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Title: The Hunterian lectures on colour-vision and colour-blindness
Author: F. W. Edridge-Green
Release date: March 4, 2025 [eBook #75519]
Language: English
Original publication: London: Kegan Paul, Trench, Trübner & Co., Ltd, 1911
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*** START OF THE PROJECT GUTENBERG EBOOK THE HUNTERIAN LECTURES ON COLOUR-VISION AND COLOUR-BLINDNESS ***
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THE HUNTERIAN LECTURES
ON
COLOUR-VISION AND COLOUR-BLINDNESS
THE
HUNTERIAN LECTURES
ON
COLOUR-VISION AND
COLOUR-BLINDNESS
_Delivered before the Royal College of Surgeons of England
on February 1st and 3rd, 1911_
BY
PROFESSOR F. W. EDRIDGE-GREEN
M.D.Durh., F.R.C.S.Eng.
BEIT MEDICAL RESEARCH FELLOW
KEGAN PAUL, TRENCH, TRÜBNER _&_ Co., Ltd.
43 GERRARD STREET, LONDON, W.
1911
PREFACE
As there are many who are interested in the subject of vision and
colour-blindness who are not acquainted with the structure of the eye,
I will give a few details so that these persons may be able to consider
the problem from the point of view of these lectures.
The eye is very similar to a photographic camera, and an actual image
is formed on the back of the eye just as it is on the plate of the
photographic camera or on the view-finder. The eye possesses a lens and
also an iris which acts as an adjustable stop and regulates the size
of the pupil. The membrane at the back of the eye upon which the image
is formed is called the retina. The retina has several layers, but the
sensitive layer consists of two elements called, from their shape, rods
and cones. The problem therefore which has to be considered is, how is
the light which forms the image on the sensitive layer of the retina
transformed into visual impulses?
Those who are interested in the subject will find further details in my
book on _Colour-Blindness and Colour-Perception_ in the International
Scientific Series. In that book there are three plates which show how
the colour-blind see colours.
I have been annoyed to find that unauthorised persons have made
lanterns professing to be mine but grossly inaccurate. The sole makers
are those mentioned on page 53 in this book.
F. W. Edridge-Green.
The Institute of Physiology,
University College,
Gower Street, London.
CONTENTS
LECTURE I
THE THEORY AND FACTS OF COLOUR-VISION AND COLOUR-BLINDNESS
PAGE
THE VISUAL PURPLE THE ESSENTIAL FACTOR IN VISION 11
1. Anatomical Evidence 11
2. Physiological Analogy with other Body Cells 12
3. The Relation between the Foveal and the Extra-Foveal
Regions 13
4. The varying Sensibility of the Fovea 13
5. Chemical Analogy 14
6. Disappearance of Lights falling upon Fovea 15
7. Illusion of Moving Light 16
8. Purple After-Image 17
9. Currents seen in the Field of Vision not due to the
Circulation 17
10. Pressure Figure 19
11. Macular Star 19
12. Entoptic Appearance of Cone Mosaic 20
13. Visual Acuity 21
THE EVOLUTION OF THE COLOUR-SENSE 26
THE FACTS OF COLOUR-BLINDNESS 34
1. Defects of Light-Perception 35
2. Defects of Colour-Perception 38
THE TWO MAIN VARIETIES OF COLOUR-BLINDNESS:
1. Dichromic Vision 40
2. Trichromic Vision 42
LECTURE II
THE DETECTION OF COLOUR-BLINDNESS FROM A PRACTICAL POINT OF VIEW
1. Object of a Test for Colour-Blindness 44
2. The Requirements of a Test for Colour-Blindness 45
3. Persons to be Excluded 47
4. The Construction of a Test for Colour-Blindness 48
5. The Lantern Test 53
6. Other Tests for Colour-Blindness 66
LECTURE I
_Delivered on February 1st_
GENTLEMEN,--Colour-blindness is not a good name for the condition
to which it is applied, and still worse is the use of the term
red-blindness or green-blindness. In the majority of cases of
colour-blindness there is no blindness to colours in the ordinary
acceptation of the term; a green, red, or yellow light produces a very
definite sensation of colour. Those who confuse red and green do so,
not because they see red as green or green as red, but because both
give rise to a similar sensation of colour. The word light must be
used in the sense of referring to those waves which excite the organ
of vision. Because two stimuli excite a sensation of light, it does
not follow that they are similar. We cannot, for instance, distinguish
by the eye polarised light from non-polarised light. We have to
distinguish between the physical stimuli by their physical properties
apart from their effect on the organ of vision. I propose to divide the
subject into two parts, and in this lecture to deal with the theory and
facts of colour-vision and colour-blindness, and in the second lecture
with the detection of colour-blindness from a practical point of view.
I. THE THEORY AND FACTS OF COLOUR-VISION AND COLOUR-BLINDNESS
The following is the theory which I have propounded in order to explain
vision and colour-vision. A ray of light impinging on the retina
liberates the visual purple from the rods and a photograph is formed.
The rods are concerned only with the formation and distribution of
the visual purple, not with the conveyance of light-impulses to the
brain. The ends of the cones are stimulated through the photo-chemical
decomposition of the visual purple by light (very probably through the
electricity which is produced), and a visual impulse is set up which is
conveyed through the optic-nerve fibres to the brain. The character of
the stimulus differs according to the wave-length of the light causing
it. In the impulse itself we have the physiological basis of the
sensation of light, and in the quality of the impulse the physiological
basis of the sensation of colour. The impulse being conveyed along
the optic nerve to the brain, stimulates the visual centre, causing
a sensation of light, and then passing on to the colour-perceiving
centre, causes a sensation of colour. But though the impulses vary
in character according to the wave-length of the light causing them,
the retino-cerebral apparatus is not able to discriminate between the
character of adjacent stimuli, not being sufficiently developed for the
purpose. At most, seven distinct colours are seen, whilst others see
in proportion to the development of their colour-perceiving centres,
only six, five, four, three, or two. This causes colour-blindness, the
person seeing only two or three colours instead of the normal six,
putting colours together as alike which are seen by the normal-sighted
to be different. In the degree of colour-blindness just preceding
total, only the colours at the extremes of the spectrum are recognised
as different, the remainder of the spectrum appearing grey. Though my
own opinion is that the ordinary form of congenital colour-blindness
is caused by a defective development of the portion of the brain which
has the function of the perception of colour, we must not exclude any
portion of the retino-cerebral apparatus, defect of which would have
exactly the same result. It will be noticed that the theory really
consists of two parts, one concerned with the retina and the other with
the whole retino-cerebral apparatus. I shall in these lectures use the
word cerebral in this sense. I am not aware of a single fact which does
not support this theory, and I have used it to predict facts which have
subsequently been rediscovered by others and now form a part of our
common knowledge.
THE VISUAL PURPLE THE ESSENTIAL FACTOR IN VISION
I will now state very briefly the evidence which supports the view that
the visual purple is the essential factor in the retina which enables
it to transform light into visual impulses.
1. _Anatomical._--In the fovea of the retina only cones are to be
found. Immediately external to this each cone is surrounded by a ring
of rods. The number of rings of rods round each cone increases as the
periphery is reached. The outer segments of the cones are situated in
a space which is filled with fluid. The external limiting membrane
retains this fluid in its place. I find[1] four depressions or canals
which lead into the larger depression of the external fovea. These
canals appear to have smaller branches, and serve to conduct the
visual purple into the part of most acute vision. The cones which are
present in the fovea have very long outer segments which would present
a greater surface for photo-chemical stimulation. The visual purple
is only to be found in the rods and not in the cones. I determined to
ascertain whether the visual purple could be seen between the cones
in the fovea. I have examined under the microscope the retinas of two
monkeys which had been kept previously in a dark room for forty-eight
hours. The yellow spot was the reddest part of the whole retina, and
the visual purple was seen to be between and not in the cones.[2]
[1] _Journal of Physiology_, vol. xli, p. 274.
[2] _Transactions of the Ophthalmological Society_, 1902, p. 300.
2. _Physiological analogy with other body cells._--It is far more
probable that the rods should produce a secretion which would affect
other cells rather than themselves. The liver cells do not form bile
in order to stimulate themselves, and the internal secretions are
produced to affect other parts of the body. I am not aware of a single
instance in which a cell produces a secretion which has the function
of stimulating the cell producing it. The visual purple is regenerated
in the rods by the pigment cells in connection with them.
3. _The relation between the foveal and the extra-foveal regions._--As
the fovea only contains cones, if any of the older theories of the
relative functions of the rods and cones were true we should expect
to find qualitative differences between the foveal and extra-foveal
regions. This is not the case, but as we should expect if the visual
purple were the visual substance, all the phenomena which have been
attributed to the visual purple should be found in the fovea. Von
Tschermak, Hering, Hess, Garten and others have found the Purkinje
phenomenon, the variation in optical white equations by a state of
light and dark adaptation, the colourless interval for spectral lights
of increasing intensity, and the varying phases of the after-image in
the fovea only gradually diminished.
4. _The varying sensibility of the fovea._--The fovea is in some
conditions the most sensitive part of the whole retina, and with other
conditions the least. Helmholtz has recorded some of these facts
and regarded them as quite inexplicable. We have, however, an easy
explanation of the facts on the assumption that when there is visual
purple in the fovea this is the most sensitive part of the whole
retina, but when there is none there time must elapse before it can
diffuse into the spot, and in the meantime it is insensitive to light.
I have devised several experiments which show the visual purple flowing
into the foveal region. The following simple experiment shows this very
well. If on awaking in the morning the eyes be directed to a dull
white surface, as for instance the ceiling, the region of the yellow
spot will appear as an irregular black spot, and light will appear to
invade this spot from without inwards. If the eyes be now closed and
covered with the hands, purple circles will form round the centre of
the field of vision and gradually contracting reach the centre. When
the circle reaches the centre it breaks up into a star-shaped figure
and becomes much brighter. It then disappears and is followed by
another contracting circle. Now it will be noticed that if one eye be
opened when the circle has broken up, a brilliant rose-coloured star
much brighter than any other part will be seen in the centre of the
field of vision. This has the exact hue of the visual purple. If we
wait until the star has disappeared before opening an eye, the macular
region appears as a black spot as before. This conclusively shows that
the central portion of the retina is sensitised from the peripheral
portions.
