEMICAL ELEMENTS IN THE SUN (ROWLAND, 1891).
Arranged according to the number of their representative Lines in the
Solar Spectrum.
Iron (2000+). Neodymium. Cadmium.
Nickel. Lanthanum. Rhodium.
Titanium. Yttrium. Erbium.
Manganese. Niobium. Zinc.
Chromium. Molybdenum. Copper (2).
Cobalt. Palladium. Silver (2).
Carbon (200+). Magnesium (20+). Glucinum (2).
Vanadium. Sodium (11). Germanium.
Zirconium. Silicon. Tin.
Cerium. Strontium. Lead (1).
Calcium (75+). Barium. Potassium (1).
Scandium. Aluminium (4).
_Doubtful Elements._--Iridium, osmium, platinum, ruthenium, tantalum,
thorium, tungsten, uranium.
_Not in Solar Spectrum._--Antimony, arsenic, bismuth, boron, nitrogen
(vacuum tube), cæsium, gold, iridium, mercury, phosphorus, rubidium,
selenium, sulphur, thallium, praseodymium.
Oxygen was added to the solar ingredients by Runge and Paschen in 1896,
gallium by Hartley and Ramage in 1899. Lithium may be admitted
provisionally, and the chromospheric constituent helium takes rank,
since 1895, as a chemical element.
TABLE III.
EPOCHS OF SUN-SPOT MAXIMUM AND MINIMUM FROM 1610 TO 1901.
+----------+----------++----------+----------+----------+----------+
| Minima. | Maxima. || Minima. | Maxima. | Minima. | Maxima. |
+----------+----------++----------+----------+----------+----------+
| 1610.8 | 1615.5 || 1712.0 | 1718.2 | 1810.6 | 1816.4 |
| 1619.0 | 1626.0 || 1723.5 | 1727.5 | 1823.3 | 1829.9 |
| 1634.0 | 1639.5 || 1734.0 | 1738.7 | 1833.9 | 1837.2 |
| 1645.0 | 1649.0 || 1745.0 | 1750.3 | 1843.5 | 1848.1 |
| 1655.0 | 1660.0 || 1755.2 | 1761.5 | 1856.0 | 1860.1 |
| 1666.0 | 1675.0 || 1766.5 | 1769.7 | 1867.2 | 1870.6 |
| 1679.5 | 1685.0 || 1775.5 | 1778.4 | 1878.9 | 1884.0 |
| 1689.5 | 1693.0 || 1784.7 | 1788.1 | 1890.2 | 1894.0 |
| 1698.9 | 1705.5 || 1798.3 | 1804.2 | 1901.9 | |
+----------+----------++----------+----------+----------+----------+
TABLE IV.
MOVEMENTS OF SUN AND STARS.
1. Translation of Solar System.
+--------------------+-------------------------+-------+
| Apex of Movement. | Authority. | Date. |
+--------------------+-------------------------+-------+
| R. A. Dec. | | |
| | | |
| 277° 30' + 35° | Newcomb | 1898 |
| 273° 36' + 29° 30' | Kapteyn | 1901 |
| 279° + 46° | Porter | 1901 |
| 275° + 45° | Boss | 1901 |
| 277° 30' + 20° | Campbell (from stellar | 1902 |
| | spectroscopic measures) | |
+--------------------+-------------------------+-------+
| Velocity = 12·4 miles per second (Campbell). |
+------------------------------------------------------+
2. Stellar Velocities.
+---------------------+------------+-------------+---------------------+
| Name of Star. | Rate. | Direction. | Remarks. |
| | Miles per | | |
| | Sec. | | |
+---------------------+------------+-------------+---------------------+
| Delta Leporis | 58 | Receding | Campbell, 1901 |
| Eta Cephei | 54 | Approaching | " 1899 |
| Theta Canis Majoris | 60 | Receding | " 1901 |
| Iota Pegasi | 47 | Approaching | " " |
| Mu Sagittarii | 47 | Approaching | " " |
| Eta Andromedæ | 52 | Approaching | " " |
| Zeta Herculis | 44 | Approaching | Bélopolsky, 1893 |
| 61 Cygni | 34 | Approaching | " " |
| Mu Cassiopeiæ | 60 | Approaching | Campbell, 1901 |
| 1830 Groombridge | 59 | Approaching | " " |
| Arcturus | 4.3 | Approaching | Keeler, 1890 |
| Arcturus | 278 | Tangential | Accepting Elkin's |
| | | | parallax of 0·024" |
| 1830 Groombridge | 150 | Tangential | Parallax = 0·14" |
| Mu Cassiopeiæ | 113 | Tangential | Parallax = 0·10" |
| | | | (Peter) |
| Z. C. 5^h 243 | 82 | Tangential | Parallax = 0·312" |
| | | | (Gill) |
| Lacaille, 2,957 | 78 | Tangential | Parallax = 0·064" |
| | | | (Gill) |
| Lacaille, 9,352 | 73 | Tangential | Parallax = 0·283" |
| | | | (Gill) |
| o_2, Eridani | 72 | Tangential | Parallax = 0·166" |
| | | | (Gill) |
| Eta Eridani | 61 | Tangential | Parallax = 0·149" |
| | | | (Gill) |
+---------------------+------------+-------------+---------------------+
TABLE V.
LIST OF GREAT TELESCOPES.
1. Reflectors--A. Metallic Specula.
+-------------+-----------+------------+-------------+------------------+
| Locality. |Aperture in|Focal Length| Constructor.| Remarks. |
| | Inches. | in Feet. | | |
+-------------+-----------+------------+-------------+------------------+
|Birr Castle, | | | Third Earl | |
|Parsonstown, | 72 | 54 | of Rosse, |Newtonian. |
|Ireland | | | 1845 | |
+-------------+-----------+------------+-------------+------------------+
|Melbourne | 48 | 28 | T. Grubb, |Cassegrain. |
|Observatory | | | 1870 | |
+-------------+-----------+------------+-------------+------------------+
| | | | Third Earl |Newtonian. |
|Birr Castle | 36 | -- | of Rosse, |Remounted |
| | | | 1839 |equatoreally 1876.|
+-------------+-----------+------------+-------------+------------------+
| | | | William |Newtonian. |
|Royal | | | Lassell, |Presented |
|Observatory | 24 | 20 | 1846 |by the Missess |
|Greenwich | | | |Lassell to the |
| | | | |Royal Observatory |
+-------------+-----------+------------+-------------+------------------+
| B. Silvered Glass Mirrors. |
+-------------+-----------+------------+-------------+------------------+
|Ealing, near | 60 | 27 |A. A. Common,|Newtonian. |
|London | | | 1891 | |
+-------------+-----------+------------+-------------+------------------+
| | | |G. W. Richey,|Can be employed |
|Yerkes | 60 | 25 | 1902 |at choice as a |
|Observatory | | | |Coudé or a |
| | | | |Cassegrain. |
+-------------+-----------+------------+-------------+------------------+
|National | 48 | -- |Martin, 1875 |Newtonian. |
|Observatory, | | | |Remodelled for |
|Paris | | | |spectrographic |
| | | | |work by |
| | | | |Deslandres in |
| | | | |1892. |
+-------------+-----------+------------+-------------+------------------+
|Meudon | 39 | 9·7 | | |
|Observatory | | | | |
+-------------+-----------+------------+-------------+------------------+
|Lick | 36 | 17·5 |Calver, 1879 |Mounted by |
|Observatory | | | |Common at |
| | | | |Ealing in 1879. |
| | | | |Sold by him to |
| | | | |Crossley, 1885. |
| | | | |Presented by |
| | | | |Crossley to the |
| | | | |Lick |
| | | | |Observatory, 1895.|
+-------------+-----------+------------+-------------+------------------+
|Toulouse | 32·5 | 16·2 | Brothers | |
|Observatory | | | Henry | |
+-------------+-----------+------------+-------------+------------------+
|Marseilles | 31·5 | -- |Foucault | |
|Observatory | | | | |
+-------------+-----------+------------+-------------+------------------+
|Royal | 30 | -- |Common, 1897 |Cassegrain. |
|Observatory, | | | |Mounted as a |
|Greenwich | | | |counterpoise |
| | | | |to the |
| | | | |Thompson |
| | | | |equatoreal. |
+-------------+-----------+------------+-------------+------------------+
|Westgate- | | | Common, |The property |
|on-Sea | 30 | -- | 1889 |of Sir Norman |
| | | | |Lockyer. |
+-------------+-----------+------------+-------------+------------------+
|Harvard | | | H. Draper, |Mounted for |
|College | 28 | -- | 1870 |spectrographic |
|Observatory | | | |work,1887. |
+-------------+-----------+------------+-------------+------------------+
|Royal | | | T. Grubb, | |
|Observatory, | 24 | -- | 1872 | |
|Edinburgh | | | | |
+-------------+-----------+------------+-------------+------------------+
|Daramona, | | | Sir H. |Remounted 1891. |
|Ireland | 24 | 10·5 | Grubb, |Owned by Mr. W. E.|
| | | | 1881 |Wilson. |
+-------------+-----------+------------+-------------+------------------+
| | | | |Can be used as a |
|Yerkes | 23·5 | 7·7 | Ritchey, |Cassegrain, with |
|Observatory | | | 1901 |an equivalent |
| | | | |focal length of |
| | | | |38 feet. |
+-------------+-----------+------------+-------------+------------------+
|Harvard | | | | |
|College | 20 | -- | Common, | |
|Observatory | | | 1890 | |
+-------------+-----------+------------+-------------+------------------+
|Crowborough, | 20 | 8·2 | Sir H. |Mounted with a |
|Sussex | | | Grubb, |7-inch |
| | | | 1885 |refractor. |
+-------------+-----------+------------+-------------+------------------+
| 2. Refractors. |
+-------------+-----------+------------+-------------+------------------+
|Palais de | | | Gautier, |Mounted as a |
|l'Optique, | 49·2 | 197 | 1900 |siderostat in |
|Paris | | | |connection with |
| | | | |a plane mirror 79 |
| | | | |inches across. |
+-------------+-----------+------------+-------------+------------------+
|Yerkes | 40 | 62 | Alvan G. | |
|Observatory | | | Clark, 1897 | |
+-------------+-----------+------------+-------------+------------------+
| | | | |For photographic |
|Lick | | | A. Clark and|purposes a |
|Observatory | 36 | 57·8 | Sons, 1888 |correcting lens is|
| | | | |available, of 33 |
| | | | |inches aperture, |
| | | | |47·8 feet focus. |
+-------------+-----------+------------+-------------+------------------+
| | | | |Mounted with a |
|Meudon | 32·5 | 55·2 | Henrys and |photographic |
|Observatory | | |Gautier, 1891|refractor of 24·4 |
| | | | |inches aperture. |
+-------------+-----------+------------+-------------+------------------+
| | | | |Photographic. |
|Astrophysical| | |Steinheil and|Mounted with a |
|Observatory, | 31·5 | 39·4 |Repsold, 1899|visual refractor |
|Potsdam | | | |20 inches in |
| | | | |aperture. |
+-------------+-----------+------------+-------------+------------------+
|Bischoffsheim| | | |Visual. Mounted |
|Observatory, | 30·3 | 52·6 | Henrys and |on Mont Gros, |
|Nice | | |Gautier, 1886|1,100 feet above |
| | | | |sea level. |
+-------------+-----------+------------+-------------+------------------+
|Imperial | | | A. Clark and|Visual. Mounted |
|Observatory, | 30 | 42 | Sons, 1885 |by Repshold. |
|Pulkowa | | | | |
+-------------+-----------+------------+-------------+------------------+
|National | | | | |
|Observatory, | 28·9 | -- | Martin | |
|Paris | | | | |
+-------------+-----------+------------+-------------+------------------+
