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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 Febi'iiary \st and ^rd, 191 1 
 
 BY 
 
 PROFESSOR F. W. ^DRIDGE-GREEN 
 
 M.D.DuRH., F.R.C.S.Eng. 
 
 BEIT MEDICAL RESEARCH FELLOW 
 
 KEGAN PAUL, TRENCH, TRUBNER & Co., Ltd. 
 
 43 GERRARD STREET, LONDON, W. 
 
 19H 
 
OPTOMETRY UBRAW 
 
ESS 
 
 OPT0MEL1 
 LIBRARY 
 
 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 trans- 
 formed into visual impulses ? 
 
 Those who are interested in the subject will 
 find further details in my book on Colour-Blindness 
 
 5 
 
 04425 
 
6 PREFACE 
 
 and Colour 'Perception in the International Scien- 
 tific 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 . ii 
 
 1. Anatomical Evidence . . . . . ii 
 
 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- 1 mage . . ... 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 
 
 7 
 
8 CONTENTS 
 
 LECTURE II 
 
 THE DETECTION OF COLOUR-BLINDNESS FROM A 
 PRACTICAL POINT OF VIEW 
 
 1. Object of a Test for Colour-BIindness 
 
 2. The Requirements of a Test for Colour-BIindness 
 
 3. Persons to be Excluded . ... 
 
 4. The Construction of a Test for Colour-BIindness . 
 
 5. The Lantern Test . ... 
 
 6. Other Tests for Colour-Blindness 
 
 PAGE 
 
 44 
 45 
 47 
 48 
 
 53 
 66 
 
LECTURE I 
 
 Delivered on February ist 
 
 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. 
 
 9 
 
10 THE HUNTERIAN LECTURES 
 
 I. The Theory and Facts of Colour- 
 Vision AND CoLOUR-BlINDNESS 
 
 The following is the theory which I have pro- 
 pounded 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 decom- 
 position of the visual purple by light (very 
 probably through the electricity which is pro- 
 duced), 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 
 
ON COLOUR-VISION 11 
 
 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 re- 
 mainder of the spectrum appearing grey. Though 
 my own opinion is that the ordinary form of con- 
 genital 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 ap- 
 paratus, 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. 
 
 I. Anatomical. — In the fovea of the retina only 
 cones are to be found. Immediately external to 
 
12 THE HUNTERIAN LECTURES 
 
 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 ^ 
 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 
 
 2. Physiological analogy with other body cells. — 
 It is far more probable that the rods should pro- 
 duce 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 
 
 1 Journal of Physiology, vol. xli, p. 274. 
 
 2 Transactions 0/ the Opthahnological Society ^ 1902, p. 300. 
 
ON COLOUR-VISION 13 
 
 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 equa- 
 tions 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. 
 
14 THE HUNTERIAN LECTURES 
 
 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 anMogy. — 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 
 
ON COLOUR-VISION 15 
 
 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 explana- 
 tion 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 dark- 
 ness. In green vision the eyes have probabty 
 remained in a condition of more or less adapta- 
 tion to darkness, and are therefore more sensitive 
 to the green rays. 
 
 6. Disappearance of lights falling upon fovea. — 
 
16 THE HUNTERIAN LECTURES 
 
 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 ex- 
 periment 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 moder- 
 ately 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 subse- 
 quently 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 
 
ON COLOUR-VISION 17 
 
 it could be grasped. The reason of this is that 
 the eye moves so that the Hght 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. 
 
 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.^ If one eye be partially 
 covered with an opaque disc whilst both eyes 
 are directed forwards in a not too brightly illumi- 
 
 ^ Journal of Physiology ^ vol. xli, p. 269. 
 
 B 
 
18 THE HUNTERIAN LECTURES 
 
 nated room, and special attention be paid to the 
 covered eye, an appearance of whirling currents 
 will be seen with this eye (see Fig. i). These 
 
 currents appear to be directed towards the centre, 
 and have a very similar appearance to a whirl- 
 pool which is fed by four main branches. These 
 
ON COLOUR-VISION 19 
 
 again are fed by smaller branches which con- 
 tinually change their paths. On closing both 
 eyes all the portion in which the whirling cur- 
 rents 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 qualit}^, 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. 
 