5. _Chemical analogy._--The visual purple gives a curve which is very
similar to that of many other photo-chemical substances. We know that
with photo-chemical substances the chemical effect is not proportional
to the intensity of the light. That is, a different curve is obtained
with weak light from that which is formed with light of greater
intensity. It is reasonable, therefore, to suppose that the visual
purple which is formed by the pigment cells under the influence of a
bright light would be somewhat different in character from that which
is formed in darkness. Again, from the chemical analogy which I have
just given, even if the visual purple were of the same character, we
should not expect similar curves with different intensities of light.
It is probable that both factors are in operation. This deduction
gives an explanation of the Purkinje phenomenon, or the fact that when
the eyes are adapted to darkness the point of greatest luminosity is
shifted more towards the violet end of the spectrum. Some dichromics
who have shortening of the red end of the spectrum have a luminosity
curve which is very similar to that of a normal-sighted person with a
spectrum of lesser intensity. We have only to assume in these cases
either that the receiving nervous apparatus is less responsive, or
that the visual purple formed at one intensity of light is similar to
that formed at a lower intensity by a normal-sighted person. We also
have an explanation of other conditions, such as erythropsia, or red
vision, white objects appearing more or less red. If we suppose that
the eye has remained in a state of light adaptation, the visual purple
produced being more sensitive to the red rays, objects appear of this
colour. As we should expect, erythropsia is frequently associated
with hemeralopia, or difficulty in seeing in the twilight, the eyes
being adapted to light and not to darkness. In green vision the eyes
have probably remained in a condition of more or less adaptation to
darkness, and are therefore more sensitive to the green rays.
6. _Disappearance of lights falling upon fovea._--If the cones are not
sensitive to light, a ray of light falling upon the fovea alone and not
upon the adjacent portion of the retina containing rods should produce
no sensation of light, provided that there is not already any visual
purple in the fovea. It has been known to astronomers for a long time
that if a small star in a dark portion of the sky be steadfastly looked
at, it will disappear from view, whilst other stars seen by indirect
vision remain conspicuously visible. The following simple experiment
shows the same thing. If a piece of black velvet about three feet
square on a door have a pin put in the centre, and the source of light
be behind the observer, the pin will be brightly illuminated; and on
looking at it (the observer not being too close) and keeping the eye
quite still, the pin will disappear, the visual substance diffused
into the fovea centralis being used up and not renewed. When viewed by
indirect vision it is impossible to make it disappear in this way. When
I have taken great care to have very dark surroundings and have used
only one eye, I have made moderately bright lights disappear in this
manner. These facts have been attributed to a defective sensibility
of the fovea for feeble light. The important point, however, that the
light is at first most clearly seen by the fovea and only subsequently
fades, has been overlooked. If these facts were due to a defective
sensibility of the fovea, the star or light would not be visible at
first.
7. _Illusion of moving light._--If a small light be looked at fixedly
in a dark room, it will appear to move until it comes apparently so
close that it could be grasped. The reason of this is that the eye
moves so that the light falls upon a more peripheral part of the
retina. I find that the movement takes place as if some photo-chemical
substance acted under the influence of gravity. For instance, when
standing the light appears to travel upwards; resting the head on one
side, it appears to travel in the opposite direction. The light appears
as if we were looking straight at it, and the eye, which is covered up,
remains directed straight at the object. When the second eye is opened
two images of the light are seen, and the image which is seen with the
periphery of the first eye rapidly coalesces with that seen directly by
the second eye.
8. _Purple after-image._--A positive after-image of a purple (rose)
colour can be obtained after white light or any spectral colour. It
will be noticed that when there is little light during the subsequent
observation the colour of an after-image inclines to blue or green,
when there is more light towards purple or red.
[Illustration: Fig. 1.]
9. _Currents seen in the field of vision not due to the
circulation._--It occurred to me that if there were canals in the
retina which promoted the easy flow of the visual purple into the
fovea, we ought to obtain evidence of the currents flowing along these
channels entoptically. I found that this was the case, and that the
currents could be seen in numerous ways.[3] If one eye be partially
covered with an opaque disc whilst both eyes are directed forwards in
a not too brightly illuminated room, and special attention be paid to
the covered eye, an appearance of whirling currents will be seen with
this eye (see Fig. 1). These currents appear to be directed towards the
centre, and have a very similar appearance to a whirlpool which is fed
by four main branches. These again are fed by smaller branches which
continually change their paths. On closing both eyes all the portion
in which the whirling currents are seen appears as dull purple. These
currents cannot be due to vessels, because we know that the centre
of the retina corresponding to the point where the greatest movement
is seen is free from vessels. The appearance is also very different
from that of the movement of blood in vessels. The currents can also
be seen in the light, in the dark, through yellow-green glass, and
with intermittent light. The main branches form a star-shaped figure
with four rays. The currents carry the visual quality, colour, and
brightness from whence they come into an after-image. They also tend
to move an after-image towards the centre. The currents behave as if
they ran in definite channels, but could also overrun, on any further
stimulus, the banks of the channels. For instance, a thin, bright line
with a little more light appears as a broad band, and the central star
figure will enlarge into a rhomboid, oval or disc. Movements of the
eyes affect the broad currents in the outer part of the field of vision.
[3] _Journal of Physiology_, vol. xli, p. 269.
10. _Pressure figure._--Pressure on the front of the eye causes the
star-shaped figure to be seen, and this changes into a rhomboid with a
little more pressure.
11. _Macular star._--It occurred to me that we ought to obtain evidence
of the canals in the retina in cases where the outflow from the retina
is obstructed, as by tumour. I find this is the case; the star-shaped
figure given by Sir Victor Horsley in his paper on tumour of the
frontal lobe[4] is almost exactly the same as that seen subjectively.
[4] _British Medical Journal_, 1910, p. 556.
[Illustration: Fig. 2.]
12. _Entoptic appearance of cone mosaic._--Appearances corresponding
to the cone mosaic of the retina may be seen in several ways[5] (see
Fig. 2). The appearance seen corresponds to the cone distribution of
the retina as viewed from its outer side, the portions occupied by rods
appearing as dark spaces.
[5] _Journal of Physiology_, vol. xli, p. 226.
13. _Visual acuity._--Visual acuity is most acute with the fovea, and
diminishes from within outwards. It corresponds very fairly with the
cone distribution of the retina. On the other hand, there is not one
single fact which points to the rods as being light-sentient organs.
This is well recognised by those best qualified to judge.[6] I could
give many more facts in support of the view that the visual purple is
the visual substance, and I have not yet had brought to my notice any
fact which is not readily explicable on that hypothesis. There may be
other photo-chemical substances in the retina, but there is not the
slightest evidence that such is the case. I regard the visual purple as
the sole visual substance. We could, of course, split the visual purple
into innumerable simpler photo-chemical substances, each of which has
its own absorption curve, having its maximum in some particular part of
the spectrum. It is difficult to say at present exactly how the visual
purple acts as a stimulus transformer, but this is because so many
plausible hypotheses immediately occur to us. It is very probable that
light acting upon the visual purple is, according to its wave-length,
absorbed by particular atoms or molecules, the amplitude of their
vibrations being increased. These vibrations may cause corresponding
vibrations in certain discs of the outer segments of the cones, which
seem especially constructed to take up vibrations. We know that when
light falls on the retina it causes an electric current. We know how
the telephone is able through electricity to convey waves of sound,
and something similar may be present in the eye, the apparatus being
especially constructed for vibrations of small wave-length. The current
of electricity set up by light may cause the sensation of light, and
the vibrations of the atoms or molecules the sensation of colour.
[6] _Nagel. Physiol. des Menschen_, vol. iii, p. 107.
In all vital processes there is a condition of katabolism or chemical
change in the protoplasm, and an anabolic or building-up process, in
which the protoplasm is restored to its normal state. We have therefore
to consider two definite processes in the visual purple--namely, a
breaking down of the visual purple photo-chemically by light and its
restoration by the pigment cells and rods. Under ordinary conditions
of light, and during the whole of the daytime, the visual purple is
continually being bleached and reformed. It is obvious, therefore,
that when the eye has been kept in the dark and is then exposed to
light, an observation taken immediately will not be comparable with
one taken a few seconds afterwards, because in the first observation
we have only to consider the katabolic change; whilst in the second
observation the anabolic change has to be considered as well, as the
visual purple has to be reformed for subsequent seeing. There appears
to be very little evidence in ordinary circumstances of this anabolic
process; for instance, if we fatigue the eye with sodium light in a
dark room, and then immediately examine a spectrum, we find that
though all the yellow has disappeared there is no increase in the blue;
in fact, the blue seems rather diminished than otherwise. Again, there
is not the slightest diminution in either the red or green, showing
conclusively that yellow cannot be a compound sensation made up by a
combination of red and green. We must therefore explain in another way
the apparent trichromatism of normal colour-vision, which is so well
known to every photographer, especially those who are concerned with
colour photography. If my theory of the evolution of the colour-sense
be the correct one, and we have cases of colour-blindness corresponding
to every degree of the evolutionary process, we have an explanation
of the facts. In past ages all saw the rainbow made up of only three
colours--red, green, and violet. When a new colour (yellow) appeared
between the red and green, it is obvious that a mixture of red and
green would give rise, not to red-green, but to the colour which had
replaced it--namely, yellow. The retina, therefore, corresponds to a
layer of photo-chemical liquid in which there are innumerable wires
each connected with a galvanometer. When light falls upon a portion of
this fluid the needle of the galvanometer corresponding to the nearest
wire is deflected. The wires correspond to the separate fibres of the
optic nerve, and the galvanometers to the visual centres of the brain.