|Royal | | | Sir H. |Visual and |
|Observatory, | 28 | 28 | Grubb, |photographic. |
|Greenwich | | | 1894 |Mounted by |
| | | | |Ransome and Simms.|
+-------------+-----------+------------+-------------+------------------+
| University | | |Sir H. Grubb,| Visual. |
| Observatory,| 27 | 34 | 1881 | |
| Vienna | | | | |
+-------------+-----------+------------+-------------+------------------+
| Royal | | |Sir H. Grubb,| The Thompson |
| Observatory,| 26 | 26 | 1897 | photographic |
| Greenwich | | | | equatoreal. |
+-------------+-----------+------------+-------------+------------------+
| Naval | | | A. Clark and| |
| Observatory,| 26 | 29 | Sons, 1873 | |
| Washington | | | | |
+-------------+-----------+------------+-------------+------------------+
| Leander | | | A. Clark and| |
| McCormick | 26 | 32·5 | Sons, 1881 | |
| Observatory,| | | | |
| Virginia | | | | |
+-------------+-----------+------------+-------------+------------------+
| Cambridge | | | T. Cooke and| Presented to the |
| University | 25 | -- | Sons, 1870 | University in |
| Observatory | | | | 1889 by |
| | | | | Mr. R. S. Newall.|
+-------------+-----------+------------+-------------+------------------+
| Meudon | | | Henrys and | Photographic. |
| Observatory | 24·4 | 52·2 | Gautier, | Mounted with a |
| | | | 1891 | visual 32.5 |
| | | | | refractor. |
+-------------+-----------+------------+-------------+------------------+
| Harvard | | | A. Clark and| Photographic |
| College | 24 | 11·3 | Sons, 1893 | doublet. The gift|
| Observatory | | | | of Miss Bruce. |
| | | | | Transfered in |
| | | | | 1896 to Arequipa,|
| | | | | Peru. |
+-------------+-----------+------------+-------------+------------------+
| Royal | | |Sir H. Grubb,| Photographic. |
| Observatory,| | | 1898 | The gift or Mr. |
| Cape of | 24 | 22·6 | | McClean. Mounted |
| Good Hope | | | | with an 18-inch |
| | | | | visual refractor.|
+-------------+-----------+------------+-------------+------------------+
| Lowell | | | Alvan G. | Visual. First |
| Observatory,| | | Clark, 1896 | mounted near the |
| Flagstaff, | 24 | 31 | | city of Mexico. |
| Arizona | | | | Installed at |
| | | | | Flagstaff, 1897. |
+-------------+-----------+------------+-------------+------------------+
| National | | | Henrys and | Visual and |
| Observatory,| 23·6 | 59 | Gautier, | photographic. |
| Paris | | | 1891 | Mounted as an |
| | | | | equatoreal Coudé.|
+-------------+-----------+------------+-------------+------------------+
| Halsted | | | A. Clark and| |
| Observatory,| 23 | 32 | Sons, 1883 | |
| Princeton, | | | | |
| N.J. | | | | |
+-------------+-----------+------------+-------------+------------------+
| City | | | | Mounted as a |
| Observatory,| 22 | 30 | | visual |
| Edinburgh | | | | equatoreal on |
| | | | | the Calton Hill, |
| | | | | 1898. |
+-------------+-----------+------------+-------------+------------------+
| Etna | | | Merz, 1897 | |
| Observatory | 21·8 | -- | | |
+-------------+-----------+------------+-------------+------------------+
| Buckingham | | | Buckingham | |
| Observatory | 21·2 | -- | and Wragge | |
+-------------+-----------+------------+-------------+------------------+
| Porro | | | Porro | |
| Observatory,| 20·5 | -- | | |
| Turin | | | | |
+-------------+-----------+------------+-------------+------------------+
| Chamberlin | | | Alvan G. | Visual. |
| Observatory,| 20 | 28 | Clark and | with a reversible|
| Colorado | | | Saegmüller, | crown lens for |
| | | | 1894 | photography. |
+-------------+-----------+------------+-------------+------------------+
| Manila | | | Merz and | Visual. |
| Observatory | 20 | -- | Saegmüller, | Provided with a |
| | | | 1894 | photographic |
| | | | | correcting lens. |
+-------------+-----------+------------+-------------+------------------+
| Strasburg | 20·5 | 23 |Merz and | |
| Observatory | | |Repsold, 1880| |
+-------------+-----------+------------+-------------+------------------+
| Brera | | | Merz and | |
| Observatory,| 19·1 | 23 | Repsold | |
| Milan | | | | |
+-------------+-----------+------------+-------------+------------------+
| Dearborn | | | A. Clark and| Mounted 1864. |
| Observatory,| 18·5 | 27 | Sons, 1862 | |
| Illinois | | | | |
+-------------+-----------+------------+-------------+------------------+
| National | | | Henrys and | Coudé Mount. |
| Observatory,| 18·1 | 29·5 | Gautier, | Visual. |
| La Plata | | | 1890 | |
+-------------+-----------+------------+-------------+------------------+
| Lowell | | | Brashear, | Mounted with a |
| Observatory,| 18 | 26·3 | 1894 | 12-inch Clark |
| Flagstaff, | | | | refractor as |
| Arizona | | | | counterpoise. |
+-------------+-----------+------------+-------------+------------------+
| Van der Zee | | | Fitz | Dismounted. |
| Observatory,| | | | |
| Buffalo, | 18 | -- | | |
| N.Y. | | | | |
+-------------+-----------+------------+-------------+------------------+
|Bischoffsheim| | | Henrys and | Coudé Mount. |
| Observatory,| 16·5 | 26·2 | Gautier, | Visual. |
| Nice | | | 1889 | |
+-------------+-----------+------------+-------------+------------------+
| University | | | Henrys and | Coudé Mount. |
| Observatory,| 16·5 | 29·5 | Gautier, | Visual. |
| Vienna | | | 1890 | |
+-------------+-----------+------------+-------------+------------------+
| Jesuit | | | Henrys and | Photographic. |
| Observatory,| 16·5 | 22·5 | Gautier, | Mounted with a |
| Zi-ka-Wei | | | 1897 | visual refractor |
| | | | |of equal aperture.|
+-------------+-----------+------------+-------------+------------------+
| Goodsell | | | Brashear, | |
| Observatory,| 16·2 | -- | 1891 | |
| Northfield, | | | | |
| Minnesota. | | | | |
+-------------+-----------+------------+-------------+------------------+
| Warner | | | A. Clark and| |
| Observatory,| 16 | 22 | Sons, 1891 | |
| Rochester, | | | | |
| N.Y. | | | | |
+-------------+-----------+------------+-------------+------------------+
| Grand-Ducal | | | Brashear and| A twin
| Observatory,| 16 | 6·6 | Grubb, 1900 | photographic |
| Königsstuhl,| | | |doublet. The gift |
| Heidelberg | | | | of Miss Bruce. |
| | | | | Mounted with a |
| | | | | visual 10-inch |
| | | | | refractor by |
| | | | | Pauly. |
+-------------+-----------+------------+-------------+------------------+
| Meudon | | | | |
| Observatory | 15·7 | 5·3 | | |
+-------------+-----------+------------+-------------+------------------+
| Washburn | | | A. Clark and| |
| Observatory,| 15·6 | 20·3 | Sons, 1879 | |
| Wisconsin | | | | |
+-------------+-----------+------------+-------------+------------------+
| Teramo | | | T. Cooke and| Formerly the |
| Observatory,| 15·5 | -- | Sons, 1885 | property of |
| Italy | | | | Mr. Wigglesworth.|
+-------------+-----------+------------+-------------+------------------+
| Royal | | | T. Grubb, | Presented by |
| Observatory,| 15·1 | -- | 1872 | Lord Crawford. |
| Edinburgh | | | | |
+-------------+-----------+------------+-------------+------------------+
| Madrid | | | Merz | |
| Observatory | 15 | -- | | |
+-------------+-----------+------------+-------------+------------------+
| Tulse Hill | | |Sir H. Grubb,| Lent by the |
| Observatory | 15 | 15 | 1870 | Royal Society to |
| | | | | Sir William |
| | | | | Huggins. Mounted |
| | | | | with an 18-inch |
| | | | | Cassegrain |
| | | | | reflector. |
+-------------+-----------+------------+-------------+------------------+
| National | | | Lerebours | |
| Observatory,| 15 | 29 | | |
| Paris | | | | |
+-------------+-----------+------------+-------------+------------------+
| Harvard | | | Merz, 1847 | |
| College | 15 | 22 | | |
| Observatory | | | | |
+-------------+-----------+------------+-------------+------------------+
| National | | | | |
| Observatory,| 15 | -- | | |
| Rio de | | | | |
| Janeiro | | | | |
+-------------+-----------+------------+-------------+------------------+
| Tacubaya | | |Sir H. Grubb,| |
| Observatory,| 15 | 15 | 1880 | |
| Mexico | | | | |
+-------------+-----------+------------+-------------+------------------+
| Stonyhurst | | |Sir H. Grubb,| |
| College | 15 | 15 | 1893 | |
| Observatory | | | | |
+-------------+-----------+------------+-------------+------------------+
| Brera | | | | |
| Observatory,| 15 | -- | | |
| Milan | | | | |
+-------------+-----------+------------+-------------+------------------+
| University | | |Sir H. Grubb,| Visual. |
| of | 15 | 15 | 1893 | Mounted with a |
| Mississippi | | | | photographic |
| | | | | 9-inch refractor.|
+-------------+-----------+------------+-------------+------------------+
| Imperial | | | Merz and | |
| Observatory,| 15 | 22·5 | Mahler, 1840| |
| Pulkowa | | | | |
+-------------+-----------+------------+-------------+------------------+
| Maidenhead | | |Sir H. Grubb,| The property of |
| Observatory | 15 | -- | 1893 | Mr. Dunn. |
| | | | | Mounted with a |
| | | | | twin photographic|
| | | | | reflector. |
+-------------+-----------+------------+-------------+------------------+
| Odessa | | | Merz, 1881 | |
| Observatory | 14·9 | -- | | |
+-------------+-----------+------------+-------------+------------------+
|Bischoffsheim| | | Henrys and | |
| Observatory,| 14·9 | 23 | Gautier | |
| Nice | | | | |
+-------------+-----------+------------+-------------+------------------+
| Brussels | | | Merz and | |
| Observatory | 14·9 | 20 | Cooke, 1877 | |
+-------------+-----------+------------+-------------+------------------+
| Observatory | | | Merz and | |
| of Bordeaux | 14·9 | 22·4 |Gautier, 1880| |
+-------------+-----------+------------+-------------+------------------+
| Observatory | | | Merz and | |
| of Lisbon | 14·9 | -- | Mahler | |
+-------------+-----------+------------+-------------+------------------+
TABLE VI.
List of Observatories employed in the Construction of the Photographic
Chart and Catalogue of the Heavens.