 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 
 
20 THE HUNTERIAN LECTURES 
 
 Horsley in his paper on tumour of the frontal lobe 
 is almost exactly the same as that seen subjec- 
 tively. 
 
 \ 
 
 / 
 
 \ 
 
 y 
 
 Fig. 2. 
 
 12. Entoptic appearance of cone mosaic. — Ap- 
 pearances corresponding to the cone mosaic of 
 the retina may be seen in several wa5^s ^ (see 
 Fig. 2). The appearance seen corresponds to the 
 
 1 British Medical Journal^ 1910, p. 556. 
 
 2 Journal of Physiology^ vol. xli, p. 226. 
 
ON COLOUR-VISION 21 
 
 cone distribution of the retina as viewed from its 
 outer side, the portions occupied by rods appear- 
 ing as dark spaces. 
 
 13. Visual acuity. — Visual acuity is most acute 
 with the fovea, and diminishes from within out- 
 wards. 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. ^ 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 par- 
 ticular 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 
 
 ^ Nagel. Physiol, des Menschen., vol. iii, p. 107. 
 
22 THE HUNTERIAN LECTURES 
 
 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 con- 
 structed 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. 
 
 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 pro- 
 cesses 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 com- 
 parable 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. 
 
ON COLOUR-VISION 2S 
 
 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 there- 
 fore explain in another way the apparent tri- 
 chromatism of normal colour-vision, which is so 
 well known to every photographer, especially 
 those who are concerned with colour photo- 
 graphy. If my theory of the evolution of the 
 colour-sense be the correct one, and we have 
 cases of colour-blindness corresj)onding 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 re- 
 placed it — namely, yellow. The retina, there- 
 fore, 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 sepa- 
 rate fibres of the optic nerve, and the galvano- 
 meters to the visual centres of the brain. 
 
 Cases of colour-blindness may be divided into 
 two classes, which are quite separate and distinct 
 
24 THE HUNTERIAN LECTURES 
 
 from each other, though both may be present in 
 the same person. In the first class there is hght 
 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 con- 
 ditions 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 com- 
 monly 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.^ 
 
 ^ " Colour Blindness and Colour Perception," International Scien- 
 i.fic Series, p. 196. 
 
ON COLOUR-VISION 25 
 
 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 ex- 
 treme red and the extreme violet — namely, those 
 colours which present the greatest physical con- 
 trast 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. 
 I will now show on the screen some representa- 
 tions 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 cha- 
 racteristic mistakes should be made, and that 
 
26 THE HUNTERIAN LECTURES 
 
 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 3^ellow object he will decide, after look- 
 ing at it, whether it be a red or green in his esti- 
 mation, and represent it accordingly. He will be 
 greatly influenced by the nature of colours in its 
 immediate proximity, because simultaneous con- 
 trast 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 
 
ON COLOUR-VISION 27 
 
 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 pro- 
 duces 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 
 
28 THE HUNTERIAN LECTURES 
 
 added to the eye and that it became able to dis- 
 criminate between different wave-lengths of light. 
 What would be the most probable commencement 
 of development of the sense of colour ? Un- 
 doubtedly 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 physi- 
 cally 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 per- 
 ceived 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. 
 
ON COLOUR-VISION 29 
 
 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 i'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 
 
30 THE HUNTERIAN LECTURES 
 
 every part, though the curve has the same general 
 shape. The three trichromics described in my 
 recent paper ^ on *' The Relation of Light-Per- 
 ception to Colour-Perception " each saw ten 
 consecutive monochromatic patches in the spec- 
 trum 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 con- 
 tinually called red or green. 
 
 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, dis- 
 tinguish 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 '' tetra- 
 chromic," 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. 
 