Cases of colour-blindness may be divided into two classes, which are
quite separate and distinct from each other, though both may be
present in the same person. In the first class there is light as well
as colour loss. In the second class the perception of light is the
same as the normal-sighted, but there is a defect in the perception
of colour. In the first class certain rays are either not perceived
at all or very imperfectly. Both these classes are represented by
analogous conditions in the perception of sounds. The first class
of the colour-blind is represented by those who are unable to hear
very high or very low notes. The second class of the colour-blind is
represented by those who possess what is commonly called a defective
musical ear. Colour-blind individuals belonging to this class can be
arranged in a series. At one end of this series are the normal-sighted,
and at the other end the totally colour-blind. The colours appear
at the points of greatest difference, and I have classified the
colour-blind in accordance with the number of colours which they see in
the spectrum. The normal-sighted may be designated hexachromic; those
who see five colours, pentachromic; those who see four, tetrachromic;
those who see three, trichromic; those who see two, dichromic; and
those who see none, totally colour-blind. There are many degrees
included in the dichromic class. There may or may not be a neutral
band, and this is widest in those cases approaching most nearly to
total colour-blindness. I have recorded a case of a patient who was
colour-blind with one eye.[7] It is an interesting fact that for
form vision the colour-blind eye was much the better of the two, and
he could recognise fine lines in the spectrum with this eye which
were not visible to the other. He saw the two ends of the spectrum
tinged with colour and the remainder grey. It will be noticed that
his colour sensations were limited to the extreme red and the extreme
violet--namely, those colours which present the greatest physical
contrast to each other. Neither the red nor the violet appeared of the
nature of a primary colour, but gave the impression that they were
largely diluted with grey. A theory of colour-vision must account for
a case of this kind, and also for the other varieties and degrees of
colour-blindness. The trichromic are a very important class, and any
theory must account for the fact that they see yellow as red-green, and
blue as violet-green. As we should theoretically expect, when there is
shortening of the spectrum the centres of the colours are moved towards
the unshortened side.
[7] “Colour Blindness and Colour Perception,” _International Scientific
Series_, p. 196.
I will now show on the screen some representations of pictures painted
by colour-blind persons. The upper picture is the copy, and the one
below is the one which has been painted by the colour-blind artist.
He has been given a selection of colours on plates, and from them
has selected the one which he thought appropriate in each case. It
will be noticed that the mistakes made are characteristic of the
colour-perception of the person painting them. Whenever I show these
pictures, I am asked why it is that these characteristic mistakes
should be made, and that the true colour of the object is not used
instead? This undoubtedly would be the case if the artist were allowed
to match the colours by directly comparing them. But he is not able to
do this; he looks at the copy and decides upon the colour of an object,
and then looks for the colour with which to paint it.
A man rarely uses a hue which he does not see as a definite colour, and
therefore it has been quite possible for me to pick out those who are
more or less colour-blind in the exhibitors of the picture gallery. For
instance, if a trichromic have to paint a yellow object he will decide,
after looking at it, whether it be a red or green in his estimation,
and represent it accordingly. He will be greatly influenced by the
nature of colours in its immediate proximity, because simultaneous
contrast is increased in the colour-blind. Thus he will certainly
represent a yellow which is adjacent to a red as green, and a yellow
which is adjacent to a green as red.
THE EVOLUTION OF THE COLOUR-SENSE
There can be no doubt that an evolution of the colour-sense has taken
place: the only point is how and when did this occur. It is obvious
that in those low forms of animal life in which the most rudimentary
sense of sight exists there can be no sense of colour. The animal which
can only perceive light and shade can only discriminate in a rough way
between varying intensities of the stimulus. It is obvious, therefore,
that the sense of light must have been developed first and then the
sense of colour. The sense of sight must have been first developed
for those waves which produce their maximum effect upon the sensitive
protoplasm. The next process of development would be for the protoplasm
to become sensitive to the waves above and below those which produced
the primary stimulus. In the physical stimulus which produces the
sensation of light there are two factors to be considered, the length
of the wave and its amplitude: the greater the amplitude within certain
limits the greater the intensity of the sensation. The wave-length of
the physical stimulus is the physical basis of the sensation of colour.
How did the sensations of colour first arise? Let us suppose that the
physiological effect of the physical stimulus differed according to the
wave-length of the physical stimulus.
Let us consider that the eye has reached a stage in which it has
become sensitive to a fair range of the spectral rays; that is to
say, evolution has proceeded to the extent of making the protoplasm
sensitive to rays of light considerably above and below those which
first caused a sensation of light. We now have an eye which is
sensitive to the greater part of the rays which form the visible
spectrum. It is, however, an eye which is devoid of the sense of
colour; no matter from what part of the spectrum the rays be taken the
only difference which will be appreciated will be one of intensity.
I however mentioned that in the physical stimulus there were two
variables, wave-length and amplitude of the wave. Let us now suppose
that a fresh power of discrimination was added to the eye and that it
became able to discriminate between different wave-lengths of light.
What would be the most probable commencement of development of the
sense of colour? Undoubtedly to my mind the differentiation of physical
stimuli which were physically most different. That is to say, the eye
would first discriminate between the rays which are physically most
different in the visible spectrum, the red and the violet, that is
presuming the eye had become sensitive to this range. It is probable
that it had not, and there has been a steady evolution as to the extent
of the spectrum perceived as well as to colour. We have examples of
this in those cases of defective light-perception in which there is
shortening of the red or violet end of the spectrum.
Let us now work out the evolution of the colour-sense on the assumption
that the rays which are physically most different, namely, red and
violet, were those which were first differentiated. We know that the
various rays differ in their effects on various substances; the red
rays are more powerful in their heating effects, whilst the violet
rays are more active actinally, as is well known by the readiness with
which they act upon a photographic plate, which is scarcely affected by
red light. We should now have an individual who would see the spectrum
nearly all a uniform grey of different degrees of luminosity, but with
a tinge of red at one end and a tinge of violet at the other. There is
a great deal of evidence to show that this is how the colour-sense was
first developed. For instance, in the degree of colour-blindness just
preceding total the spectrum is seen in this way. I have also examined
a woman who became totally colour-blind, apparently through disease
of the ear. I examined her when she had recovered a certain amount of
colour sensation; her sensations were confined to the extreme red and
violet. As the colour sense developed it was not necessary that the
rays should differ so much in refrangibility before a difference was
seen, and so the red and violet gradually invaded the grey or neutral
band, until at a certain point they met in the centre of the spectrum.
Such cases are called dichromics.
The next stage of evolution of the colour-sense is when the
colour-perceiving centre is sufficiently developed to distinguish three
main colours in the spectrum. The third colour, green, appears in the
centre of the spectrum, that is, at the third point of the greatest
physiological difference. In accordance with the prediction of the
theory, I found a considerable number of persons who saw the spectrum
in this way, about 1·5 per cent of men. The trichromic see three main
colours in the spectrum--red, green, and violet. They usually describe
the spectrum as consisting of red, red-green, green, green-violet, and
violet. They do not see yellow and blue as distinct colours, and are
therefore in continual difficulty over them. There are very few of the
tests in general use which can detect them, especially if names be not
used. They will usually pass a matching test with ease. An examination
with the spectrum shows that their colour-perception is less than the
normal in every part, though the curve has the same general shape.
The three trichromics described in my recent paper[8] on “The Relation
of Light-Perception to Colour-Perception” each saw ten consecutive
monochromatic patches in the spectrum instead of the eighteen or
nineteen seen by those who see six colours in the spectrum. It is easy
to show that the trichromic are dangerously colour-blind. They will
mark out with my colour-perception spectrometer a patch containing
greenish yellow, yellow, and orange-yellow, and declare that it is
absolutely monochromatic. When tested with coloured lights they find
great difficulty with yellow and blue. Yellow is continually called red
or green.
[8] _Proceedings of the Royal Society_, vol. B 82, 1910, p. 458.
There are several other degrees of colour perception, and it may
be well to say a word or two about them, though I class all above
the trichromic with the normal-sighted for practical purposes, as
they are not dangerously colour-blind, and can always, in ordinary
circumstances, distinguish signal lights correctly. In the next
stage of evolution four colours are seen in the spectrum, and the
fourth colour appears at the fourth point of greatest physiological
difference, namely, at the orange-yellow of the hexachromic or
six-colour people. These persons I have designated “tetrachromic,”
because they see four distinct colours in the spectrum, that is, red,
yellow, green, and violet. They do not see blue as a definite colour,
and are continually classing blues with greens; they usually prefer
to call blue, purplish green. In the next stage of evolution there
appeared those who see five colours in the spectrum--red, yellow,
green, blue, and violet, blue being now recognised as a definite
colour. These are the pentachromic group. These people pass all the
tests in general use with ease. They, however, have a definitely
diminished colour-perception compared with the normal, or those who see
six colours in the spectrum. They mark out in the spectrum only fifteen
monochromatic patches instead of eighteen. They cannot see orange as a
definite colour; for instance, they can never tell whether a strontium
light, which is red, or a calcium light, which is orange, is being
shown them.