All are provided with 13-inch photographic, coupled with 11-inch visual
refractors:
----------------------------------------------------------
| Name of Observatory. | Constructors of Instruments.|
| |-------------------------------|
| | Optical Part.|Mechanical Part.|
|------------------------|--------------|----------------|
|Paris | Henrys | Gautier |
|Algiers | " | " |
|Bordeaux | " | " |
|Toulouse | " | " |
|San Fernando (Spain) | " | " |
|Vatican | " | " |
|Cordoba | " | " |
|Montevideo | " | " |
|Perth, Western Australia| " | " |
|Helsingfors | " | Repsold |
|Potsdam | Steinheil | " |
|Catania | " | Salmoiraghi |
|Greenwich | Sir H. Grubb | Sir H. Grubb |
|Oxford | " | " |
|The Cape | " | " |
|Melbourne | " | " |
|Sydney | " | " |
|Tacubaya (Mexico) | " | " |
----------------------------------------------------------
INDEX
Abbe, Cleveland, corona of 1878, 176, 177
Aberdour, Lord, solar chromosphere, 68
Aberration, discovered by Bradley, 3, 15;
cause of, 31, 231;
investigations of, 241, 438
Abney, daylight coronal photographs, 179;
infra-red photography, 210, 223, 438
Absorption, terrestrial atmospheric, 134, 211, 214-216, 225;
solar, 134-136, 172, 213, 221, 222, 225, 277;
correlative with emission, 135, 136, 140
Adams, discovery of Neptune, 79-82;
lunar acceleration, 271;
orbit of November meteors, 331
Aerolites, falls of, 339, 340
Airy, solar translation, 39;
observations during eclipses, 62, 64, 70;
Astronomer-Royal, 79;
search for Neptune, 80, 81;
corona of 1851, 175;
solar parallax, 227, 236;
transit of Venus, 233;
Mercurian halo, 235;
lunar atmosphere, 264
Aitken, double star discoveries, 419
Albedo, of Mercury, 246;
of Venus, 255;
of Mars, 283;
of minor planets, 288;
of Jupiter, 290;
of Saturn, 303;
of Uranus, 304
Alexander, spiral nebulæ, 118;
observation during eclipse, 245
Algol, variability of light, 10, 390;
eclipses, 390;
nature of system, 391
Altitude and azimuth instrument, 120 _note_, 121
Amici, comet of 1843, 103
Anderson, discovery of Nova Aurigæ, 396;
of Nova Persei, 400
Andrews, conditions of liquefaction, 151
Ångström, C. J., _Optical Researches_, 138;
spark spectrum, 139;
nature of photosphere, 152;
solar spectroscopy, 210, 212;
hydrogen in sun, 211;
temperature of stars, 375
Ångström, K., infra-red solar spectrum, 210;
solar constant, 225
Arago, eclipse of 1842, 62, 64, 65;
prominences, 69;
polarization in comets, 103;
magnetic relations of auroræ, 130;
nature of photosphere, 151;
meteor-systems, 329
Arai, photographs of corona of 1887, 185
Arcturus, spectrum, 373, 383;
radial movement, 387
Argelander, Bonn Durchmusterung, 32, 423;
solar motion, 39;
centre of Milky Way, 40;
comet of 1811, 100
Aristotle, description of a comet, 350
Arrhenius, light-pressure theory of comets, 348
Asten, movements of Encke's comet, 94
Asteroids, so designated by Herschel, 75
Astronomical circles, 121, 122
Astronomical physics, 7, 141, 142
Astronomical Society founded, 6;
Herschel its first President, 14
Astronomy, classification, 1;
popularity and progress, 5;
in United States, 6;
in Germany, 28;
practical reform, 32;
of the invisible, 42;
physical, 141
Atmosphere, solar, 94, 182, 192, 221, 225;
of Venus, 236, 239, 253, 254;
of Mercury, 246-248;
of the moon, 263, 264;
of Mars, 276;
of minor planets, 288
Auroræ, periodicity, 129, 162;
excited by meteors, 335
Auwers, reduction of Bradley's observations, 39;
system of Procyon, 42;
opposition of Victoria, 238;
solar parallax, 240;
new star in Scorpio, 395
Babinet, nebular hypothesis, 314
Backlund, movements of Encke's comet, 94, 360
Baden-Powell, Sir George, eclipse expedition, 188
Bailey, nebulosity round Pleiades, 411;
stellar photometric observations, 421;
discovery of variable clusters, 436
Baily, early life and career, 59-61;
observations of eclipses, 61-64;
density of the earth, 60, 261
Baily's Beads, 61, 62
Bakhuyzen, rotation of Mars, 275
Ball, Sir Robert, parallaxes of stars, 36 _note_, 416;
contacts in transits, 239
Balmer's Law, 198, 383
Barnard, micrometrical measures of Neptune, 84;
of minor planets, 288;
of Saturn's rings, 301;
photographs of solar corona, 186, 190;
transit of Mercury, 245;
halo round Venus, 254;
surface of Mars, 280;
ellipticity of Jupiter's first satellite, 292;
of Uranus, 304;
discovery of inner Jovian satellite, 293, 434;
red spot on Jupiter, 296;
eclipse of Japetus, 300;
attendants on comet of 1882, 363;
on Brooks's comet, 366, 367;
Swift's comet, 368;
photographic discovery of a comet, 369;
observations of Nova Aurigæ, 398, 399;
Hind's variable nebula, 403;
exterior Pleiades nebulosities, 411;
galactic stars, 423;
photographs of Milky Way, 424, 425;
cluster variables, 433;
horizontal telescope, 438
Bartlett, photograph of a partial eclipse, 166
Basic lines, 206, 207
Baxendell, meteors of 1866, 331
Becker, drawings of solar spectrum, 211
Beckett, Sir E. (Lord Grimthorpe), value of solar parallax, 232
Beer and Mädler, surveys of lunar surface, 265, 267;
studies of Mars, 275
Bélopolsky, coronal photographs, 185;
theory of corona, 191;
rotation of Venus, 252;
of Jupiter, 297;
spectroscopic determinations of Saturn's rings, 300;
spectrum of Gamma Cassiopeiæ, 378;
system of Castor, 389, 391;
detection of variable stars as spectroscopic binaries, 391
Berberich, mass of asteroids, 287;
orbit of Holmes's comet, 337
Berkowski, daguerrotype of eclipsed sun, 166
Bessel, biographical sketch, 28-30;
reduction of Bradley's observations, 32;
parallax of 61 Cygni, 36;
disturbed motion of Sirius and Procyon, 41;
trans-Uranian planet, 79;
Halley's comet, 102;
theory of instrumental errors, 122;
personal equation, 123;
rotation of Mercury, 246;
lunar atmosphere, 263;
cometary emanations, 325, 345;
multiple tails, 347;
comet of 1807, 352
Betelgeux, remoteness, 37, 417;
spectrum, 373, 381, 383, 384;
radial movement, 387
Bianchini, rotation of Venus, 250
Biela, discovery of a comet, 95
Bigelow, magnetic and solar disturbances, 161;
theory of corona, 191
Bigourdan, eclipse of 1893, 187;
velocity of comet of 1882, 364
Bird's quadrants, 4, 112, 121
Birmingham, colours of stars, 375 _note_;
discovery of T Coronæ, 393
Birt, rotation of a sun-spot, 144;
Selenographical Society, 266
Bischoffsheim, Coudé telescope, 436
Black Ligament, 235
Bode, popular writings, 5;
solar constitution, 57;
missing planet, 72, 73
Bode's Law, 71, 83, 286
Boeddicker, heat-phases during lunar eclipses, 269, 270;
drawings of Jupiter, 296;
of the Milky Way, 424
Boehm, solar observations, 146, 148
Boguslawski, centre of sidereal revolutions, 41;
observation of Halley's comet, 102
Bolometer, principle of construction, 222
Bond, G. P., his father's successor, 86;
light of Jupiter, 289;
Saturn's rings, 298;
Donati's comet, 324, 325;
Andromeda nebula, 409;
double-star photography, 409
Bond, W. C., observation of Neptune's satellite, 84;
discovery of Hyperion, 85;
of Saturn's dusky ring, 86;
resolution of nebulæ, 119;
celestial photography, 153, 409;
satellite-transit on Jupiter, 291
Borda, repeating circle, 121
Boss, solar translation, 40;
observations on comets, 352, 356
Bossert, proper motions of stars, 415
Bouguer, solar atmospheric absorption, 221
Boulliaud, period of Mira, 10
Bouvard, tables of Uranus, 78;
Encke's comet, 90
Boys, radio-micrometer, 220;
density of the earth, 261
Bradley, discoveries of aberration and nutation, 3;
solar translation, 10;
star-distances, 10, 16;
observation
on Castor, 17;
instruments, 28, 120;
observations reduced by Bessel and Auwers, 32, 39
Brahe, Tycho, star of 1572, 24
Brandes, observations of meteors, 327, 334;
Braun, prominence photography, 197;
density of the earth, 261
Brayley, meteoric origin of planets, 311
Brédikhine, theory of cometary appendages, 100, 348;
repulsive forces, 346, 347;
chemical differences, 347, 348;
formative types, 351, 352, 355, 363, 369;
structure of chromosphere, 199;
red spot on Jupiter, 294;
Andromede meteors, 337;
stationary radiants, 341;
spectrum of Coggia's comet, 343
Bremiker, star maps, 81
Brenner, rotation of Venus, 252
Brester, _Théorie du Soleil_, 152
Brewster, diffraction theory of corona, 67;
telluric lines in solar spectrum, 134;
absorption spectra, 136
Brinkley, ostensible stellar parallaxes, 33
Brisbane, establishment of Paramatta Observatory, 6, 90
Brooks, fragment of 1882 comet, 363;
cometary discoveries, 365, 366
Brünnow, stellar parallaxes, 113, 416
Bruno, Giordano, motion of stars, 9
Buffham, rotation of Uranus, 303
Buffon, internal heat of Jupiter, 289
Bunsen, discovery of spectrum analysis, 132
Burchell, magnitude of Eta Carinæ, 48
Burnham, stellar orbits, 46;
coronal photographs, 186;
measures of Nova Aurigæ, 399;
of planetary nebulæ, 404;
discoveries of double stars, 418, 430, 433, 435;
catalogue, 419;
system of 61 Cygni, 419
Burton, canals of Mars, 279;
rotation of Jupiter's satellites, 292
Calandrelli, stellar parallaxes, 33
Callandreau, capture theory of comets, 98
Campani, Saturn's dusky ring, 86
Campbell, Lieutenant, polarisation of corona, 170
Campbell, Professor, stellar radial velocities, 39, 406, 434;
flash spectrum, 189;
spectroscopic observations of Saturn's rings, 300;
Wolf-Rayet stars, 380;
spectroscopic binaries, 389;
Nova Aurigæ, 398;
translation of solar system, 406;
stellar diffraction-spectra, 440
Canals of Mars, 278-280
Cannon, Miss A. J., spectrographic researches, 386
Canopus, remoteness, 37, 417;
spectrum, 416
Capella, spectrum, 373, 383, 384;
a spectroscopic binary, 389
Carbon, material of photosphere, 152;
absorption by, in sun, 212;
in stars, 374
Carbonelle, origin of meteorites, 340
Carinæ, Eta, light variation, 48, 49;
spectrum, 379
Carrington, astronomical career, 144, 145;
sun-spot observations, 146;
solar rotation, 147;
spot-distribution, 148;
luminous outburst on sun, 159, 160;
Jovian and sun-spot periods, 163;
origin of comets, 370
Cassini, Domenico, discoveries of Saturnian satellites, 84;
of division in ring, 85;
solar rotation period, 146;
solar parallax, 228;
rotation of Venus, 250;
of Mars, 274;
of Jupiter, 290, 295;
satellite of Venus, 256;
satellite-transit on Jupiter, 291
Cassini, J. J., stellar proper motions, 10;
sun-spots on limb, 54;
theory of corona, 66;
rotation of Venus, 250;
structure of Saturn's rings, 299
Castor, system of, 18, 389
Cavendish experiment, 60, 261
Ceres, discovery, 73, 74;
diameter, 75, 288
Chacornac, observation of sun-spot, 156;
star-maps, 284, 413;
variable nebula, 403
Challis, search for Neptune, 81, 82;
duplication of Biela's comet, 96
Charlois, discoveries of minor planets, 283
Charroppin, coronal photographs, 186
Chase, photographic discovery of a comet, 338;
stellar parallaxes, 416
Chladni, origin of meteors, 327, 332
Christie, Mercurian halo, 245
Chromosphere, early indications, 68;
distinct recognition, 69, 70, 167;
depth, 174, 175, 200;
metallic injections, 195;
eruptive character, 199;
spectrum, 200
Clark, Alvan, large refractors, 114, 429, 430, 436
Clark, Alvan G., discovery of Sirian companion, 42, 430;
40-inch refractor, 433
Clarke, Colonel, figure of the earth, 262
Clarke, F. W., celestial dissociation, 206
Clausen, period of 1843 comet, 105;
cometary systems, 362
Clerihew, secondary tail of 1843 comet, 103
Clusters, variable stars in, 436
Coggia, discovery of a comet, 343
Comet, Halley's, return in 1759, 4, 88;
orbit computed by Bessel, 29;
capture by Neptune, 98, 365;
return in 1835, 101-103, 345;
type of tail, 346, 352;
of 1843, 7, 103-105;
type of tail, 346, 352;
relationships, 349-351;
Newton's, 88, 364;
Encke's, 90;
changes of volume, 92;
of brightness, 95;
acceleration, 93, 94;
capture by Mercury, 99;
Winnecke's, 94, 342;