 1 Proceedings of the Royal Society^ vol. B 82, 1910, p. 458. 
 
ON COLOUR-VISION 81 
 
 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 recog- 
 nised as a definite colour, and thus we get the 
 hexachromic or normal group, and, as we should 
 theoretically expect, the yellow of the penta- 
 chromic 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 hepta- 
 chromic group, and this seventh colour appears at 
 the exact point at which it should appear, accord- 
 ing 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 
 
32 THE HUNTERIAN LECTURES 
 
 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 evolu- 
 tion, and we could go on splitting up the spectrum 
 indefinitely if only we had the power to dis- 
 tinguish 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 mono- 
 chromatic 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 evolu- 
 tion 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. 
 
ON COLOUR-VISION 33 
 
 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. Whei? 
 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 com- 
 parison 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 mono- 
 chromatic patches and the discs in the outer seg- 
 ments of the cones. These are about sixteen in 
 number in the guinea-pig. As in photography, 
 
34 THE HUNTERIAN LECTURES 
 
 the intensity of the Hght is a very important factor 
 in vision. With colours of moderate intensity, 
 the periphery of the retina is found to be colour- 
 bUnd, but this apparent colour-bHndness dis- 
 appears when more intense Hghts are used. A 
 person may have shortening of the spectrum with 
 light of moderate intensity, but when the Ught is 
 increased be able to recognise the spectrum to its 
 normal limit. The change in steepness of grada- 
 tion, 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 photo- 
 graphy, 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 
 
ON COLOUR-VISION 35 
 
 of light-perception are quite distinct from defects 
 of colour-perception. 
 
 I. 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 dis- 
 tinguish between the two he would be in con- 
 tinual difficulty with blues and greens. All 
 coloured objects reflecting rays occupying the 
 missing portion appear darker than they do to 
 
36 THE HUNTERIAN LECTURES 
 
 the normal-sighted, and are always matched with 
 darker colours belonging to a point more in- 
 ternal. 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 inter- 
 fere 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 equalty re- 
 flect or transmit these rays can be correctly com- 
 pared, 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 appre- 
 ciation of shade. Again, if neither colour reflects 
 or transmits rays occupying the shortened portion 
 of the spectrum, there will obviously be no inter- 
 ference 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 occupy- 
 ing 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 there- 
 fore matched with a violet or blue very much 
 darker than itself. 
 
ON COLOUR-VISION 37 
 
 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 
 w^hich 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 re- 
 marks which I have made for a shortened spec- 
 trum 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 
 
38 THE HUNTERIAN LECTURES 
 
 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 defec- 
 tive light-perception can be explained by a de- 
 fective 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-per- 
 ception 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 consider- 
 able 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 Holm- 
 gren'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 com- 
 pared with the normal. We find that the colour- 
 blind are much more dependent on the luminosity 
 of the colour than the normal-sighted ; they re- 
 
ON COLOUR-VISION 89 
 
 quire 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 evi- 
 dently 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, tetra- 
 chromic, trichromic, dichromic, or totally colour- 
 blind. When I first gave this classification of 
 colour-bhndness, the facts that I discovered were 
 so at variance with those generally stated that it 
 was very difficult for those who were not well ac- 
 quainted 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 
 
40 THE HUNTERIAN LECTURES 
 
 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. 
 
 I. 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 ^ 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 re- 
 mainder 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 ac- 
 cordance 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 
 
ON COLOUR-VISION 41 
 
 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-percep- 
 tion. 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 correspond- 
 ing 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, how- 
 ever, 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 satu- 
 rated 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 
 
42 THE HUNTERIAN LECTURES 
 
 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 correspond- 
 ing to the sodium flame and a mixture of thallium- 
 green and lithium-red make a mixture which is 
 different to that of the normal. ^ 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 
 
 1 Proceedings of the Royal Society^ vol. B 76, 1905. 
 
ON COLOUR-VISION 43 
 
 had all the symptoms attributed to the anoma- 
 lous trichromatics, but they were not anomalous 
 trichromatics, as they made an absolutely normal 
 match. ^ 
 
 ^ Transactions of the Ophthalmological Society^ 1907, p. 255. Pro- 
 ceedings of the Royal Society^ vol. B 82, 1910. 
 