In the next stage of evolution orange is recognised as a definite
colour, and thus we get the hexachromic or normal group, and, as
we should theoretically expect, the yellow of the pentachromic is
now split up into two colours--orange and yellow. The last stage of
evolution which we appear to have reached are those who see seven
colours in the spectrum, and the additional one is called indigo. These
constitute the heptachromic group, and this seventh colour appears at
the exact point at which it should appear, according to my theory,
namely, between the blue and violet. Persons belonging to this class
have a marvellous colour-perception and memory for colours. They will
indicate a certain shade of colour in the spectrum, and then next day
will be able to put the pointer at precisely the same point, a feat
which is quite impossible to the ordinary normal-sighted person. They
see a greater number of monochromatic patches in the spectrum than the
hexachromic, but the curve has the same form. The marking out of the
heptachromic does not appear correct to those who see six colours; for
instance, the blue appears to invade the green, and the indigo does
not appear a definite colour at all. If, however, we bisect the blue
of the seven-colour man, and then bisect his indigo, on joining the
centres we get the blue of the six-colour man, showing most definitely
that the blue has been split up into two fresh colours. It will be
noticed that there is room for much further evolution, and we could go
on splitting up the spectrum indefinitely if only we had the power to
distinguish these finer differences, but as a matter of fact I have
never met with a normal-eyed man who could see more than twenty-nine
monochromatic patches in the spectrum, and there are really millions,
though by monochromatic patches I do not mean twenty-nine separate
colours. Not only are all the details of the process of the evolution
of the colour-sense supported by all the facts that we can obtain
from literature and museums, but the theory accounts for facts which
were previously inexplicable. The distinction between light-sensation
and colour-sensation is explained, and all facts of colour-mixing,
complementary colours, and simultaneous contrast. We can understand
how, as in many cases which have been recorded, a man may lose his
colour-perception and still have an unaltered sense of luminosity and
visual acuity.
The explanation of complementary colours is a fundamental part of the
theory. It is obvious that the two colours of the dichromic are only
recognised as different because they are seen in contrast to each
other, and that when they are mixed they neutralise each other. It is
the same with the other colour-sensations, when they are developed they
replace the colours occupying their positions. Therefore green which
replaces the grey of the dichromic should be, and is, complementary to
the other two colour-sensations, red and violet combined. In the same
way when the yellow sensation replaces the red-green of the trichromic
it should be possible to compound it of both. Also, when the green
sensation is in a feeble state of development it will not have the
value that it has at a subsequent stage, and, therefore, yellow will be
a much redder colour to those persons than the normal, and in a colour
match of red and green forming yellow, more green will be required.
Simultaneous contrast is also explained. When two colours are
contrasted each appears to be a colour higher or lower, as the case
may be, in the spectrum scale; that is to say, the close comparison
exaggerates the difference. As the colour-blind have fewer colours,
simultaneous contrast should be greater with them, and this I have
found to be the case.
There may be some relation between the monochromatic patches and the
discs in the outer segments of the cones. These are about sixteen in
number in the guinea-pig. As in photography, the intensity of the
light is a very important factor in vision. With colours of moderate
intensity, the periphery of the retina is found to be colour-blind,
but this apparent colour-blindness disappears when more intense lights
are used. A person may have shortening of the spectrum with light
of moderate intensity, but when the light is increased be able to
recognise the spectrum to its normal limit. The change in steepness of
gradation, according to the intensity of the light, is well known to
photographers. The Purkinje effect, a change in maximum sensitiveness
of the eye according to the intensity of the light, is, in my opinion,
a photo-chemical effect. I find that the Purkinje effect is found
for small portions of the retina if a black object has been situated
in the corresponding part of the field of vision. The yellow pigment
which is found in the yellow spot probably acts like the yellow screen
in photography, which, by absorbing the blue and violet rays of the
atmosphere, renders visible that which would otherwise be invisible.
This is further borne out by the fact that hunters in India are able to
hunt later in the day than usual by using spectacles glazed with golden
yellow glass.
THE FACTS OF COLOUR-BLINDNESS
When we consider the path along which a visual impulse has to pass,
and that each cell has probably some special function in connection
with that impulse, it is not surprising that we meet with a large
number of different defects of colour-perception and light-perception.
Defects of light-perception are quite distinct from defects of
colour-perception.
1. _Defects of light-perception._--The person having the defect is
placed in a similar position to a normal-sighted person with those
particular rays removed or reduced to the same intensity. Defects of
light-perception may be caused by absorption or by some defect in
the visual purple or cerebro-retinal apparatus. The chief defect of
light-perception which is found is shortening of the red or the violet
end of the spectrum. Let us consider the influence of a shortened
spectrum upon colour-vision. The first evident fact is that bodies
reflecting only light, the rays of which occupy the missing portion of
the spectrum, appear black.
Nearly all colours are compound; that is to say, the coloured body
reflects other rays than those of the colour seen. Thus a blue-green
glass may transmit the green, blue, and violet rays of the spectrum.
Let us suppose that we have a substance reflecting the green, blue,
and three-quarters of the violet, the colour of the body to a normal
person being green. Then if we had another substance which reflected
the whole of the violet, it would appear blue. But with a person who
could not perceive the terminal fourth of the violet the colour would
look exactly the same as the green one, and as he could not distinguish
between the two he would be in continual difficulty with blues and
greens. All coloured objects reflecting rays occupying the missing
portion appear darker than they do to the normal-sighted, and are
always matched with darker colours belonging to a point more internal.
Thus a dichromic with a shortened red end of the spectrum matches a red
with a darker green.
It will be noticed that a shortened spectrum, especially if one
end only be affected, may interfere very little with the general
appreciation of shade. If, for instance, we take a case in which the
red end of the spectrum is shortened, so that only three-quarters
of the red of the normal-sighted is seen, then all bodies which
equally reflect or transmit these rays can be correctly compared,
because a similar portion of light has been removed from each. It is
only when one colour reflects or transmits the rays occupying the
shortened portion, and the other does not, that there is any definite
interference with the appreciation of shade. Again, if neither colour
reflects or transmits rays occupying the shortened portion of the
spectrum, there will obviously be no interference with the appreciation
of shade.
A very common mistake due to shortening of the red end of the spectrum
is the confusion of pink and blue. If a person with considerable
shortening of the red end of the spectrum is shown a pink which is made
up of a mixture of red and violet, the red consisting of rays occupying
the missing portion of the spectrum, only the violet is visible to him,
and so the pink appears a violet without a trace of red. This pink is
therefore matched with a violet or blue very much darker than itself.
Mistakes which are due to shortening of the spectrum may be remedied
if we subtract the rays occupying the missing portion from the colour
of confusion. For instance, if we take a blue and a pink which have
been put together as identical by a person with a shortened red end of
the spectrum, and look at them through a glass which is opaque to the
red, but transparent to the remaining rays of the spectrum, both will
appear alike in hue and shade. A person with considerable shortening
of the red end of the spectrum will look at a red light (which is so
dazzlingly bright to a normal-sighted person as to make his eyes ache
after looking at it closely for a few seconds), at a distance of a few
inches, and remark that there is nothing visible, and that the whole
is absolutely black. It is obvious that the light must consist only of
rays occupying the missing portion of the spectrum. The same remarks
which I have made for a shortened spectrum apply to cases in which
there is defect of light-perception through absorption or any other
cause. The person having the defect is placed in a similar position to
a normal-sighted person with those particular rays removed or reduced
to the same intensity.
Another effect of shortening of the spectrum when it is sufficient to
interfere with the difference-perception which appears to be inherent
in the central nervous system, is that the colours appear to be moved
in the direction of the unshortened portion. For instance, we find the
neutral point of the dichromic, with shortening of the red end of the
spectrum, in most cases further towards the violet end of the spectrum
in comparison with a case in which the spectrum is of normal length. In
the same way a trichromic with a shortened red end of the spectrum has
the junction of the red and green nearer the violet end than in a case
where there is no shortening.
The point that I specially wish to emphasise is that, though every
case in which there is defective light-perception can be explained
by a defective sensibility to light of certain wave-length, not a
single case of the very large number of persons that I have examined
can be explained on the older theories; that is, the defect of
light-perception cannot be explained on the assumption that there is
a defect in a light-perceiving substance which is sensitive to rays
of light from a considerable range of the spectrum. A large number
of cases in which there is shortening of the red end of the spectrum
escape detection when only the green test is used, as is usual
according to Holmgren’s instructions.
2. _Defects of colour-perception._--The colour-blind have a diminished
hue-perception and see a less number of colours than the normal. All
the symptoms of colour-blindness are such as we should expect from want
of development of the retino-cerebral apparatus for the perception
for colour. This is evident even in the slighter cases which show a
diminished colour-perception compared with the normal. We find that
the colour-blind are much more dependent on the luminosity of the
colour than the normal-sighted; they require a stronger stimulus; they
fatigue more easily with colours than the normal-sighted; they have a
more marked simultaneous contrast; the visual angle subtended by the
coloured object requires to be larger, and they have a very bad memory
for colours. The diminution of colour-perception with a diminished
visual angle evidently depends upon several causes. It is very marked
when there is diminished light-perception for those rays which are
imperfectly seen. It is also dependent upon certain retinal conditions,
as in scotoma and allied conditions. There are colour-blind persons,
however, who are able to recognise colours under as small a visual
angle as the normal-sighted, and I have examined one dichromic (said
to be red-blind by a physicist) who recognised red easily through the
thickest neutral glass of my lantern, and who had no difficulty with
this colour at a distance.
Apart from any other defect of light or colour-perception, every case
with which I have met has fallen naturally into one of the classes
I have given; that is to say, every person is either heptachromic,
hexachromic, pentachromic, tetrachromic, trichromic, dichromic,
or totally colour-blind. When I first gave this classification of
colour-blindness, the facts that I discovered were so at variance with
those generally stated that it was very difficult for those who were
not well acquainted with the subject to compare the two sets. The
general knowledge of the subject has, however, steadily increased, and
the facts which I had so great a difficulty in getting recognised now
form part of our common knowledge. It would be well, therefore, to
describe the two main varieties of colour-blindness which are of chief
practical importance, and to show the relation which they bear to the
writings of other persons. These two main varieties have dichromic and
trichromic vision.