Biela's, 95-97, 333;
Brorsen's, 97;
Vico's, 97, 367;
Faye's, 98;
of 1811, 99-101, 346;
of 1807, 100, 347, 352;
of 1819, 101, 103;
Lexell's, 106, 367;
Tewfik, 178, 358, 362, 369;
Donati's, 323-325, 347, 348;
of 1861, 326, 327, 346;
Perseid, 327, 332;
Leonid, 327, 332, 333, 343;
Klinkerfues's, 335;
Holmes's, 337, 343, 369;
Coggia's, 343, 346, 347;
of 1901, 343;
of 1880, 349, 351;
Aristotle's, 350;
Tebbutt's, 352-355;
Schaeberle's, 355, 356;
Wells's, 356, 357;
of September, 1882, 358-361, 362-364;
Thome's, 361;
Pons-Brooks, 365, 366;
Sawerthal's, 366;
Brooks's, of 1889, 366, 367;
Swift's, 368
Cometary tails, repulsive action upon, 100, 103, 104, 346-348;
coruscations in, 105;
three types, 346-348, 355, 363;
multiple, 347, 348, 351, 352, 355, 363, 368
Comets, subject to gravitation, 88;
of short period, 92, 93;
translucency, 95, 105, 106, 353;
small masses, 96, 106;
capture by planets, 98, 306, 367;
changes of volume, 102, 365, 369;
polarisation of light, 103, 354, 355;
refractive inertness, 106, 353;
relations to meteor-systems, 327, 332-336;
disintegration, 333, 339, 362, 363;
spectra, 342-344, 354, 355, 362-364;
luminous by electricity, 344, 355, 357;
systems, 353, 355, 357, 362, 365;
origin, 369-371
Common, reflectors for eclipse photography, 187;
Jupiter's inner satellite, 293;
detection of great comet near the sun, 358;
its five nuclei, 362;
photographs of Andromeda nebula, 395;
of Orion nebula and Jupiter, 407, 408;
great reflectors, 412, 429;
cluster variables, 436
Common, Miss, drawing of eclipsed sun, 187
Comstock, lunar atmosphere, 264
Comte, celestial chemistry, 140;
astronomy, 142
Cooke, 25-inch refractor, 430
Copeland, comets of 1843 and 1880, 349;
spectrum of comet of 1882, 364;
of Gamma Cassiopeiæ, 378;
of Nova Andromedæ, 395; of Orion nebula, 407;
discoveries of gaseous stars, 379;
Nova Aurigæ, 396, 398
Copernicus, stellar parallax, 16
Cornu, telluric lines in solar spectrum, 202;
velocities in prominences, 205;
ultra-violet solar spectrum, 210, 215;
velocity of light, 232 _note_, 241;
spectrum of hydrogen, 383;
of Nova Cygni, 393
Cornu and Bailie, density of the earth, 261
Corona of 1842, 62-64, 67;
early records and theories, 65-67;
photographs, 166, 173, 178, 181, 185-190;
spectrum, 170, 173, 178, 188, 190, 193;
varying types, 174-176, 193;
of 1877, 175-177;
of 1882, 177;
of 1869, 183;
of 1886, 185;
of 1889, 185-187;
of 1893, 188;
of 1898, 189;
of 1900, 189;
of 1901, 190;
daylight photography of, 178-180;
glare theory, 182;
mechanical theory, 191;
electro-magnetic theories, 191, 192
Coronium, 171, 174, 193
Cortie, movements in sun-spots, 157;
their spectral changes, 208
Cotes, corona of 1715, 176
Croll, secular changes of climate, 259, 260;
derivation of solar energy, 313
Crookes, chemical elements, 210
Crova, solar constant, 225
Cruls, comet of 1882, 358, 364
Cusa, solar constitution, 57
Cysatus, Orion nebula, 21;
comet of 1618, 362
Damoiseau, theory of Halley's comet, 101
D'Arrest, orbits of minor planets, 285;
Andromede meteors, 334;
ages of stars, 375;
variable nebulæ, 403;
measures of nebulæ, 404
Darwin, G. H., rigidity of the earth, 258;
Saturn's ring system, 301;
origin of the moon, 316-318;
development of solar system, 318, 319, 322;
solar tidal friction, 319
Daubrée, falls of aerolites, 339
Davidson, satellite-transit on Jupiter, 292
Davis, stellar parallaxes, 417
Dawes, prominences in 1851, 70;
Saturn's dusky ring, 86;
a star behind a comet, 106;
solar observations, 143, 164;
observations and drawings of Mars, 276, 278, 280;
satellite-transits on Jupiter, 291, 292
De Ball, markings on Mercury, 248
Delambre, Greenwich observations, 3;
solar rotation, 146;
light-equation, 231
De la Roche, Newton's law of cooling, 217
De la Rue, celestial photography, 152, 153, 268;
solar investigations, 154;
expedition to Spain, 166, 167
De la Tour, experiments on liquefaction, 151
Delaunay, tidal friction, 271, 272;
Coudé telescope, 436
Delisle, diffraction theory of corona, 67;
transits of Venus, 233, 239
Dembowski, double star measurements, 418
Denning, observations of Mercury, 246, 247;
mountain on Venus, 253;
rotation of Jupiter, 290;
red spot, 295;
periodicity of markings, 297;
rotation of Saturn, 302;
meteors of 1885, 336;
of 1892, 338;
stationary radiants, 341
Denza, meteors of 1872, 334
Derham, theory of sun-spots, 53;
ashen light on Venus, 255
Deslandres, eclipse expedition, 187;
rotation of corona, 188;
prominence photography, 198;
hydrogen spectrum in prominences, 198, 383;
photographs of Jupiter, 297;
radial movements of Saturn's rings, 300;
helium absorption in stars, 376;
stellar radial velocities, 406
Diffraction, corona explained by, 67, 70, 181;
spectrum, 139, 210, 223, 439
Dissociation in the sun, 152, 206-210;
in space, 312
Doberck, orbits of double stars, 38, 418
Dollond, discovery of achromatic telescope, 4, 112
Donati, discovery of comet, 323;
spectra of comets, 342;
of stars, 372
Doppler, effect of motion on light, 200
Douglass, observations of Jupiter's satellites, 292
Downing, perturbations of the Leonids, 338
Draper, H., ultra-violet spectrum, 210;
oxygen in sun, 213;
photographs of the moon, 268;
of Jupiter's spectrum, 291;
of Tebbutt's comet, 354;
of spectrum of Vega, 382;
of Orion nebula 407
Draper, J. W., lunar photographs, 152;
distribution of energy in spectrum, 223 _note_
Draper Memorial, 384-386
Dreyer, Catalogue of Nebulæ, 50
Dulong and Petit, law of radiation, 217, 219
Dunér, spectra of sun-spots, 156;
spectroscopic measurement of solar rotation, 203;
spectroscopic star catalogue, 381
Dunkin, solar translation, 39
Duponchel, sun-spot period, 163
Durchmusterung, Bonn, 33, 412;
Cape photographic, 412;
parallax, 418;
photometric, 421
Dyson, coronal photographs, 190
Earth, mean density, 60, 261;
knowledge regarding, 257;
rigidity, 257, 259;
variation of latitude, 258, 259;
figure, 261, 262;
effects of tidal friction, 271-273;
bodily tides, 316;
primitive disruption, 317
Easton, structure of Milky Way, 423, 424
Ebert, coronoidal discharges, 192
Eclipse, solar, of 1836, 61;
of 1842, 62-65, 67, 69;
of 1851, 69, 70, 166;
of 1860, 166, 167;
of 1868, 167-170;
of 1869, 170;
of 1870, 171;
of 1871, 173;
of 1878, 174-177;
of 1882, 177, 178;
of 1883, 180, 181;
of 1885, 183;
of 1886, 184;
of 1887, 185;
of 1889, 185-187;
of 1893, 187, 188;
of 1896, 188;
of 1898, 189;
of 1900, 189, 190;
of 1901, 190
Eclipses, lunar, heat-phases during, 269, 270
Eclipses, solar, importance, 59;
ancient, 60, 273;
classification, 61;
results, 192, 193
Eddie, comet of 1880, 349;
of 1882, 363
Edison, tasimeter, 177
Egoroff, telluric lines in solar spectrum, 211, 214
Elements, chemical, dissociation in sun, 206, 209, 210
Elkin, star parallaxes, 37, 416, 417;
photography of meteors, 338;
transit of great comet, 358, 360;
secondary tail, 363;
triangulation of the Pleiades, 410
Elliot, opinions regarding the sun, 57
Elvins, red spot on Jupiter, 296
Encke, star maps, 78;
calculation of short-period comet, 90;
resisting medium, 93;
distance of the sun, 230, 232;
period of Pons's comet, 365
Engelmann, rotation of Jupiter's satellites, 292
Ericsson, solar temperature, 218
Erman, meteoric rings, 330
Eros, measures of, for solar parallax, 238;
discovery, 284;
variability, 285
Ertborn, mountain in Venus, 253
Espin, spectra of variable stars, 379;
stars with banded spectra, 381;
Nova Aurigæ, 397, 398
Euler, resisting medium, 93
Evershed, eclipse photographs, 189, 200
Evolution, of solar system, 308, 309, 313-316, 322;
of earth-moon system, 316-318;
of stellar systems, 420
Fabricius, David, discovery of Mira Ceti, 10
Fabricius, John, detection of sun-spots, 52
Faculæ, relation to spots, 53, 155, 158;
solar rotation from, 155;
photographed, 197, 198, 377
Faye, nature of prominences, 70, 166;
discovery of a comet, 98;
cyclonic theory of sun-spots, 144, 157;
solar constitution, 150-152;
maximum of 1883, 163;
velocities in prominences, 205;
distance of the sun, 240;
planetary evolution, 314, 315, 321
Feilitsch, solar appendages, 70
Fényi, solar observations, 184, 204
Ferrel, tidal friction, 272
Ferrer, nature of corona, 67;
prominences, 69
Fessenden, electrical theory of comets, 348
Finlay, transit of great comet, 358, 360
Fizeau, daguerrotype of the sun, 153;
Doppler's principle, 201;
velocity of light, 232
Flammarion, canals of Mars, 280;
trans-Neptunian planet, 306
Flamsteed, solar constitution, 57;
distance, 228
Flaugergues, detection of 1811 comet, 99;
transit of Mercury, 244
Fleming, Mrs., spectrum of Beta Lyræ, 379;
preparation of Draper Catalogue, 386;
discoveries of new stars, 399
Flint, star-parallaxes, 417
Fontana, mountains of Venus, 252;
satellite, 256;
spots on Mars, 274
Forbes, George, trans-Neptunian planets, 306, 307
Forbes, James D., spectrum of annularly eclipsed sun, 134;
solar constant, 225
Foucault, spectrum of voltaic arc, 137;
photograph of the sun, 153;
velocity of light, 232, 240;
silvered glass reflectors, 429
Fraunhofer, early accident, 33;
improvement of refractors, 34;
clockwork motion, 121;
spectra of flames, 131;
of sun and stars, 133, 134, 372;
objective prism, 385;
diffraction gratings, 439
Fraunhofer lines, mapped, 133, 136;
origin, 135-137, 171, 172;
reflected in coronal spectrum, 170, 173, 181;
in cometary spectra, 354, 357;
shifted by radial motion, 201
Freycinet, distribution of minor planets, 287
Fritz, auroral periodicity, 162
Frost, solar heat radiation, 222
Galileo, descriptive astronomy, 2;
double-star method of parallaxes, 16;
discovery of sun-spots, 52;
solar rotation, 146;
planets and sun-spots, 163;
darkening at sun's edge, 221
Galle, discovery of Neptune, 81, 82;
Saturn's dusky ring, 86;
distance of the sun, 237;
path of Andromede meteors, 334
Galloway, solar translation, 39
Gambart, discovery of comet, 95
Gauss, orbits of minor planets, 74;
_Theoria Motus_, 77;
magnetic observations, 126, 127;
cometary orbits, 370
Gautier, sun-spot and magnetic periods, 126, 128;
sun-spots and weather, 129
German Astronomical Society, 6, 414
Gill, star-parallaxes, 37, 42, 416, 417;
expedition to Ascension, 237;
distance of the sun, 237, 238, 240;
constant of aberration, 241;
arc measurements, 261, 262;
comet of 1882, 359, 412;
oxygen-absorption in stars, 384;
photograph of Argo nebula, 404;
Cape Durchmusterung, 412;
photographic celestial survey, 413;
actinic intensity of galactic stars, 425;
Coudé telescope, 438
Gladstone, J. H., spectrum analysis, 134, 136
Glaisher, occultation by Halley's comet, 106
Glasenapp, coronal photographs, 185;
light equation, 231, 241;
double star measures, 419
Glass, optical, excise duty on, 112, 115;
Guinand's, 113, 114;
Jena, 431
Gledhill, spot on Jupiter, 294
Goldschmidt, nebulæ in the Pleiades, 411
Goodricke, periodicity of Algol, 390
Gore, catalogue of variable stars, 391;
of computed binaries, 418
Gothard, bright-line stellar spectra, 378, 379;
spectrum of Nova Aurigæ, 398;
photographs of nebulæ, 409
Gould, variation of latitude, 258;
photograph of Mars, 281;
comets of 1807 and 1881, 349, 352;
luminous instability of stars, 392;
photographic measures of the Pleiades, 410;
_Uranometria Argentina_, 415;
solar cluster, 423, 426
Graham, discovery of Metis, 77
Grant, solar envelope, 70, 167;
transit phenomena, 254
Green, observation of Mars, 280
Greenwich observations, 3, 27, 32
Gregory, David, achromatic lenses, 112 _note_
Gregory, James, double star method of parallaxes, 16;
reflecting telescopes, 109
Groombridge, star catalogue, 31
Grosch, corona of 1867, 176
Grubb, Sir Howard, photographic reflector, 409;
great refractors, 430, 433;
siderostat, 437
Grubb, Thomas, Melbourne reflector, 110 _note_, 428