LECTURE II 
 
 Delivered on February ^rd 
 
 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 ascertain- 
 ing 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. 
 
 44 
 
ON COLOUR-BLINDNESS 45 
 
 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 danger- 
 ous. 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 ex- 
 pressing 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-blind- 
 ness. He has complained bitterly to the superin- 
 tendent, 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 superin- 
 tendent 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 with- 
 
46 THE HUNTERIAN LECTURES 
 
 out making any of the mistakes which he subse- 
 quently 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 dis- 
 tinguish 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 observa- 
 tion out loud. Therefore, from the very com- 
 mencement 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, 
 
ON COLOUR-BLINDNESS 47 
 
 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 he excluded. — We wish to exclude 
 all those individuals who are included in the 
 following three classes : (i) 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 
 
J8 THE HL XTERIAX UECTLRKS 
 
 irf led at die extreme left of the ^ectmm are the 
 most pcnetialii^ as may be se»i by lookiiig at a 
 I^t or tdie son on a foggy day, or thioi^ 
 sei^ial thicknesses of neatral ^ass. It is chiefly 
 by fliese lays Idiat we leoognise a red light at a 
 dfeftanoe; and it is tiheii^OEe of gieat impcxtaiice 
 lliat a sailor or a^ine-driiier shoaM be aUe to 
 ponceii^ tfaan. The thiid da^ contains poscMis 
 who aic abie to dfetingoish cc^oms easily when 
 tiiey aie <Jose to. but hal to distii^Dish tl^m at a 
 Astanoe, cming to Idie nenre-fibies si^i^^^ii^ the 
 ccntial portion of the letina bdng in^aiied. As 
 a t^tA at a Astance ocdqsies the central portion 
 ai the Tisoal fidd, it is essential that the ccHie- 
 spooAng; portion of the retina shoold be lUMinaL 
 There are cases of central scotrana for coloors with 
 peiiectfoon-irision; these woold, therefcHe, not be 
 detected by a test for visual acuity. This dass 
 also inrfntfcs tiiose who without having a scotoma 
 are unable to Astingoish between ci^oois at the 
 nonnal <fetanre when flie image on the retina is 
 (fiminislied in size. 
 
 I¥. Thcanuinuiiamof a Usi for cobmr-biimbuss. 
 — Inthecoastmctionof atestforcdoor-Uindness, 
 tihe fads erf cidoar-Uindness most be utilised so 
 that the ol^ect and requirements of the test are 
 fnWilifd The ioSkmiDg fads are of practical 
 uupottance. 
 
 I. Masi cobmr-hUnd make mistakes wHk cerUdn 
 cokmrs, htd are correct wUh regard io others. This 
 wesj be ilfaistrated in the following way. Let us 
 take an ordmary ^^bronaac, and, having given 
 
o^ coLocmrmjsMSSBSs 
 
 H 
 
 H 
 
 It 
 
 of 
 
50 THE HUNTERIAN LECTURES 
 
 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 defi- 
 nitely 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 appar- 
 
ON COLOUR-BLINDNESS 51 
 
 ently 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, hut 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 
 
52 THE HUNTERIAN LECTURES 
 
 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. 
 
ON COLOUR-BLINDNESS 53 
 
 14. The colour-blind may have a sense of lumin- 
 osity 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 pre- 
 senting colours of different saturation in succes- 
 sion. 
 
 16. Colour-blindness is frequently associated with 
 very high intelligence and exceptional ability. 
 
 V. The Lantern Test.'^ — i. Description of appara- 
 tus. The lantern contains four discs : three carrying 
 seven coloured glasses, and one with seven modi- 
 fying 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 repr 
 sent a 5j-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. 
 
 
 Modifying glasses. 
 
 I. Red (A and B). 
 
 7- 
 
 Ground 
 
 glass. 
 
 2. Yellow. 
 
 8. 
 
 Ribbed 
 
 glass. 
 
