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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.