Gruithuisen, snow-caps of Venus, 255;
lunar inhabitants, 265
Gully, detection of Nova Andromedæ, 394
Guthrie, nebulous glow round Venus, 253
Hadley, Saturn's dusky ring, 86;
reflecting telescope, 109
Haerdtl, Winnecke's comet, 94
Hale, luminous outburst on sun, 161;
daylight coronal photography, 179;
spectrum of prominences, 195, 198;
prominence photography, 197, 198;
photographs of faculæ, 198, 377;
carbon in chromosphere, 200;
bright lines in fourth-type stars, 381;
reflectors and refractors, 432
Hall, Asaph, parallax of the sun, 241;
discovery of Martian satellites, 282;
rotation of Saturn, 302;
double star measurements, 419
Hall, Chester More, invention of achromatic telescope, 112
Hall, Maxwell, rotation of Neptune, 305
Halley, stellar proper motions, 9;
composition of nebulæ, 22;
observation of Eta Carinæ, 48;
eclipse of 1715, 66, 68;
predicted return of comet, 88;
magnetic theory of auroræ, 130;
transits of Venus, 233;
lunar acceleration, 271;
origin of meteors, 327
Halm, magnetic relations of latitude variation, 259
Hansen, solar parallax from lunar theory, 230
Hansky, coronal photographs, 188, 189
Harding, discovery of Juno, 75;
celestial atlas, 77
Harkness, spectrum of corona, 170;
corona of 1878, 175;
shadow of the moon in solar eclipses, 182;
light equation, 231;
distance of the sun, 237, 240, 241, 242
Harriot, observations on Halley's comet, 29
Hartley, gallium in the sun, 200, 213
Hartwig, Nova Andromedæ, 394
Hasselberg, metallic spectra, 211;
spectra of comets, 342, 357;
of Nova Andromedæ, 395
Hastings, composition of photosphere, 152;
observations at Caroline Island, 181;
Saturn's dusky ring, 299
Hegel, number of the planets, 73
Heis, radiant of Andromedes, 334
Heliometer, 34, 234, 237, 238, 240
Helium, a constituent of prominences, 194, 195, 199;
no absorption by, in solar spectrum, 213;
absorptive action in first-type stars, 376;
bright in gaseous stars, 377, 378, 380;
in Orion nebula, 407
Helmholtz, gravitational theory of sun-heat, 311-313
Hencke, discoveries of minor planets, 76
Henderson, parallax of Alpha Centauri, 36, 416;
observation of chromosphere, 68
Henry, Paul and Prosper, lunar twilight, 264;
markings on Uranus, 303;
photograph of Saturn, 408;
photographs of nebulæ in the Pleiades, 410, 411;
stellar photography, 413;
plane mirrors, 438
Herrick and Bradley, duplication of Biela's comet, 96
Herschel, Alexander S., cometary and meteoric orbits, 332
Herschel, Caroline, her brother's assistant, 12;
observation of Encke's comet, 90
Herschel, Colonel, spectrum of prominences, 168;
of reversing layer, 172;
of corona, 174
Herschel, Sir John, life and work, 45-50;
Magellanic clouds, 47, 422;
sun-spots, 58, 59, 144;
solar flames, 68;
anticipation of Neptune's discovery, 81;
status of Hyperion, 85;
Biela's comet, 95;
Halley's, 102;
comet of 1843, 103;
sixth star in "trapezium," 113;
grinding of specula, 116;
spectrum analysis, 136;
solar photography, 145, 154;
solar constitution, 151;
shadow round eclipsed sun, 182;
actinometrical experiments, 216;
solar heat, 217;
climate and eccentricity, 259;
lunar atmosphere, 263;
surface of Mars, 276;
Andromeda nebula, 396;
observations of nebulæ, 404;
double nebulæ, 412
Herschel, Sir William, discovery of Uranus, 5;
founder of sidereal astronomy, 9;
biographical sketch, 11-14;
sun's motion in space, 15, 39, 425;
revolutions of double stars, 18, 442;
structure of Milky Way, 19-21, 423;
nature of nebulæ, 21-26, 401;
results of his observations, 25;
centre of sidereal system, 40;
theory of the sun, 54-56, 70;
asteroids, 75;
discoveries of Saturnian and Uranian satellites, 84, 87, 110;
comet of 1811, 99;
reflecting telescopes, 109-111;
sun-spots and weather, 129;
transit of Mercury, 244;
refraction in Venus, 252;
lunar volcanoes, 266;
terrestrial affinity of Mars, 274;
Jovian trade-winds, 289;
rotation of Jupiter's satellites, 292;
ring of Saturn, 298;
rotation of Saturn, 302;
origin of comets, 369;
stellar photometry, 420
Herz, comets' tails, 348
Hevelius, "Mira" Ceti, 10;
contraction of comets, 92;
granular structure of comet, 362
Higgs, photographs of solar spectrum, 211, 214
Hind, solar flames, 70;
Iris and Flora discovered by, 77;
distortion of Biela's comet, 96;
transit of a comet, 101;
earth in a comet's tail, 326;
comets of 1843 and 1880, 349;
Schmidt's comet, 363;
new star, 392;
variable nebula, 403
Hirn, solar temperature, 220;
resistance in space, 348
Hodgson, outburst on the sun, 160
Hoeffler, star-drift in Ursa Major, 426
Hoek, cometary systems, 362
Holden, Uranian satellites, 87;
eclipse expedition, 180;
coronal extensions, 186;
solar rotation, 203;
transit of Mercury, 245;
intra-Mercurian planets, 250;
drawing of Venus, 252;
lunar photographs, 268;
canals on Mars, 279;
surface of Mars, 281;
transits of Jupiter's satellites, 292;
markings on Uranus, 304;
disintegration of comet, 362;
colours of double stars, 374;
Nova Aurigæ, 398;
Orion and Trifid nebulæ, 403, 404;
director of Lick Observatory, 435
Holden and Schaeberle, observations of nebulæ, 433
Holmes, discovery of a comet, 337
Homann, solar translation, 406
Hooke, solar translation, 10;
stellar parallax, 16;
repulsive action on comets, 102 _note_;
automatic movement of telescopes, 120;
spots on Mars, 274, 275
Hopkins, solidity of the earth, 257
Horrebow, sun-spot periodicity, 125;
satellite of Venus, 256
Hough, G. W., red spot on Jupiter, 295, 430;
observations of double stars, 419
Houzeau, solar parallax, 240
Howlett, sun-spot observations, 155
Hubbard, period of comet of 1843, 105, 351
Huggins, Sir William, spectroscopic observations of prominences,
170, 195;
hydrogen spectrum in stars, 178, 198;
daylight coronal photography, 178, 179;
repulsive action in corona, 191;
stellar motions in line of sight, 201, 386, 387;
transit of Mercury, 245;
occultation of a star, 263;
snowcaps on Mars, 276;
spectrum of Mars, 277;
of Jupiter, 290;
Jovian markings and sun-spots, 297;
spectrum of Uranus, 304;
of comets, 342, 343;
photographs, 354, 357;
stellar spectroscopy, 373;
colours of stars, 374;
classification of star spectra, 376;
photographs, 382, 383, 438;
stellar chemistry, 381, 382;
spectra of new stars, 393, 395;
theory of Nova Aurigæ, 397;
spectra of nebulæ, 401, 402, 407;
nebular radial movement, 405
Huggins, Sir William and Lady, photograph of Uranian spectrum, 305;
spectra of Wolf-Rayet stars, 380;
ultra-violet spectrum of Sirius, 383;
nitrogen in stars, 384;
spectrum of Nova Aurigæ, 396-398;
of Andromeda nebula, 403;
of Orion nebula, 407
Humboldt, sun-spot period, 126;
magnetic observations, 127;
meteoric shower, 329
Hussey, T. J., search for Neptune, 79
Hussey, W. J., cloud effects on Mars, 281;
cometary appendages, 369;
period of Delta Equulei, 419;
discoveries of double stars, 419, 433
Huygens, stellar parallax, 16;
Orion nebula, 22;
discovery of Titan, 84;
Saturn's ring, 85, 301;
spot on Mars, 275
Hydrogen, a constituent of prominences, 168, 195, 199;
spectrum, 178, 198, 383, 384;
absorption in stars, 198, 373, 381-383;
in sun, 211;
theoretical material of comets' tails, 347;
emissions in stars, 377-380, 384, 393, 397;
in nebulæ, 402, 407
Innes, Southern double stars, 419
Jacoby, measurement of Rutherfurd's plates, 410;
Pritchard's parallax work, 417
Janssen, photographs of the sun, 165;
spectroscopic observations of prominences, 168, 169;
escape from Paris in a balloon, 171;
coronal spectrum, 173, 181;
coronal photographs, 181;
rarefaction of chromospheric gases, 182;
oxygen absorption in solar spectrum, 214;
transit of 1874, 234;
spectrum of Venus, 254;
of Saturn, 303;
photographs of Tebbutt's comet, 353, 354;
of Orion nebula, 407
Japetus, eclipse of, 300;
variability in light, 302
Jewell, solar spectroscopy, 200, 211
Joule, heat and motion, 309
Jupiter, mass corrected, 77, 92;
conjectured influence on sun-spot development, 163;
physical condition, 289, 290;
spectrum, 290, 291;
satellite-transits, 291, 292;
discovery of inner satellite, 293;
red spot, 293-296;
photographs, 297, 408;
periodicity of markings, 297
Kaiser, rotation of Mars, 275;
map of Mars, 278
Kammermann, observation of Maia nebula, 410
Kant, status of nebulæ, 14;
Sirius the central sun, 40;
planetary intervals, 71;
tidal friction, 272;
condition of Jupiter, 289;
cosmogony, 308
Kapteyn, solar translation, 40;
Cape Durchmusterung, 412;
stellar parallaxes, 417, 418;
actinic intensity of galactic stars, 425;
solar cluster, 426
Kayser and Runge, spectroscopic investigations, 211, 213
Keeler, red spot on Jupiter, 296;
spectroscopic determination of movements in Saturn's rings, 300;
spectrum of Uranus, 304;
of third type stars, 382;
of nebulæ, 402;
photographs of nebulæ, 403, 411, 412, 432;
nebular radial movements, 405, 434, 440;
grating spectroscope, 440
Kepler, star of 1604, 25;
solar corona, 66;
missing planets, 71;
cometary decay, 91, 339;
comet of 1618, 96;
physical astronomy, 141
Kiaer, comets' tails, 348
Kirchhoff, foundation of spectrum analysis, 132, 135-137, 372;
map of solar spectrum, 137;
solar constitution, 149, 151, 172
Kirkwood, distribution of minor planets, 286;
grouped orbits, 287;
divisions in Saturn's rings, 301, 302;
origin of planets, 314;
their mode of rotation, 321;
comets and meteors, 333, 339
Kleiber, Perseid radiants, 341
Klein, Hyginus N., 267, 268
Klinkerfues, comet predicted by, 335, 339;
apparitions of Southern comet, 350;
tidal theory of new stars, 397
Knobel, cloud effects on Mars, 281
Konkoly, spectrum of Gamma Cassiopeiæ, 378;
spectroscopic survey, 381 _note_
Kreil, lunar magnetic action, 130
Kreutz, period of 1843 comet, 105;
orbit of 1861 comet, 327;
period of great September comet, 361;
cause of disintegration, 363;
eclipse-comet of 1882, 362
Krüger, segmentation of great comet, 362
Küstner, variation of latitude, 258
Kunowsky, spots on Mars, 275
Lacaille, southern nebulæ, 22;
Eta Carinæ, 48
Lagrange, theory of solar system, 2;
planetary disruption, 76
Lahire, diffraction theory of corona, 67;
distance of the sun, 228;
mountains of Venus, 252
Lalande, popularisation of astronomy, 5;
revolving stars, 18;
_Histoire Céleste_, 31, 415;
nature of sun-spots, 53;
observations of Neptune, 83
Lambert, solar motion, 10;
construction of the universe, 14, 40;
missing planets, 71
Lamont, magnetic period, 127, 128
Lamp, ashen light on Venus, 256
Langdon, mountains of Venus, 253
Langley, solar granules, 165;
corona of 1878, 176;
spectroscopic effects of solar rotation, 202;
infra-red spectrum, 210, 223, 224;
experiments at Pittsburg, 221;
bolometer, 222;
distribution of energy in spectrum, 224, 225;
atmospheric absorption, 224, 225, 276;
solar constant, 225;
lunar heat-spectrum, 269;
temperature of lunar surface, 270;
age of the sun, 312
Laplace, lunar acceleration, 2, 271;
_Système du Monde_, 5;
nebular hypothesis, 25, 308, 309, 313, 314, 322;
stability of Saturn's rings, 85, 298;
solar atmosphere, 94, 221;
Lexell's comet, 106, 367;
solar distance by lunar theory, 230;
origin of meteors, 328;
of comets, 370
Lassell, discovery of Neptune's satellite, 83;
of Hyperion, 85;
Saturn's dusky ring, 86;
observations at Malta, 87, 434;
reflectors, 114;
equatoreal mounting, 121
Latitude, variation of, 258, 259
Laugier, period of 1843 comet, 105;
solar rotation, 146
Le Chatelier, temperature of the sun, 219
Lescarbault, pseudo-discovery of Vulcan, 248;
halo round Venus, 254
Lespiault, orbits of minor planets, 285
Le Sueur, spectrum of Jupiter, 291
Leverrier, discovery of Neptune, 80-82;
Lexell's comet, 98, 367;
distance of the sun, 230, 240;
revolutions of Mercury, 248;
supposed transits of Vulcan, 249;
mass of asteroids, 287;