1. _Dichromic vision._--The cases which come under this head form
the class of the ordinary red-green blind. It is under this head
that nearly every one of the recorded cases may be classed. Vision,
as far as colour is concerned, is dichromic, the neutral point being
situated in the green of the normal-sighted at about λ 500. All the
colours on the red side tend to be confused with each other; therefore
red, orange, yellow and half of the green are seen as one colour, the
remainder of the green, blue and violet as the other. The luminosity
curve in uncomplicated cases is similar to the normal. There may be
shortening of the spectrum at either the red or the violet end of
a varying degree. All degrees of shortening of the red end of the
spectrum may be found. Dichromics with normal luminosity curves are
those which were formerly designated green-blind; but this designation
is not in accordance with the facts, because there is no defect of
light-perception in the green, and the so-called diagnostic mistakes,
as, for instance, putting a bright green with a dark red, are not made.
Cases of so-called red-blind are dichromics with shortening of the red
end of the spectrum. I have shown that the defective perception of
the red end of the spectrum will not account for the dichromic vision
which is found in these cases. We may also meet with shortening of the
spectrum with otherwise normal colour-perception. We also meet with
dichromic cases forming a series from almost total colour-blindness
to those bordering on the trichromic. Any theory must account for the
fact that there are varying degrees of colour-blindness in dichromic
vision, and why there is a large neutral band corresponding to the
colours of the centre of the spectrum in some cases, and in others the
neutral band is so small that the dichromic cannot mark it out. The
two colours seen by the dichromic are red and violet, though where no
distinction is seen between yellow and red, and blue and violet, the
brighter colour will often be selected; that is why so many dichromics
say that they see yellow and blue in the spectrum. Those who have had
practical experience of colour-blindness will know, however, that many
dichromics make no mistakes with the red test. The following will
give the normal-sighted the best idea of colour-blindness, and it is
how the dichromic see the spectrum, and explains why they are able to
distinguish between colours. Let him regard the dichromic as a man who
has two colours--red and violet and white. The purest red is at the
red end of the spectrum; this becomes less and less saturated as the
violet is approached until the neutral or white point is reached; then
violet comes into the white, and this increases in saturation to the
termination of the violet. The ordinary dichromic therefore sees green
as a much whiter and less saturated colour than red.
2. _Trichromic vision._--These persons see three distinct colours
in the spectrum--red, green and violet. They describe the region
intermediate between red and green; that is to say, the orange and
the yellow as red-green, and blue as violet-green. It will be seen,
therefore, that their chief difficulty is distinguishing yellows and
blues. A yellow, for instance, which is situated next to a green will
be called red, and the same yellow when adjacent to a red will be
called green. There are various degrees of trichromic vision, varying
from those who are little better than dichromic to those who are
tetrachromic. The trichromic rarely find any difficulty with their
three main colours--red, green and violet.
These cases have been described under the name of anomalous
trichromatics. This name is one which has been given to those persons
who in making a match between a yellow corresponding to the sodium
flame and a mixture of thallium-green and lithium-red make a mixture
which is different to that of the normal.[9] A man who puts too much
green in the mixture is called a green anomaly; whilst a man who puts
too much red in the mixture is called a red anomaly. The red anomaly
is only a trichromic with shortening of the red end of the spectrum,
and this may be as extensive as in any case of dichromic vision. I
have, however, described trichromic cases which had all the symptoms
attributed to the anomalous trichromatics, but they were not anomalous
trichromatics, as they made an absolutely normal match.[10]
[9] _Proceedings of the Royal Society_, vol. B 76, 1905.
[10] _Transactions of the Ophthalmological Society_, 1907, p. 255.
_Proceedings of the Royal Society_, vol. B 82, 1910.
LECTURE II
_Delivered on February 3rd_
THE DETECTION OF COLOUR-BLINDNESS FROM A PRACTICAL POINT OF VIEW
I. _Object of a test for colour-blindness._--Tests for colour-blindness
are of two kinds; namely, those which are used for the purpose of
ascertaining the special phenomena of colour-blindness, and those which
are employed when the inquiry is made for some practical purpose. As
with visual acuity, it is necessary to fix an arbitrary standard. As we
do not wish to exclude a greater number than is absolutely necessary,
the object of the test should be to exclude dangerous persons and
dangerous persons only. These persons may have other duties to perform
which do not require them to possess a perfect colour-sense. I should,
however, like to see those persons who are specially qualified for
a certain position, occupy it, for instance, men who have to keep a
look-out on our most important ships being selected because of their
accurate colour-vision and visual acuity. I do not mean that I would
select only those men and reject the others, but that I should like
to see a second object of a test, namely, to select those who are
specially efficient so that the Captain might know on whom to rely in
conditions of exceptional difficulty.
II. _The requirements of a test for colour-blindness._--A test for
colour-blindness, when it is to be employed for some definite and
specific purpose, as, for instance, excluding dangerous persons from
certain callings, should be such as to show definitely that the
persons rejected are dangerous. It is very useful to demonstrate to
the men and their fellows that a rejected candidate is dangerous. The
colleagues of a rejected candidate would refuse to risk their lives
with a man who before their eyes called a red light, green. I was
expressing these views when a superintendent of a railway company,
who is using my lamp, told me that he had adopted this method with
great satisfaction to himself and to the men. A man, for instance, who
has been working twenty years on the railway has been rejected for
colour-blindness. He has complained bitterly to the superintendent,
at the same time declaring emphatically that he is normal-sighted.
The superintendent has replied, “You know red?”--“Yes.” “You know
green?”--“Yes.” “You will therefore agree that if you call green,
red; or red, green, you ought to be rejected. Bring two or three of
the other men in with you and I will test you.” The man has readily
agreed to this. The superintendent has then tested him by asking him to
name various coloured objects in the room, and knowing by experience
exactly the coloured objects which are miscalled by the colour-blind
readily exposes his defect. It is noteworthy that on some occasions a
colour-blind man has been tested by another person in the same room
without making any of the mistakes which he subsequently made, because
none but coloured objects which he could readily recognise were shown
to him. This is an example of the necessity of a practical knowledge
of colour-blindness in an examiner. On account of the arrangement of
signals by sea and land, it is necessary that persons employed in the
marine and railway services should be able to recognise and distinguish
between the standard red, green, and white lights in all conditions
in which they are likely to be placed. An engine-driver or sailor has
to name a coloured light when he sees it, not to match it. He has to
say to himself, “This is a red light, therefore there is danger”; and
this is practically the same as if he made the observation out loud.
Therefore, from the very commencement we have colour-names introduced,
and it is impossible to exclude them. The engine-driver is told that
red is a “danger” signal, green a “caution” signal, and white an “all
right” signal. Therefore, it is absolutely necessary that he should
know the meaning of these colour-names. A test should be such as to
make it impossible for the examinee to be coached through it. This is
one of the most important requirements of a test for colour-blindness
and one that is rarely fulfilled. Nearly every one of the tests in
general use fail on this account.
A test should be one which can be carried out as rapidly as is possible
with absolute efficiency; of two equally efficient tests the one which
takes the least time must be selected. A test, therefore, should have
no unnecessary details which though of theoretical interest are not
concerned with the object in hand. The test should be made as easy and
as little complicated for the examiner as possible.
III. _Persons to be excluded._--We wish to exclude all those
individuals who are included in the following three classes: (1)
Those who see three or less colours in the spectrum. (2) Those who,
whilst being able to perceive a greater number of colours than three,
have the red end of the spectrum shortened to a degree incompatible
with their recognition of a red light at a distance of two miles. (3)
Those who are unable to distinguish between the red, green, and white
lights at the normal distance through defect or insensitiveness of the
cerebro-retinal apparatus when the image on the retina is diminished in
size.
I will now explain why these three classes of persons should be
excluded. The first class includes the trichromic, the dichromic, and
the totally colour-blind, in accordance with the facts previously
stated. The trichromic never, in ordinary circumstances, mistake green
for red, but confuse yellow with green or red. Colour is a feeble
quality of objects to them, and nervousness or excitement may reduce
them to the condition of the dichromic. The dichromic are liable to
mistake a green light for red, and vice versa. It is very important
that persons belonging to the second class should be excluded, and
yet none of the ordinarily used tests detect them. The rays of red
at the extreme left of the spectrum are the most penetrating, as may
be seen by looking at a light or the sun on a foggy day, or through
several thicknesses of neutral glass. It is chiefly by these rays that
we recognise a red light at a distance; and it is therefore of great
importance that a sailor or engine-driver should be able to perceive
them. The third class contains persons who are able to distinguish
colours easily when they are close to, but fail to distinguish them at
a distance, owing to the nerve-fibres supplying the central portion
of the retina being impaired. As a light at a distance occupies
the central portion of the visual field, it is essential that the
corresponding portion of the retina should be normal. There are cases
of central scotoma for colours with perfect form-vision; these would,
therefore, not be detected by a test for visual acuity. This class also
includes those who without having a scotoma are unable to distinguish
between colours at the normal distance when the image on the retina is
diminished in size.
IV. _The construction of a test for colour-blindness._--In
the construction of a test for colour-blindness, the facts of
colour-blindness must be utilised so that the object and requirements
of the test are fulfilled. The following facts are of practical
importance.
1. _Most colour-blind make mistakes with certain colours, but are
correct with regard to others._ This may be illustrated in the
following way. Let us take an ordinary dichromic, and, having given
him the set of wools belonging to the Classification Test, ask him to
pick out all the reds. On examining the pile of wools selected as reds,
it will be found that the majority are red, but in addition there will
be some browns and yellow-greens. If he be then told to pick out the
whole of the greens the greater number of those selected will be green,
but there will be also greys, browns, and reds. In each case, it will
be seen that the majority of wools are of the desired colour.
If another dichromic be examined in the same way it will be found
that, though he may not make exactly the same mistakes, he will in all
probability pick out the same greens to put with the reds, and the same
reds to put with the greens. The same result will be obtained if the
colour-blind persons be asked to name a large number of colours. They
will in most cases name the colour correctly. It will be noticed that
the greens which were put with the reds when classifying the colours,
will be called red in naming them. It is evident that the same idea has
guided the colour-blind in each case. This shows that, though a person
may be red-green blind, he is not absolutely red-green blind in the
sense of being totally unable to distinguish between the two colours.
