Part 1

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The text of this e-book has been preserved as in the original, except for the correction of a single typo (ests → test), the insertion of a missing footnote anchor and the shifting of footnotes below the relevant paragraph.

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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.