orbit of November meteors, 332;
Perseids and Leonids, 333
Lexell, comet of 1770, 98, 106, 367
Liais, supposed transit of Vulcan, 249;
comet of 1861, 326;
division of a comet, 339
Librations, of Mercury, 247;
of Venus, 251;
of the moon, 266
Lick, foundation of observatory, 434
Light, velocity, 38, 232, 241;
extinction in space, 45;
refrangibility changed by movement, 201
Light-equation, 231, 241
Ligondès, development of solar system, 316
Lindsay, Lord, expedition to Mauritius, 234
Line of sight, movements in, 201, 386;
of solar limbs, 202, 203;
in prominences, 204, 208;
of stars, 201, 386, 387;
binaries detected by, 387-391
Listing, dimensions of the globe, 262
Littrow, chromosphere, 70;
sun-spot periodicity, 126
Liveing and Dewar, carbon in the sun, 212
Lockyer, solar spectroscopy, 156, 212;
theory of sun-spots, 159, 163;
daylight observations of prominences, 169, 194, 204, 205;
eclipse of 1870, 171;
slitless spectroscope, 173;
corona of 1878, 175;
glare theory of corona, 182;
eclipse of 1886, 184;
chromospheric spectrum, 195;
classification of prominences, 196;
their radial movements, 204;
celestial dissociation, 206-210;
chemistry of sun-spots, 207;
spots on Mars, 275;
meteoritic hypothesis, 376, 402;
equatoreal Coudé, 438
Loewy, constant of aberration, 241, 438;
lunar photographs, 268;
director of Paris Observatory, 414;
equatoreal Coudé, 436, 437
Lohrmann, lunar chart, 265;
Linné, 267
Lohse, J. G., spectrum of great comet, 364
Lohse, O., daylight coronal photography, 178 _note_;
spectral investigations, 211;
twilight on Venus, 256;
red spot on Jupiter, 294;
periodicity of Jupiter's markings, 297;
motion of Sirius, 386;
spectrum of Nova Cygni, 393
Louville, nature of corona, 67;
chromosphere, 68
Lowell, rotation of Mercury, 248;
of Venus, 252;
markings on Venus, 255;
observations of Mars, 280, 281;
satellites, 283
Lyman, atmosphere of Venus, 254
McClean, photographs of solar spectrum, 211, 215;
helium stars, 377;
oxygen stars, 384;
equipment of Cape Observatory, 433
Macdonnell, luminous ring round Venus, 254
Maclaurin, eclipse of 1737, 65
Maclear, Admiral, observations during eclipses, 172, 182
Maclear, Sir Thomas, maximum of Eta Carinæ, 49;
observation of Halley's comet, 102
Mädler, central sun, 41;
observations
of Venus, 253;
lunar rills, 263;
aspect of Linné, 267;
common proper motions, 426
Magellanic clouds, 47, 422;
spiral character, 425
Magnetism, terrestrial, international observations, 126;
periodicity, 127, 128;
solar relations, 128, 160, 161, 163, 205;
lunar influence, 130
Mann, last observation of Donati's comet, 325
Maraldi, solar corona, 67;
rotation of Mars, 274;
satellite-transits on Jupiter, 291;
spot on Jupiter, 295
Marius, Andromeda nebula, 21;
sun-spots, 52
Mars, oppositions, 228;
solar parallax from, 228, 237, 240;
polar spots, 274, 276, 277, 280, 281;
permanent markings, 274-276;
rotation, 274, 275;
atmosphere, 276, 277;
climate, 277, 278;
canals, 278-281;
photographs, 281;
satellites, 282, 283, 314, 320, 321
Marth, revolutions of Neptune's satellite, 305
Maskelyne, components of Castor, 18;
Astronomer-Royal, 27;
experiment at Schehallien, 261;
comets and meteors, 332
Maunder, photographs of corona of 1886, 185;
comparative massiveness of stars, 375;
constitution of nebulæ, 403
Maunder, Mrs., coronal photographs, 189, 190
Maury, director of Naval Observatory, 7;
duplication of Biela's comet, 96
Maury, Miss A. C., spectrographic investigations, 386;
discoveries of spectroscopic binaries, 387, 388
Maxwell, J. Clerk, structure of Saturn's rings, 298, 300
Mayer, C., star satellites, 17
Mayer, Julius R., tidal friction, 272;
meteoric sustentation of sun's heat, 310
Mayer, Tobias, stellar motions, 10;
solar translation, 15;
repeating circle, 122;
solar distance, 230;
satellite of Venus, 256;
lunar surface, 263
Mazapil meteorite, 340
Meldrum, sun-spots and cyclones, 164
Melloni, lunar heat, 269
Melvill, spectra of flames, 131
Mercury, mass, 92;
luminous phenomena during transits, 244, 245;
spectrum, 245;
mountainous conformation, 246, 247;
rotation, 247, 248;
theory of movements, 248, 250
Mersenne, reflecting telescope, 108
Messier, catalogue of nebulæ, 22
Meteoric hypothesis of solar sustentation, 310;
of planetary formation, 311
Meteoritic hypothesis of cosmical constitution, 376, 402
Meteors, origin, 327, 328;
relations to comets, 327, 332-334, 340;
Leonids, 328-334, 338;
Perseids, 329, 332, 333, 341;
Andromedes, 334-338;
stationary radiants, 341
Meunier, canals of Mars, 280
Meyer, divisions of Saturn's rings, 302;
comet of 1880, 351;
cometary refraction, 353;
comet Tewfik, 362
Michell, double stars, 17;
torsion balance, 261;
star systems, 426
Michelson, velocity of light, 241
Milky Way, grindstone theory, 14;
clustering power, 20, 26;
structure, 20, 41, 45, 47, 423-425;
centre of gravity, 40, 41;
frequented by Wolf-Rayet, temporary, and helium stars, 380, 399, 425;
by gaseous nebulæ, 402;
drawings and photographs, 424, 425
Miller, W. A., spectrum analysis, 132, 136, 137;
stellar chemistry, 373
Mira, light changes, 10;
spectrum, 374, 379
Mitchel, lectures at Cincinnati, 6
Mitchell, photograph of reversing layer, 190
Möller, theory of Faye's comet, 98
Mohn, origin of comets, 370
Moll, transit of Mercury, 245
Monck, Perseid meteors, 341;
new stars, 395
Moon, acceleration, 2, 271, 272;
magnetic influence, 130;
photographs, 152, 153, 268;
solar parallax from disturbed motion, 230, 240;
study of surface, 263;
atmosphere, 263-265;
charts, 265-267;
librations, 266;
superficial changes, 267, 268;
thermal radiations, 269, 270;
rotation, 272;
tables, 272, 273;
origin, 316-318
Morinus, celestial chemistry, 140
Morstadt, Andromede meteors, 332
Mouchez, photographic survey of the heavens, 413;
death, 414
Müller, phases of Mercury, 246;
of minor planets, 288;
albedo of Mars, 283;
of Jupiter, 290;
of Saturn, 303;
variability of Neptune, 305;
of Pons's comet, 366;
stellar photometry, 421
Munich, Optical Institute, 28, 34
Myer, solar eclipse, 183
Nasmyth, Lassell's reflector, 83;
solar willow-leaves, 164;
comparative lustre of Mercury and Venus, 255;
condition of Jupiter, 289
Nasmyth and Carpenter, _The Moon_, 265
Nebula, Andromeda, early observations, 21;
new star in, 394, 395;
photographs, 395, 409;
structure, 396;
spectrum, 402, 403;
visibility at Arequipa, 435
Nebula, Orion, observed by Herschel, 12;
mentioned by Cysatus, 21;
apparent resolvability, 119;
suspected variability, 403;
radial movement, 405;
spectrum, 407;
photographs, 407, 408, 436
Nebulæ, first discoveries, 22;
catalogues, 22, 46, 50, 412;
distribution, 23, 48, 422;
composition, 24, 47, 401, 402;
resolution, 47, 117, 119;
double, 48, 412;
spiral, 118, 410, 412;
new stars in, 394-396, 399, 401;
spectra, 401-403, 407;
variability, 403, 404;
radial movements, 405;
photographs, 407-409, 425
Nebular hypothesis, Herschel's, 24, 25;
Laplace's, 25, 308, 309, 322;
objections, 313-315
Neison, atmosphere of Venus, 254;
rills on the moon, 263;
_The Moon_, 265
Neptune, discovery, 78-83;
satellite, 83, 305;
density, 84;
comets captured by, 98, 306, 365;
mode of rotation, 305, 313, 315, 322
Newall, F., duplicity of Capella, 389;
stellar radial motions, 430
Newall, R. S., 25-inch refractor, 430
Newcomb, runaway stars, 39;
solar translation, 40;
origin of minor planets, 76;
telescopic powers, 119;
corona of 1878, 176;
of 1869, 183;
distance of the sun, 231-233;
velocity of light, 241;
variation of latitude, 259;
lunar atmosphere, 263;
lunar theory, 272, 273;
disturbance of Neptune's satellite, 305;
formation of planets, 314;
star catalogue, 415;
structure of Milky Way, 423
Newton, H. A., capture of comets by planets, 98;
falls of aerolites, 311;
November meteors, 330, 331;
meteors of 1885, 336, 337;
orbits of aerolites, 340
Newton, Sir Isaac, founder of theoretical astronomy, 1, 141;
comets subject to gravitation, 88;
first speculum, 109;
solar radiations, 216;
law of cooling, 217-219;
telescopes and atmosphere, 434
Niesten, volume of asteroids, 287;
red spot on Jupiter, 293
Nobert, diffraction gratings, 439
Noble, observations of Mercury, 246;
secondary tail of comet, 355
Nolan, origin of the moon, 317;
period of Phobos, 320
Norton, expulsion theory of solar appendages, 193 _note_;
comets' tails, 345, 347
Nova Andromedæ, 394, 395
Nova Aurigæ, 396-399
Nova Cygni, 393, 394, 398
Nova Persei, 400, 401
Nutation, discovered by Bradley, 3, 15;
a uranographical correction, 31
Nyrén, constant of aberration, 241
Observatory, Greenwich, 3, 27, 433;
Cape of Good Hope, 6, 36, 433;
Paramatta, 6, 90;
Harvard College, 7, 85;
Königsberg, 30;
Dorpat, 43;
Pulkowa, 44;
Palermo, 72;
Berlin, 90;
Anclam, 149;
Potsdam, 149;
Kew, 153;
Arequipa, 264, 435, 436;
Yerkes, 433;
Lick, 435
Occultations of stars by comets, 95, 105, 106;
by the moon, 263;
by Mars, 276;
of Jupiter by the moon, 264
Olbers, Bessel's first patron, 29, 30;
discoveries of minor planets, 74, 75;
origin by explosion, 75, 76;
career, 89, 90;
Biela's comet, 95;
comet of 1811, 99;
electrical theory of comets, 100, 104, 324, 347;
multiple tails, 100;
comet of 1819, 101;
cometary coruscations, 105;
November meteors, 329
Olmsted, radiant of Leonids, 328;
orbit, 329
Oppenheim, calculation of Schmidt's comet, 363
Oppolzer, E. von, theory of sun-spots, 159;
variability of Eros, 285
Oppolzer, Th. von, Winnecke's comet, 94;
comet of 1843, 350
Oxygen in sun, 213-215;
telluric absorption, 214;
in stars, 384
Packer, variable stars in cluster, 436
Palisa, search for Vulcan, 181, 250;
discoveries of minor planets, 283
Pallas, discovery, 74;
inclination of orbit, 75, 286;
diameter, 75, 287, 288
Pape, Donati's comet, 345
Parallax, annual, of stars, 10, 16, 33, 36, 416-418;
horizontal, of sun, 227;
Encke's result, 230, 232;
improved values from oppositions of Mars, 231, 237;
from light velocity, 231, 232, 241;
from recent transits, 236, 240;
from observations of minor planets, 238, 239;
general result, 242
Paris Catalogue of Stars, 415
Paschen, oxygen in sun, 215;
solar temperature, 220
Pastorff, drawings of the sun, 101
Peirce, structure of Saturn's rings, 298
Perrine, eclipse photographs, 190;
nature of corona, 191;
observation
of Holmes's comet, 369;
nebula round Nova Persei, 401
Perrotin, rotation of Venus, 252;
markings on, 255;
canals of Mars, 279;
clouds on Mars, 281;
striation of Saturn's rings, 299;
rotation and compression of Uranus, 303, 304;
changes of Pons's comet, 366;
Maia nebula, 410;
measures of double stars, 419
Perry, eclipse of December, 1889, 187
Personal equation, 123, 235
Peter, star-parallaxes, 417
Peters, C. A. F.,
parallax of 61 Cygni, 36;
disturbed motion of Sirius, 42
Peters, C. F. W., orbit of Leonid meteors and comet, 332
Peters, C. H. F., sun-spot observations, 147, 148;
discoveries of minor planets, 283;
star maps, 284, 415
Peytal, description of chromosphere, 69
Phobos, rapid revolution, 282, 283, 314;
tidal relations, 320, 321
Photography, solar, 145, 153, 154, 165;
of corona, 166, 173, 175, 178, 181, 185-190;
without an eclipse, 178-180;
of prominences, 167, 197, 198;
of coronal spectrum, 171, 188, 190;
of prominence-spectrum, 195, 198;
of arc-spectrum, 206, 211;
of solar spectrum, 210, 211, 215, 439, 440;
of Uranian spectrum, 305;
of cometary spectra, 354, 357;
of stellar and nebular spectra, 382-384, 396, 398, 400, 407;
lunar, 152, 153, 268;
detection of comets by, 178, 338, 369;
of asteroids, 284;
of new stars, 399;
use of, in transits of Venus, 234, 236, 240;
Mars depicted by, 277, 281;
Jupiter, 297, 408;
comets, 353, 354, 368, 412;
nebulæ, 395, 401, 407-409, 411, 425;
Milky Way, 424, 425;
star-charting by, 413, 414;