The fact that they are actually judging by colour may be demonstrated
by giving them coloured materials of different kinds, or by asking them
to name a large number of coloured objects.
It will be seen that if we take a dichromic and ask him to name a
number of red and green wools, in the majority of instances he will
name them correctly. But as, almost invariably, the same wools are
chosen, for all practical purposes the same result would be obtained
by asking a person to name a few of these wools. What more decided and
brighter greens could we have than Nos. 76 and 94 of my Pocket Test?
yet these are two of the greens which are called reds by the dichromic.
We should have accomplished as much by asking a colour-blind person to
name Nos. 76 and 94 as if we had asked him to name a large number of
greens. The colours in a test should, therefore, be those which the
colour-blind are particularly liable to miscall. At the same time,
their nature should be unmistakable to the normal-sighted.
2. _The colour-blind name colours in accordance with their
colour-perception, and thus show definitely to which class they
belong._ I have not come across a man who has guessed correctly when
examined with my test. A man who did guess would know that he was
incompetent. As the colour-blind are often not aware of their defect
they answer as they see, only guessing when they feel uncertain as to
the nature of the colour shown. There is probably more misapprehension
on this point than on any other in the practical testing of
colour-blindness.
3. _Colours may be changed to the colour-blind, whilst leaving them
unaltered to the normal-sighted._
4. _The phenomena of simultaneous and successive contrast are much more
marked for the colour-blind than for the normal-sighted._ Two colours,
which have not changed in the slightest degree to the normal-sighted
on being contrasted, have apparently altered very considerably to the
colour-blind. As an example of this, let us take a pure deep yellow,
a bright red, and a bright green. To the normal-sighted the yellow
will be altered very little by comparison with the red or the green,
but a trichromic would say that the colour was green when contrasted
with the red, red when contrasted with the green. This principle of
exaggerated contrast must be borne in mind when examining a candidate.
Thus if a trichromic be doubtful about a yellow, but seems inclined to
call it green, he should be given a pure green to compare with it. In
the same way, in showing the coloured lights, the same colour produced
in a different way should often be shown. Thus an orange-red may be
shown immediately after a pure red. This will not alter the colour
to the normal-sighted, but greatly facilitate the examination of the
colour-blind.
5. _Many colour-blind match correctly, but name the principal colours
wrongly._ Therefore the test must be a naming test, the examinee being
rejected if he confuse the colours which it is essential he should
distinguish between in his occupation.
6. _Many colour-blind recognise colours easily when they are close to
them, or the surface is large, but fail to distinguish between them
when they are at a distance or the image on the retina is small._ The
test must be constructed in conformity with these facts.
7. _The colour-blind are more dependent upon luminosity than the
normal-sighted, and are liable to mistake a change in luminosity for a
change of colour._ The test should have a means of rapidly changing the
luminosity of a colour.
8. _The colour-blind find special difficulty with faint and dim
colours._ The test should have colours of this kind.
9. _The colour-blind who have shortening of the red end of the spectrum
cannot see reds reflecting or transmitting only rays corresponding to
the shortened portion._ It is essential that reds of this kind should
form part of the test.
10. _The colour-blind find more difficulty in comparing colours when
different materials are used, than when the coloured objects are all of
the same nature._
11. _Most colour-blind find more difficulty with transmitted than with
reflected light._
12. _The colour-blind have a defective memory for colours._
13. _Colours may be changed to the normal-sighted whilst leaving
them unchanged to the colour-blind._ When three colours of the
normal-sighted are included in one of the colour-blind, it is obvious
that a change from one colour to another of the three will make no
difference to the colour-blind. Also when the spectrum is shortened,
the addition or rays corresponding to the shortened portion to another
colour will not alter its appearance to the person with the shortened
spectrum. For instance, to a person with shortening of the red end of
the spectrum, a blue will still remain blue, when so many red rays from
the shortened portion have been added to it as to make it appear rose
to the normal-sighted.
14. _The colour-blind may have a sense of luminosity similar to that of
the normal-sighted._
15. _The dichromic distinguish between the colours of the
normal-sighted, which are included in one of theirs by their relative
luminosity and the difference of saturation which is apparent to
them._ A test should therefore have the means of presenting colours of
different saturation in succession.
16. _Colour-blindness is frequently associated with very high
intelligence and exceptional ability._
V. _The Lantern Test._[11]--1. _Description of apparatus._ The lantern
contains four discs: three carrying seven coloured glasses, and one
with seven modifying glasses. Each disc has a clear aperture. The
other mechanical details are: an electric or oil lamp with projecting
accessories, a diaphragm for diminishing the size of the light
projected, handles for moving the discs and the indicator showing the
colour or modifier in use. The diaphragm is graduated in respect to
three apertures to represent a 5-1/2-inch railway signal bullseye at
600, 800, and 1000 yards respectively when the test is made at 20 ft.
The glasses are as follows:---
_Coloured glasses._
1. Red (A and B).
2. Yellow.
3. Green.
4. Signal Green.
5. Blue.
6. Purple.
_Modifying glasses._
7. Ground glass.
8. Ribbed glass.
9. Neutral (No. I).
10. " ( " II).
11. " ( " III).
12. " ( " IV).
13. " ( " V).
[11] Made by Reiner and Keeler, 9, Vere Street, W.; and Meyrowitz, 1a,
Old Bond Street, W.
[Illustration: Fig. 3.]
It will be noticed that three of the discs are similar in every
respect. In some of my lanterns the two reds are put at the end of the
series of colours and numbered Red 1 and Red 2. This makes no important
difference, but the arrangement given here is more convenient. It
should be noted that Red 1 corresponds to Red B and Red 2 to Red A. If
the electric lamp should get broken the projecting apparatus can be
removed and an ordinary kerosene lamp placed behind the aperture.
[Illustration: Fig. 4.]
2. _Reasons for special construction._--The lantern has been
constructed conformably with the requirements and facts of
colour-blindness. All the facts I have given have been considered in
constructing the lantern.
The examiner, on possessing a lantern for the first time, should
carefully test himself with it and ascertain how the different lights
appear to him with different conditions of general illumination.
It is probable that certain improvements may suggest themselves to
him, therefore, I think it will be advisable to deal with certain of
these points, as it will help the examiner in the use of the lantern.
The colours have never been altered, and I certainly should have
altered them if I could have improved the lantern by doing so. I
have never met a single colour-blind person who has not been readily
and easily detected with my lantern, though I have examined many who
have passed other lanterns and in some cases a number of other tests
for colour-blindness. In most cases one turn of the wheel will be
sufficient to make a colour-blind person disclose his defect.
The examiner may be dissatisfied with the colour of the blue; let us,
therefore, compare an examination of a normal-sighted person with that
of an ordinary dichromic. The normal-sighted person will name every
one of the colours with ease and certainty, with perhaps the exception
of the blue, with which he is in some doubt. Here is the result of an
examination of an ordinary dichromic: he called the yellow, green; the
green, red; the signal green, no colour; the blue, blue; the purple,
green; red A, no colour or light; red B, green. It will be noticed that
the only colour that he has correctly named is the blue. We can try
him again and again, and though he will mistake all the other colours
he will always name the blue correctly. The examiner will have learnt
from this several important facts. He will see that the colour-blind
are really guided by their sensations of colour, and that it is not
simply a matter of guessing. The more an examiner has practical
experience of colour-blindness, the more will he recognise the fact
that the colour-blind are guided by their sensations of colour. He
will notice that the dichromic has readily recognised the blue which
was scarcely apparent to him (the examiner), and therefore cannot have
overlooked as a matter of carelessness colours which are much more
apparent to the normal-sighted. The blue is a valuable colour for other
reasons, for though it is not a colour on which I reject candidates,
anyone miscalling it must be very carefully examined before he is
passed. The trichromic generally call this blue, green. If we wish to
obtain a purer blue, we can do so by combining the blue or the purple
with the signal green.
Again, the examiner may think that it might be better to have an
apparatus which showed two or three lights instead of only one. I
will therefore give my reasons for adopting only one. This point was
one which occupied my attention for a considerable time, especially
in view of the fact--which, as far as I am aware, I was the first to
discover--that simultaneous contrast is increased to the colour-blind.
I was naturally anxious to turn this fact to account. I found,
however, that I gained nothing by increasing the number of lights, and
that in many cases it was a source of error. A second or third light
could have been easily added to my lantern, but besides unnecessarily
complicating the apparatus it would have served no useful purpose. All
the results which are obtained with simultaneous contrast are obtained
even more effectively with successive contrast. It will be noticed
that when lights are seen in ordinary conditions they are conditions
of successive contrast, and not of simultaneous contrast. An observer
rarely keeps his eyes definitely fixed on one light whilst he names
those adjacent to it, but moves his eyes so that the images of the
respective lights fall successively on his foveas. When more than one
light is employed, all the disadvantages of matching as against naming
are introduced. It will be seen that by presenting one light after
another we are fulfilling all the necessary conditions, only that the
light is moved instead of the eyes, sufficient time being allowed to
elapse to enable a normal-sighted person to readily recognise the true
colour of the next light without being confused by the after-image of
the one he has just seen. Many nervous normal-sighted would name a
yellow light seen between two red lights as green, and it does look
green to them from ordinary physiological conditions. They look first
at one red light, then immediately at the yellow light, then at the
second red light, and then again at the yellow light until they feel
sure that the centre light is a green light, and say so. I have never
met with a normal-sighted person who has miscalled the unmodified light
of my lantern, either red or green. Many humble, nervous normal-sighted
persons are under the impression that they are colour-blind, and yet
would make perfectly efficient officers. Many of these men have been
told by their wives or other persons that they are colour-blind, and,
believing this, try to see colours which are not visible to them. I
have examined many persons of this description, and have noted the
ease and accuracy with which they have gone through the tests for
colour-blindness when they have been assured by me that they were
normal-sighted. On the other hand, it is often very difficult to
convince a self-reliant, colour-blind person that he is colour-blind.
He is on the look-out for the small differences which he notices
between colours, and the fact of having another light for comparison
gives him the desired clue, and, though colour-blind, he passes the
test.
The material is the best possible, as it will not fade like all dyed
substances, and therefore all records made with one set of apparatus
will be uniform. Again, a coloured light has none of the accessory
qualities which enable the colour-blind to pass through other tests.
Thus many dichromics will call the yellow glass red or green, who would
not think of putting a yellow with a green or red wool, on account of
the difference in luminosity. He will, in the same way, if told to pick
out colours in the Classification Test to match the colour of the light
shown, have to depend upon his colour-perception. This is a useful
method with nervous and undecided candidates. The objection to it is
that it cannot be carried out in the dark or in a dark room. The Test
is not open to any of the objections which may be urged against the
method of simply naming colours, because the character and intensity of
the colour may be changed at will.
The method is better than that of direct comparison, because the
candidate is forced to use his colour-perception, and has to compare
the colour seen with previous impressions of colour in his mind. By the
use of neutral glasses, etc., I have obviated the fallacy of the method
of naming colours (namely, that these can be distinguished by their
saturation and luminosity), and forced the individual to depend upon
his colour-perception, and not upon some other accessory quality of the
object seen.
No amount of coaching will enable a colour-blind person to pass this
test, whilst almost any other may be passed in this way. I have tried
on many occasions to coach a man so as to pass my lantern, and without
success. The combinations are so numerous that the only result is to
make the colour-blind man nervous and doubtful and more easily detected
than before. This has occurred with men who could pass other tests with
ease.
The test also has a quality possessed by no other--namely, that of
enabling the examiner to reject dangerous persons and dangerous persons
only, the lower degrees of colour-blindness being allowed to pass.
3. _Special directions for conducting the test._--(1) The candidate
should be seated at a distance of twenty feet from the lantern. (2) He
should be asked to name the colour of the light produced by a coloured
glass (1 to 6) alone, or in combination with another coloured glass
or glasses, or with the modifying glasses (7 to 13). (3) A candidate
should be rejected (i) if he call the red, green, or the green,
red, in any circumstances; (ii) if he call the white light, in any
circumstances, red or green, or vice versa; (iii) if he call the red,
green, or white lights, black, in any circumstances. (4) A candidate
who makes mistakes, other than those mentioned above, should be put
through a very searching examination. It is not necessary to have the
room absolutely dark; in fact, I prefer a certain amount of light. The
examiner can, if he wish, make the test at night in the open air.
The examiner should on no account conduct the examination on any
regular plan, because the candidate, anxious to pass, finds out
from persons who have already passed the order and method of the
examination, and so, though colour-blind, might obtain a certificate.
Any one of the glasses may be shown first, and the candidate required
to name the colour of the light. The following will serve as an example
of the method to be employed in testing a candidate. A red being shown,
the candidate is required to name its colour. Then a blue or green may
be substituted. It is best to use the largest aperture at first and
to show all the colours on one disc. This will give confidence to
the normal-sighted candidate, whilst most of the colour-blind will be
detected. In the case of candidates who appear to be normal-sighted
and yet very nervous, there is no harm in telling them after they have
named all the colours on the disc correctly that this is the case.
No comment should, however, be made on individual answers. Then one
of the neutral, ground, or ribbed glasses should be inserted, not
the slightest intimation being given to the candidate of the nature
of the colour. He should be asked to name or describe the light, and
the answer, if incorrect, together with his other replies, carefully
recorded. The other glasses may then be shown, a combination of the
neutral, ground, ribbed, and coloured glasses being used at irregular
intervals.
When the candidate has been examined with the largest aperture,
the examiner can go through the same procedure with one of the
smaller apertures. I have found the third aperture the one which is
most generally useful. On account of the great diminution of total
luminosity caused by the diminished area of the light source, the three
smallest apertures can only be used in a dark room.
If a candidate hesitate about a colour and ultimately name it
correctly, a second and, if necessary, a third glass of the same
colour should be combined with the first. The fact that in one case a
single glass is used, and in another two or three of the same coloured
glass, makes very little difference in the colour of the light to
the normal-sighted. This is not the case with the colour-blind; a
dichromic who has hesitated about a green and then correctly named it
may emphatically call the light red when another green glass is put in
front of the first.
Care must be taken when the candidate is going to be examined with two
glasses at once, such as one of the neutral, ground, or ribbed glasses,
and a coloured glass, that he does not see the light until both are in
position, or else he may see the colour before it is modified in the
necessary way.
If the candidate call the standard red, green; or the standard green,
red, in any circumstances--that is, either alone or in combination with
the modifying glasses--he is to be rejected.
The examiner should ascertain for himself how far the various colours
are visible when modified with the neutral glasses. If the red and
green be not visible with the thickest neutral in the conditions of
luminosity and external lighting which the examiner is employing, he
should use the darkest neutral which allows the colours to be plainly
visible to the normal-sighted. In all cases of doubt the examinee
should be asked to walk towards the lantern and told to say when the
light is visible, and asked to name its colour. The distance at which
the light is visible, and then that at which the colour is visible,
should be noted and compared with the normal.
Particular attention should be paid to the answers given to the
combination of the thickest neutral glass with the standard red and
green respectively.
The examiner should utilise the fact that successive contrast is
increased in the colour-blind, as this is an easy method of detecting
the trichromics. The red having been shown, the light should be quickly
changed to yellow or clear, the examiner’s hand being placed over
the aperture if there be any intervening colours. It is necessary
that the yellow should be shown immediately after the red without
any intervening colours being first seen by the candidate. The
normal-sighted do not see any change in the yellow or clear when they
are shown after the red light, but the trichromic call the yellow
light, green. The examinee should then be shown the green light, and
then the yellow or clear, in the same way as mentioned for the red. The
normal-sighted will easily recognise the yellow, but the trichromic
will call it red. This portion of the examination must never be omitted
in any examination in which the candidate is passed. The two divisions
of the test--that is, showing yellow immediately after red and after
green--may be used at different periods of the examination, and, if
there be any doubt, repeated.
An examiner should, as far as possible, with the exceptions given in
the instructions, avoid all conversation with the candidate, simply
asking, “What colour is this?” and recording the answer without
comment. If an examiner after each answer say, “Quite right,” or some
such expression, the following is likely to occur. The candidate after,
say, six correct answers, makes a mistake; the examiner says, “Are you
sure?” Then the candidate knows at once that he has made a mistake,
and makes a guess, very probably a correct one. When a similar colour
is shown subsequently, he remembers the mistake he made, and gives the
second, and probably the correct answer.
In addition to being an efficient test, it is a very rapid test, as
many men who have been certified as normal after a lengthy examination
with other tests have at once disclosed their defect by calling the
green light of the lantern red. Many are under the impression that in
an examination with the lantern the dichromics simply guess. This is
entirely wrong. A man who did guess would know that he was incompetent.
I find that men have named the coloured lights in strict accordance
with their colour-perception. A man may, however, guess if examined
by an inexperienced and ignorant examiner, who when the examinee has
made a mistake promptly corrects it in a cross tone. A normal-sighted
person will guess when examined in this way. The examiner must receive
the examinee with a smiling face and courteous manner, and appear
pleased and satisfied with the answers, no matter what they may be.
The candidate is then placed at his ease, and answers according to
his colour-perception. It will be noticed that the lantern detects
those who have a slightly diminished colour-perception, as well as
the dangerous varieties of colour-blindness. The former undoubtedly
are not as efficient as those who have a normal colour-perception,
so that a definite standard will have to be fixed, as in the case
of visual acuity. Further details will be found in my book on
Colour-Blindness.[12]
[12] _International Scientific Series._ Kegan Paul & Co., 1909.
_Summary of method of examination._--(1) Show all the colours on one
disc with the largest aperture. (2) Show the reds, greens, and yellow
modified by the neutral glasses. (3) Show all the colours on one disc
with Number 3 aperture. (4) Show red, then immediately afterwards
yellow with largest aperture. Then show green and yellow immediately
afterwards. (5) Test the candidate with the red, green, and yellow with
the smallest aperture. (6) Show the neutrals or ground glass alone. (7)
Show blue made by combining blue or purple with the signal green. (8)
Show a colour, for instance, green, and then combine another glass of
the same colour. (9) Show the red produced by the combination of purple
with red A. (10) Give the combination of red A and signal green.
VI. _Other tests for colour-blindness._--I have three other tests for
colour-blindness: the Classification Test, the Pocket Test, and the
Colour-perception Spectrometer. I have also devised an instrument for
estimating the exact amount of red, at different wave-lengths, which is
necessary to neutralise the complementary in different persons.
1. _The Classification Test._--(_a_) _Description._--This test
consists of 4 test colours and 180 confusion colours; 150 coloured
wools, 10 skeins of silk, 10 small squares of coloured cardboard, and
10 small squares of coloured glass. The whole series of colours is
represented. In addition, there are a large number of colours which
have been chosen by colour-blind persons as matching the test colours.
The test colours are Orange, Violet, Red, and Blue-green, labelled I,
II, III, and IV respectively. The colours are chosen with the view
of presenting as much difficulty as possible to the colour-blind,
and as little as possible to the normal-sighted. The colour-blind
find especial difficulty in matching or naming a colour lying at the
junction of two of their colours. As the normal-sighted often find
difficulty in saying which colour predominates in a blue-green, so do
the tetrachromic with their purple-green, or the trichromic with their
red-green. A colour-blind person may, however, match a colour correctly
which corresponds to the centre of one of his colours. In addition to
choosing those colours for tests which are particularly liable to be
mistaken for other colours by the colour-blind, I have used coloured
materials of different kinds--wools, silks, glass, and cards--so as to
force the colour-blind to judge by colour, and not by saturation or
luminosity. (See Fig. 5.)