star-parallaxes by, 417;
rapid improvement, 438
Photometry, stellar, 49, 420, 421;
of planetary phases, 245, 288;
of Saturn's rings, 299;
photographic, 421
Photosphere, named by Schröter, 55;
structure, 151, 152, 164, 165
Piazzi, star catalogues, 31;
parallaxes, 33;
motion of 61 Cygni, 35;
birth and training, 72;
5-foot circle, 72, 121;
discovery of Ceres, 73, 74
Picard, Saturn's dark ring, 86;
sun's distance, 228
Pickering, E. C., photometric measures of Martian satellites, 282;
of minor planets, 287;
variability of Japetus, 302;
of Neptune, 305;
meteoric photography, 339;
gaseous stars, 379;
hydrogen spectrum in stars 383;
spectrographic results, 385;
eclipses of Algol, 390;
photographic celestial surveys, 399;
star density in Pleiades, 411;
photometric catalogues, 420, 421;
photographic photometry, 421;
white stars in Milky Way, 425;
climate of Arequipa, 435;
horizontal telescope, 437
Pickering, W. H., corona of 1886, 185;
coronal photographs, January 1, 1889, 186;
lunar twilight, 264;
lunar volcanic action, 267;
melting of snow on Mars, 277;
Martian snowfall, 281;
Jupiter's satellites, 292;
photographs of comets, 368;
of Orion nebula, 408;
observatory at Arequipa, 435
Pingré, phenomena of comets, 92, 96
Planets, influence on sun-spots, 163;
periods and distances, 228;
intra-Mercurian, 248-250;
inferior and superior, 288;
trans-Neptunian, 306, 307;
origin, 309, 313;
relative ages, 314, 315
Planets, minor, existence inferred, 71, 72;
discoveries, 73-75, 77, 283, 284;
solar parallax from, 237-239;
distribution of orbits, 286, 287;
collective volume, 287;
atmospheres, 288
Plantade, halo round Mercury, 244
Pleiades, community of movement near, 41;
photographed spectra, 385;
measurements, 410;
photographs, 410, 411;
nebulæ, 410, 411
Plücker, hydrogen in sun, 212
Plummer, solar translation, 39;
Encke's comet, 99
Plutarch, solar corona, 65
Pogson, prominence spectrum, 168;
reversing layer, 172;
discovery of a comet, 335, 339;
new star in cluster, 395
Pond, errors of Greenwich quadrant, 28;
controversy with Brinkley, 33
Pons, discoveries of comets, 90, 94, 365
Pontécoulant, return of Halley's comet, 101
Poor, C. Lane, calculation of Lexell's comet, 367
Porter, solar translation, 40
Pouillet, solar constant, 216, 225;
temperature of the sun, 217;
of space, 270
Poynting, mean density of the earth, 261
Prince, glow round Venus, 253
Pritchard, parallax of Beta Aurigæ, 388;
photographic determinations of stellar parallax, 417;
photometric catalogue, 420
Pritchett, corona of January, 1889, 186;
red spot on Jupiter, 294
Proctor, glare theory of corona, 182;
speed of ejections from sun, 205;
transit of Venus, 233;
distance of sun, 236;
atmosphere of Venus, 254;
rotation of Mars, 275;
map and canals of Mars, 278, 279;
condition of great planets, 289;
Nova Andromedæ, 403;
status of nebulæ, 422, 423;
structure of Milky Way, 424;
star drift, 426
Procyon, satellite, 42; parallax, 417
Prominences, observed in 1842, 63, 64, 68;
described by Vassenius, 68;
observed in 1851, 70;
photographed during eclipse, 167, 188, 190;
without eclipse, 197, 198;
spectrum, 168, 178, 194, 195, 198, 199;
spectroscopic method of observing, 168-170, 194-196;
white, 183, 184;
chemistry, 195, 199;
classification, 196;
distribution, 199;
movements in, 204-206;
heat of development, 220
Quetelet, periodicity of August meteors, 329
Ranyard, drawing of sun-spot, 101;
coronal types, 175, 185;
lunar atmosphere, 265;
Jupiter's markings, 297;
meteors from fixed radiants, 341;
cometary trains, 348;
tenuity of nebulæ, 409
Rayet, spectrum of prominences, 168, 170
Red spot on Jupiter, 293, 296
Reduction of observations, 31;
Bessel's improvements, 32, 122;
Baily's, 60
Refraction, atmospheric, 31;
effects looked for in comets, 106, 353;
Cytherean, 235, 253, 254;
lunar 263, 264
Reichenbach, foundation of Optical Institute, 28, 34, 122
Repsold, astronomical circles, 41, 122;
Cape heliometer, 416
Resisting medium, 93, 94, 360
Respighi, slitless spectroscope, 173;
prominences and chromosphere, 194, 196, 199;
solar uprushes, 205;
spectrum of Gamma Argûs, 380
Reversing layer, detected, 171, 172;
photographed, 172, 189;
depth, 173
Riccioli, secondary light of Venus, 255
Riccò, trials with coronagraph, 180;
distribution of prominences, 199;
spectrum of Venus, 254;
spot on Jupiter, 294;
spectrum of great comet, 364
Richer, distance of the sun, 228
Ristenpart, solar translation, 40
Ritchey, nebula round Nova Persei, 401;
photographs of nebulæ, 432
Ritter, development of stars, 375
Roberts, A. W., southern variables, 392
Roberts, Isaac, search for ultra-Neptunian planet, 306;
photographs of Orion nebula, 408;
of Andromeda nebula, 409;
of the Pleiades, 411
Roberval, structure of Saturn's rings, 299
Robinson, reflectors and refractors, 431
Roche, inner limit of satellite-formation, 301;
modification of nebular hypothesis, 321
Römer, star places, 10;
invention of equatoreal and transit instrument, 120;
of altazimuth, 121;
velocity of light, 231;
satellite transit on Jupiter, 291
Rosenberger, return of Halley's comet, 101
Rosetti, temperature of the sun, 219
Rosse, third Earl of, biographical sketch, 114;
great specula, 115-117;
discovery of spiral nebulæ, 118;
resolution of nebulæ, 119;
climate and telescopes, 434
Rosse, fourth Earl of, experiments on lunar heat, 269
Rost, nature of sun-spots, 54
Roszel, mass of asteroids, 287
Rowland, photographic maps of solar spectrum, 210, 440;
elements in run, 213;
concave gratings, 439, 440
Rümker, observation of Encke's comet, 90
Russell, H. C., red spot on Jupiter, 295;
change in Argo nebula, 404;
photographs of Nubeculæ, 425
Russell, H. N., atmosphere of Venus, 254
Rutherfurd, lunar photography, 268;
star spectra, 372;
photographs of the Pleiades, 410;
diffraction gratings, 439
Sabine, magnetic and sun-spot periods, 127, 128, 130
Safarik, secondary light of Venus, 256;
compression of Uranus, 304
Satellites, discoveries, 110, 282, 293;
transits, 291, 292;
variability, 292, 302;
origin, 309, 318
Saturn, low specific gravity, 298;
rotation, 302;
spectrum, 303
Saturn's rings, first disclosure, 85;
dusky ring, 86;
stability, 298, 300;
meteoric constitution, 300;
eventual dispersal, 301
Savary, orbits of double stars, 46
Savélieff, solar radiation, 164, 225
Sawerthal, discovery of a comet, 366
Schaeberle, discovery of Procyon's satellite, 42;
coronal photographs, 187, 188;
theory of corona, 191;
meteoric photography, 339;
discovery of a comet, 355
Schaeberle and Campbell, observations of Jupiter's satellites, 292
Scheiner, Father, nature of sun-spots, 52, 54;
equatoreal instrument, 120 _note_;
solar rotation, 146;
darkening of sun's limb, 221
Schiener, Dr. J., photospheric structure, 165;
spectrographic researches, 384, 405;
spectrum of Andromeda nebula, 403;
stars and nebulæ in Orion, 407
Schiaparelli, rotation of Mercury, 247;
of Venus, 251, 252;
spots on Mars, 275;
snow-cap, 277;
canals, 278-280;
compression of Uranus, 304;
comets and meteors, 327, 331, 332, 338;
anomalous tail of great comet, 364;
Pons's comet, 365;
origin of comets, 370;
measures of double stars, 419
Schmidt, A., circular refraction in sun, 159
Schmidt, J., sun-spot period, 126;
lunar rills, 263;
lunar maps, 265;
disappearance of Linné, 267;
cometary appendages, 363;
new stars, 393
Schönfeld, extension of Bonn Durchmusterung, 412, 414
Schrader, construction of reflectors, 243
Schröter, a follower of Herschel, 5;
motions of sun-spots, 146;
biographical sketch, 243, 244;
observations on Mercury, 244, 246, 247;
on Venus, 250-253, 255;
on the moon, 263;
a lunar city, 265;
Linné, 267;
spots on Mars, 275;
Jovian markings, 290
Schülen, perspective effects in sun-spots, 54
Schuster, photographs of corona, 178, 185;
spectra of oxygen, 214
Schwabe, sun-spot periodicity, 125, 126
Secchi, chromosphere, 70;
Biela's comet, 97;
cyclonic movements in sun-spots, 144;
distribution, 148;
profundity, 154;
nature, 156, 158;
constitution of photosphere, 151;
eclipse observations, 166, 167;
reversing layer, 171;
observations of prominences, 194, 196, 199;
absence of helium absorption, 213;
temperature of the sun, 218;
solar atmospheric absorption, 221;
Martian canals, 279;
spectrum of Uranus, 304;
of Coggia's comet, 343;
stellar spectral researches, 372, 373;
carbon stars, 372, 381;
gaseous stars, 377
See, stellar orbits, 42, 46;
measures of Neptune, 84;
measures of Uranus, 304;
belts of Neptune, 306;
colour of Sirius, 375 _note_;
southern double stars, 419;
evolution of stellar systems, 420
Seeliger, photometry of Saturn's rings, 299;
rationale of new stars, 396
Seidel, stellar photometry, 420
Sherman, spectrum of Nova Andromedæ, 395
Short, reflectors, 4, 109, 115, 121;
chromosphere, 68;
satellite of Venus, 256;
striation of Saturn's rings, 299
Sidereal science, foundation, 9, 442;
condition in 1785, 10;
progress, 50
Sidgreaves, spots and faculæ, 159
Siemens, regenerative theory of the sun, 312
Simony, photographs of ultra-violet spectrum, 215
Sirius, a binary star, 41;
mass, 42;
parallax, 42, 416;
spectrum, 133, 373, 383;
former redness, 375 _note_;
radial movement, 386, 387
Smyth, Admiral, Donati's comet, 324
Smyth, Piazzi, oxygen spectrum, 215;
lunar radiations, 269;
expedition to Teneriffe, 434
Solar constant, 216, 225
Solar spectrum, fixed lines in, 133-135;
maps, 133, 136, 206, 210, 211, 224, 440;
distribution of energy, 222, 223
Solar system, translation through space, 15, 39, 40, 406;
development, 308, 309, 313-316, 322;
complexity, 441
Soret, solar temperature, 218
South, observations of double stars, 45;
12-inch lens, 113;
Rosse reflector, 117;
occultation by Mars, 276
Spectroscopic binaries, 387-391
Spectrum analysis, defined, 130;
first experiments, 131, 132;
applied to the sun, 133-135, 156;
to the stars, 133, 372, 373;
Kirchhoff's theorem, 135;
elementary principles, 139, 140;
effects on science, 141, 142;
radial motion determined by, 201, 386;
investigations of comets by, 342, 343;
of new stars, 393, 399;
of nebulæ, 401-403
Spencer, position of nebulæ, 422
Spitaler, attendants on Brooks's comet, 366
Spitta, transits of Jupiter's satellites, 292
Spörer, solar rotation, 148, 149;
chromosphere, 199, 200
Stannyan, early notice of chromosphere, 68
Star catalogues, 28, 31, 32, 60, 414, 415;
spectroscopic, 381, 385, 386;
photographic, 412-414;
photometric, 420, 421
Star-drift, 426
Star-gauging, 13, 19, 47
Star-maps, 77, 78, 81, 284, 413, 415;
photographic, 413, 414
Stars, movements, 9, 10, 35, 39, 415, 426;
radial, 386, 387, 406;
comparative brightness, 13, 49, 50, 420, 421;
distances, 35-37, 416-418;
chemistry, 372, 381, 382;
spectroscopic orders, 373;
colours, 374;
development, 375-377;
actual magnitudes, 422;
gregarious, 426
Stars, double, physical connection surmised, 17;
proved, 18, 442;
masses, 38, 42;
catalogues, 43, 45, 47, 50, 418, 419;
orbits, 46, 418;
discoveries, 43, 46, 47, 418, 419, 435;
photographs, 409;
evolution, 420
Stars, gaseous, 377-380
Stars, temporary, 24, 392-401
Stars, variable, early discoveries, 9;
Eta Carinæ, 48, 49, 379;
sun-spot analogy, 128, 392;
spectra, 379;
Algol class, 390, 391;
catalogues, 391, 392
Stefan, law of cooling, 219
Steinheil, stellar photometry, 420;
silvered glass reflectors, 429
Stewart, Balfour, Kirchhoff's principle, 135 _note_;
solar investigations, 154, 155
Stewart, Matthew, solar distance by lunar theory, 230
Stokes, prevision of spectrum analysis, 138
Stone, E. J., reversal of Fraunhofer spectrum, 172;
distance of the sun, 231, 232, 236;
transit of Venus, 240;
Cape catalogue, 415;
proper motions, 426
Stone, O., star catalogues, 415;
measures of double stars, 419