[Illustration: Fig. 5.]
(_b_) _Method of examination._--The candidate should be given the four
test colours, and, having named each, he should be told to select all
those which are similar in colour to the test colour. He should be told
to pay no attention to the fact of a colour being lighter or darker; as
long as it is the same colour it should be put with the test skein. The
examiner should not go through the test before the candidate first of
all, neither should one candidate be allowed to watch another making
his selection. A shrewd colour-blind person might pass the test if he
had seen a normal-sighted person go through it previously. In order to
show the candidate the difference between a shade and a colour, the
examiner should take one of the wools which is not a test colour--blue,
for instance--and pick out four or five shades of the colour. The wools
should be arranged without the knowledge of the candidate, so that a
yellow or a grey is placed beside a red and the examinee asked to name
its colour. At another period of the examination the yellow should be
placed adjacent to a green, and the examinee again asked to name it.
The examinee may pick out a certain number of colours correctly, and
then stop, saying that there are no more exactly like the test colour.
This may embarrass the examiner; he should, however, examine any
candidate who has omitted any colours as carefully as if mistakes had
been made. He should ask the candidate to match one of the omitted
colours.
The examiner will soon find out from experience those colours which are
named and matched wrongly by colour-blind persons; he should ask the
examinee to name some of these colours.
Any candidate should be rejected who calls an orange or red, green
or brown; black, red or vice versa; or green, either purple, rose,
red, grey, brown, or violet. Similar mistakes in matching necessitate
objection. A candidate who puts purple, rose, or blue with violet, or
yellow-brown with orange is most probably dangerously colour-blind
and should be very carefully examined. There are cases which pass the
Holmgren test with ease that fail in the most conclusive manner with my
Classification Test. They put green with orange, brown and black with
red, and grey with blue-green. This is due to a different selection
both of test colours and confusion colours. Orange is by far the most
important test colour, and its confusion with green by the dichromics
is very conclusive. The three other test colours, violet, red, and
blue-green, represent both ends of the spectrum and the neutral point
in dichromic cases, and practically these colours are those with which
most mistakes are made. This test can only be regarded as supplementary
to the Lantern Test.
2. _The Pocket Test._--This consists of nineteen cards, on nine of
which are 112 single threads of wool, and 14 pieces of twisted silk,
similar to those in the Classification Test. These are numbered
consecutively, with the exception of the first thread of the first four
cards, and the last thread of the next four cards. The end threads of
the first four cards, I to IV, form the tests; they are Orange, Violet,
Red, and Blue-green. There are also cards on which red, orange, green,
blue, violet and purple, and grey, respectively are to be found. There
are also two special cards marked “Without Red” and two special cards
marked “Without Green.” (See Fig. 6.)
[Illustration: Fig. 6.]
Many normal-sighted persons might object to the inclusion of some of
the colours on the orange card, but this card clearly shows the colours
which may be taken as a match. Fine distinctions are not wanted. The
series of colours I have selected and arranged so as to confuse the
colour-blind and force them to be guided by their colour-perception,
whilst the quantity of colour is amply sufficient for the
normal-sighted to pick out the colours with the greatest ease. The
cards should be arranged irregularly on a white cloth in a good light.
The two most important tests colours are the Orange and Violet, Nos. I
and II. The person examined should be asked to point out the shades of
colour similar to No. I (Orange). A piece of paper rolled to a point
should be used for this purpose. If he do this correctly, he probably
possesses normal colour-perception. If, however, he match the test
with reds or pinks, he is more or less colour-blind, at best belonging
to the pentachromic class. If, in addition, he match the Violet test,
No. II, with blue, he at least belongs to the tetrachromic class. The
trichromic, in addition, may match the Blue-green test, No. III, with
brown and grey. The dichromic will match the Orange test, No. I, with
yellow-green and yellow-brown. Similar mistakes will be made to those
described in connection with the Classification Test. The examinee
should be asked to name all the colours on one of the cards. He should
also be asked to point out on which of the cards the four test colours
are to be found, and which contain none of the test colour.
The examiner should continually change the order of the cards. Most of
the varieties of the colour-blind will be readily detected in this way.
The special advantages of this test are: (1) The colour-blind can be
ranged definitely in their proper classes. (2) Central scotoma can be
detected with its aid. (3) The series of colours are arranged so as
to confuse the colour-blind, whilst the normal-sighted easily match
the test colours. (4) On account of the introduction of different
materials, the relative luminosity and saturation of colours does not
serve as a guide to the colour-blind. (5) Portability. (6) The wools
and silks are kept clean. (7) An important colour is not likely to be
lost.
3. _The Colour-Perception Spectrometer._[13]--(_a_) _Description of
apparatus._--This instrument is a spectrometer so arranged as to
make it possible to expose to view in the eyepiece the portion of a
spectrum between any two desired wave-lengths. In the focal plane of
the telescope are two adjustable shutters with vertical edges; the
shutters can be moved into the field from right and left respectively,
each by its own micrometer screw, and to each screw is attached a drum,
the one being on the right and the other on the left of the telescope.
On each of these drums is cut a helical slot in which runs an index,
and the drum is engraved in such a manner that the reading of the index
gives the position in the spectrum of the corresponding shutter in
wave-lengths direct. (See Fig. 9.) Thus it will be seen that if, for
instance, the reading on the left drum-head is 5320 and that on the
right drum-head is 5920, the region of the spectrum from wave-length
5320 to wave-length 5920 is exposed to view in the eyepiece.
[13] Made by A. Hilger, 75a, Camden Road, London, N.W.
[Illustration: Fig. 7.]
[Illustration: Fig. 8.]
[Illustration: Fig. 9.]
(_b_) _Directions for using the instrument._--It should be used as
far as possible with a known quality and intensity of light. A small
oil-lamp is quite suitable for the purpose. The observer should
first ascertain the exact position of the termination of the red
end of the spectrum, the left-hand shutter being moved across until
every trace of red just disappears. The position of the pointer on
the left-hand drum is noted, and the wave-length recorded. The left
drum is then moved so that the shutter is more towards the middle of
the spectrum. The right-hand drum is then moved, until the pointer
indicates the wave-length recorded as the termination of the red end
of the spectrum. The observer then moves the left-hand shutter in
and out until he obtains the largest portion of red, which appears
absolutely monochromatic to him, no notice being taken of variations
in brightness, but only in hue. The position of the index on the
left-hand drum is recorded. The left-hand shutter is then moved towards
the violet end of the spectrum, the right-hand shutter being placed at
the position previously occupied by the left-hand shutter. In this way
the whole of the spectrum is traversed until the termination of the
violet end of the spectrum is finally ascertained with the right-hand
shutter. The variation of the size of the patches and the terminations
of the spectrum with different intensities of light can be noted. The
instrument can also be used for ascertaining the exact position and
size of the neutral patch in dichromics, the position of greatest
luminosity, and the size and extent of pure colours. When it is used
to test colour-blindness, the examinee should first be shown some
portion of the interior of the spectrum, and then asked to name the
various colours which he sees. In this way he will have no clue to the
colours which are being shown him.
_Objections to other tests for colour-blindness._--The tests which
have been proposed for colour-blindness are very numerous, but some
are so defective that it is rare to detect a single colour-blind
person with them. I have, for instance, tested men whom I knew to be
colour-blind with certain lanterns with the result that not a single
one was detected. In these so-called tests all the requirements of
a test and facts of colour-blindness have been neglected. I must,
however, refer to three tests constructed by exceptionally able men,
each with considerable knowledge of the subject. I refer to the tests
of Professor Holmgren, Professor Stilling, and Professor Nagel.
All these tests can be passed at the first attempt without coaching by
certain dangerously colour-blind persons, chiefly varieties not known
to the inventors, but the chief defect of each is that it is very easy
to coach a colour-blind person to pass it. The surgeon to one of our
largest railway companies told me that when they used Holmgren’s test
they rejected one man in three hundred, but with my lantern twelve in
the same number. All these three tests are much better tests when the
persons to be examined have not seen them before. A colour-blind man
may make only one mistake, say for instance, as in a case I examined
the other day, with Nagel’s test (last edition), he passes the test
perfectly with the exception of one mistake, that of calling a grey on
one card, green. All he has to do is to look for some distinguishing
mark on this card in order to go through the test with the ease and
certainty of a normal-sighted person. It is the same with Stilling’s
letters, he has only to note the letter which he was not able to read
and the appearance of the card. A normal-sighted man or woman would
readily help him. The confusion of green and grey does not appeal
to the average man as a serious defect, especially when he sees his
friend go through the rest of the test perfectly. He says to himself,
“I suppose he sees a tinge of green in that grey.” The same man would
rightly regard it as a most iniquitous proceeding to endeavour to coach
his friend through a test when he had seen him mistake a red for a
green light.
Holmgren’s test rejects a large number of normal-sighted persons, as
may be seen by the reports of the Board of Trade; about 50 per cent
of those who appeal are found to be normal-sighted and to have been
rejected wrongly.
WILLIAM BRENDON AND SON, LTD.
PRINTERS, PLYMOUTH
Reiner and Keeler, L^{td.}
OPTICIANS
----AND----
INSTRUMENT MAKERS
_MANUFACTURERS OF_
The Edridge-Green Colour Perception Lantern
The Edridge-Green Classification Test
The Edridge-Green Pocket Test
and other
Optical and Scientific Instruments
THE ABOVE COLOUR TESTS ARE CERTIFIED BY PROF. F. W. EDRIDGE-GREEN
9 Vere Street,
Cavendish Square, =London, W.=
Telephone: 447 Mayfair.
*** END OF THE PROJECT GUTENBERG EBOOK THE HUNTERIAN LECTURES ON COLOUR-VISION AND COLOUR-BLINDNESS ***
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