Stoney, carbon in photosphere, 152;
dynamical theory of planetary atmospheres, 288;
perturbations of Leonids, 338;
status of red stars, 375
Stratonoff, star counts in Pleiades, 411
Stroobant, satellite of Venus, 256
Struve, F. G. W., stellar parallax, 35;
career and investigations, 43-45;
occultation by Halley's comet, 106;
Russo-Scandinavian arc, 261, 262
Struve, Ludwig, solar translation, 40
Struve, Otto, parallax of Eta Cassiopeiæ, 38;
solar velocity, 40;
his father's successor at Pulkowa, 45;
eclipse of 1842, 62, 64;
Neptune's satellite, 84;
research on Saturn's rings, 300, 301;
variable nebula, 403
Stumpe, solar translation, 40
Sun, Herschel's theory, 54-57, 70, 149;
atmospheric circulation, 58, 59;
chemical composition, 135, 211-213;
mode of rotation, 146, 147;
Kirchhoff's theory, 149;
Faye's, 150-152;
convection currents in, 150, 152, 165;
dissociation, 152, 206-210;
luminous outbursts, 159-161;
explosions, 205;
heat emission, 216, 217, 221, 222, 225, 226;
temperature, 217-220, 226;
problem of distance, 227;
results from transits, 230, 232, 236, 240;
from oppositions of Mars, 231, 237;
from light-velocity, 232, 241;
from measurements of minor planets, 238;
concluded value, 242;
maintenance of heat supply, 310-313;
past and future duration, 312
Sun-spots, speculations regarding, 52, 53;
Wilson's demonstration, 53, 154;
distribution, 53, 58, 148;
cyclonic aspect, 58, 144, 157, 158;
periodicity, 126, 128, 162, 163;
magnetic relations, 127, 160, 161;
meteorological, 129, 164;
auroral, 129, 130, 160, 162;
photographs, 145, 154;
level, 155;
spectra, 156, 207, 208;
volcanic hypothesis, 158;
Lockyer's rationale, 159;
planetary influence, 163;
relation to Jovian markings, 297
Swan, chromosphere, 70;
sodium line, 132
Swift, E., discovery of a comet, 368
Swift, L., fallacious glimpse of Vulcan, 181, 250;
discovery of a comet, 368
Tacchini, eclipse of 1883, 181;
white prominences, 184;
prominences and chromosphere, 199, 200;
spectrum of Venus, 254
Talbot, Fox, spectrum analysis, 131;
spectroscopic method of determining stellar orbits, 387
Tarde, nature of sun-spots, 52
Taylor, eclipse expedition, 187;
spectrum of Uranus, 305;
achromatic lenses, 431
Tebbutt, comets discovered by, 326, 352;
comet of 1882, 359
Telescopes, achromatic, 112, 431, 432
Telescopes, equatoreal, 84, 120, 121
Telescopes, reflecting, Short's, 4, 109, 115, 121;
Herschel's, 12, 109-111;
Lassell's, 83, 114, 121;
varieties of construction, 109, 110;
Rosse's, 115-119, 434;
Common's, 407, 412, 429
Telescopes, refracting, Fraunhofer's, 34, 35, 121;
Clark's, 114, 429, 430, 433, 436;
Grubb's, 430, 433;
with bent and horizontal mountings, 436-438
Tempel, red spot on Jupiter, 294;
comet discoveries, 327;
cometary observations, 352, 362;
Andromeda nebula, 394;
discovery of Merope nebula, 410
Temperature, of the sun, 217-220, 226;
of the moon, 269, 270;
of space, 270;
on Mars, 277
Tennant, eclipse observations, 168, 169, 174
Terby, surface of Mars, 278, 279, 281;
secondary tail of comet, 355
Thalén, basic lines, 207;
map of solar spectrum, 210;
solar elements, 212
Thollon, line-displacements by motion, 202, 364;
atlas of solar spectrum, 211, 440;
lunar atmospheric absorption, 264
Thome, comet discovered by, 361
Thomson, Sir William (Lord Kelvin), solar chemistry, 138;
magnetic influence of the sun, 161;
tidal strains, 257;
rotation of the earth, 273;
dynamical theory of solar heat, 311, 312
Thraen, period of Wells's comet, 357
Tidal friction, effects on moon's rotation, 271, 272, 318;
month lengthened by, 316, 318;
influence on planets, 319-322;
on development of binary systems, 420
Tietjen, asteroidal orbits, 284
Tisserand, capture of comets, 98;
lunar acceleration, 273;
revolutions of Neptune's satellite, 305;
stationary radiants, 341;
perturbations of Algol, 391;
director of Paris Observatory, 414
Titius, law of planetary intervals, 71, 72, 85
Todd, eclipse of 1887, 185;
solar distance, 236, 241;
trans-Neptunian planet, 306
Tornaghi, halo round Venus, 254
Transit instrument, 120
Trépied, reversal of Fraunhofer spectrum, 172
Troughton, method of graduation, 122
Trouvelot, veiled spots, 148;
chromosphere in 1878, 175;
intra-Mercurian planets, 181, 250;
observations of prominences, 184, 196, 204;
of Mercury, 245, 247;
rotation of Venus, 252;
red spot on Jupiter, 296
Trowbridge and Hutchins, carbon in sun, 212
Tschermak, origin of meteorites, 339
Tupman, transit expedition, 235;
results, 236
Turner, polariscopic coronal photography, 189;
employment of coelostat, 190, 438;
stationary radiants, 341
Ulloa, eclipse of 1778, 69
United States, observatories founded in, 6, 7
Uranus, discovery, 5, 74, 111;
unexplained disturbances, 78, 79, 307;
satellites, 87, 303;
equatoreal markings, 303, 304;
spectrum, 304, 305;
retrograde rotation, 313, 315, 322
Valerius, darkening of sun's limb, 221
Vassenius, description of prominences, 68
Venus, transits, 4, 229, 232;
of 1874, 233-236;
of 1882, 239, 240;
atmosphere, 236, 253, 254;
mountains, 252, 253;
spectrum, 254;
albedo, 255;
ashen light, 255, 256;
pseudo-satellite, 256;
effects upon, of solar tidal friction, 320
Very, temperature of sun, 220;
lunar heat, 270
Vesta, discovery, 75, 76;
diameter, 287;
spectrum, 288
Vicaire, solar temperature, 218
Vico, comet discovered by, 97;
rotation
of Venus, 251;
Cytherean mountain, 253
Violle, solar temperature, 218, 219;
solar constant, 225
Vogel, H. C., solar rotation, 202;
solar atmospheric absorption, 222, 224;
spectrum of Mercury, 245;
of Venus, 255;
of Vesta, 288;
of Jupiter, 290;
of Jupiter's satellites, 293;
of Uranus, 304;
rotation of Venus, 252;
ashen light, 256;
intrinsic light of Jupiter, 291;
cometary spectra, 342, 343, 355, 357;
carbon in stars, 374;
stellar development, 375, 376;
spectrum of Gamma Cassiopeiæ, 378;
of Nova Cygni, 393;
of Nova Andromedæ, 395;
spectroscopic star catalogue, 381;
radial motion of Sirius, 386;
period of Mizar, 388;
eclipses of Algol, 390;
components of Nova Aurigæ, 397;
spectrographic determinations of radial motion, 405, 406
Vogel, H. W., spectrum of hydrogen, 206 _note_, 383
Vulcan, existence predicted, 248;
pseudo-discoveries, 249, 250
Wadsworth, coronal photography, 189
Ward, Nova Andromedæ, 394
Waterston, solar temperature, 218;
meteoric infalls, 311
Watson, fallacious observations of Vulcan, 181, 250;
asteroidal discoveries, 284
Webb, comet of 1861, 326
Weber, Baily's Beads, 62;
illusory transit of Vulcan, 249
Weinek, study of lunar photographs, 268
Weiss, comets and meteors, 332, 334
Wells, comet discovered by, 356
Wesley, drawings of corona, 175
Wheatstone, spectrum of electric arc, 132;
method of ascertaining light-velocity, 232
Whewell, stars and nebulæ, 422
Williams, A. Stanley, canals of Mars, 279;
markings on Jupiter, 295, 297; rotation, 296;
Nova Persei, 400
Wilsing, solar rotation from faculæ, 155;
density of the earth, 261;
system of, 61 Cygni, 419
Wilson, Alexander, perspective effects in sun-spots, 53, 154
Wilson, H. C., red spot on Jupiter, 295;
compression of Uranus, 304;
exterior nebulosities of Pleiades, 411
Wilson, W. E., solar temperature, 220, 222;
ultra-Neptunian planets, 306
Winnecke, comet discovered by, 94;
distance of the sun, 231;
Donati's comet, 324, 347
Wisniewski, last glimpse of 1811 comet, 99
Witt, discovery of Eros, 284
Wolf, C., objections to Faye's cosmogony, 315;
origin of Phobos, 321
Wolf, Max, photographic discoveries of minor planets, 283, 284;
Nova Andromedæ, 394;
Nova Aurigæ, 396;
nebula near Nova Persei, 401;
photographic nebular survey, 412;
galactic nebulosity, 425
Wolf, R., sun-spot and magnetic periodicity, 128, 162, 163;
analogy of variable stars, 128, 392;
auroræ, 129;
suspicious transits, 249
Wollaston, ratio of moonlight to sunlight, 49;
flame spectra, 131;
lines in solar spectrum, 133
Woods, coronal photography, 179, 180;
Cape Durchmusterung, 412
Wrangel, auroræ and meteors, 335
Wright, G. F., Ice Age in North America, 260
Wright, Thomas, theory of Milky Way, 14;
structure of Saturn's rings, 299
Wright, W. H., polarisation of cometary light, 355;
spectrum of nebulæ, 400
Yerkes, donation of a telescope, 433
Young, Miss Anne, nebular hypothesis, 314
Young, C. A., spectrum of sun-spots, 156;
origin, 158;
spectrum of corona, 170, 177;
detection of reversing layer, 171, 172;
prominences and chromosphere, 194-196, 200;
photograph of a prominence, 197;
spectroscopic measurement of sun's rotation, 202;
solar cyclones and explosions, 204, 205;
basic lines, 207;
spectrum of Venus, 254;
red spot on Jupiter, 294;
observations of Uranus, 303, 304;
Andromedes of 1892, 337;
spectrum of Tebbutt's comet, 355;
of Nova Andromedæ, 395
Young, Thomas, absorption spectra, 136
Zach, Baron von, promotion of astronomy, 5, 6, 28;
Baily's Beads, 62;
search for missing planet, 72;
rediscovery of Ceres, 74;
use of a heliostat, 120
Zantedeschi, lines in solar spectrum, 134;
lunar radiation, 269
Zenger, observations on Venus, 253, 255
Zenker, cometary tails, 348
Zezioli, observation of Andromedes, 334
Zodiacal light, relation to medium of space, 94;
to solar corona, 176;
meteoric constitution, 310
Zöllner, electrical theory of comets, 99, 344, 346, 347;
solar constitution, 158;
observations of prominences, 194, 196;
reversion spectroscope, 202;
solar temperature, 220;
Mercurian phases, 245;
albedo of Venus, 255;
of Jupiter, 290;
of Saturn, 303;
of Uranus, 304;
condition of Venus, 256;
of great planets, 289;
Jovian markings, 297;
ages of stars, 375;
polarising photometer, 420, 421
THE END
BILLING AND SONS, LTD., PRINTERS, GUILDFORD
THE SYSTEM OF
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Transcriber's notes:
Original
page line Original text left as is (sic)
--- ---------- --------------------------------------------------
072 13 The search for it, through confessedly scarcely
196 24 The first description are tranquil
page line Original text Replaced with
--- ---------- ----------------------- ---------------------------
003 footnote 1 xviiie xviii^e
(the e is superscript)
009 11 byeways byways
024 46 concentation concentration
043 37 Is appears from It appears from
062 37 appearances seem by him appearances seen by him
072 42 Ecole Militaire École Militaire
082 3 forgotton forgotten
092 footnote 1 11/9647000 1/9647000 (confirmed by
looking up reference quoted)
093 7 phenenoma phenomena
100 17 Bredikhin Brédikhine
131 13 identifiying identifying
140 40 terrestial terrestrial
143 25 appearence appearance
149 27 bloodvessel blood vessel
152 12 Angström Ångström
169 3 undimished undiminished
171 42 sympton symptom
172 18 familar familiar
173 42 photograpic photographic
182 37 by which i structure by which its structure
199 37 Bredikhine Brédikhine
220 26 stata strata
246 30-31 of its orbit 24 hours of its orbit in 24 hours
53 seconds. 53 seconds
260 13 garden at its seasons garden as its seasons
284 21 throngh through
284 13 oparator operator
376 42 recognised. in a recognised in a
377 footnote 3 applie applied
395 42 the gaseous fields o the gaseous fields of
423 35 relatiouship relationship
434 footnote 2 Optice Optics
436 42 ofter some years after some years
436 footnote 1 (two references given, (split into two footnotes,
within a single footnote. and corrected references
In the text footnote 1 in the text)
used twice)
450 27 1862 Conclusion of a 1872 Conclusion of a
454 40 spectographically spectrographically
454 18 spectographic spectrographic
456 4 Lyrae Lyræ
488 index Wolf, R., sun-spot and Wolf, R., sun-spot and
magnetic periodicity, magnetic periodicity,
128, 164, 162; 128, 162, 163;
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