 3. Green. 
 
 9- 
 
 Neutral 
 
 (No. I). 
 
 4. Signal Green. 
 
 10. 
 
 >> 
 
 ( .. 11). 
 
 5. Blue. 
 
 II. 
 
 >f 
 
 ( ,. III). 
 
 6. Purple. 
 
 12. 
 
 >> 
 
 ( „ IV). 
 
 
 13. 
 
 y> 
 
 ( „ V). 
 
 * Made by Reiner and Keeler, 9, Vere Street, W. ; and Meyrowitz, 
 I a, Old Bond Street, W. 
 
54 THE HUNTERIAN LECTURES 
 
 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 i and Red 2. This 
 
 Fig. 3. 
 
 makes no important difference, but the arrange- 
 ment given here is more convenient. It should be 
 noted that Red i corresponds to Red B and 
 Red 2 to Red A. If the electric lamp should 
 get broken the projecting apparatus can be 
 
ON COLOUR-BLINDNESS 
 
 55 
 
 removed and an ordinary kerosene lamp placed 
 behind the aperture. 
 
 IF 
 
 o 
 
 o 
 
 o 
 o 
 
 o 
 
 XJ 
 
 Fig. 4. 
 
 2. Reasons for special construction. — The lantern 
 has been constructed conformably with the re- 
 
56 THE HUNTERIAN LECTURES 
 
 quirements 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 wdth 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 
 
ON COLOUR-BLINDNESS 57 
 
 that he has correctly named is the blue. We can 
 try him again and again, and though he will mis- 
 take 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 in- 
 creased to the colour-blind. I was naturally 
 anxious to turn this fact to account. I found. 
 
58 THE HUNTERIAN LECTURES 
 
 however, that I gained nothing by increasing the 
 number of Hghts, and that in many cases it was 
 a source of error. A second or third hght could 
 have been easily added to my lantern, but be- 
 sides 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 match- 
 ing 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, suffi- 
 cient time being allowed to elapse to enable a 
 normal-sighted person to readily recognise the 
 true colour of the next light without being con- 
 fused 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 ordi- 
 nary 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 
 
ON COLOUR-BLINDNESS 59 
 
 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 effi- 
 cient 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 di- 
 chromics 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 
 
60 THE HUNTERIAN LECTURES 
 
 useful method with nervous and undecided candi- 
 dates. 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 in- 
 tensity of the colour may be changed at will. 
 
 The method is better than that of direct com- 
 parison, 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 dis- 
 tinguished by their saturation and luminosity), 
 and forced the individual to depend upon his 
 colour-perception, and not upon some other ac- 
 cessory 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 com- 
 binations are so numerous that the only result is 
 to make the colour-blind man nervous and doubt- 
 ful 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. 
 
ON COLOUR-BLINDNESS 61 
 
 3. Special directions for conducting the test. — 
 (i) 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 (i to 6) alone, or in combina- 
 tion 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 circum- 
 stances. (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 em- 
 ployed 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 
 
62 THE HUNTERIAN LECTURES 
 
 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 in- 
 dividual answers. Then one of the neutral, 
 ground, or ribbed glasses should be inserted, not 
 the slightest intimation being given to the candi- 
 date 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 re- 
 plies, 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 aper- 
 tures. 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- 
 
ON COLOUR-BLINDNESS 63 
 
 sighted. This is not the case with the colour- 
 bhnd ; a dichromic who has hesitated about a 
 green and then correctly named it may emphati- 
 cally 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 
 w^hich 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. 
 
64 THE HUNTERIAN LECTURES 
 
 The examiner should utiUse the fact that suc- 
 cessive contrast is increased in the colour-bUnd, 
 as this is an easy method of detecting the tri- 
 chromics. The red having been shown, the Hght 
 should be quickly changed to yellow or clear, 
 the examiner's hand being placed over the aper- 
 ture if there be any intervening colours. It is 
 necessary that the yellow should be shown imme- 
 diately 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 
 Hght, 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 exami- 
 nation 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 ex- 
 pression, the following is likely to occur. The 
 candidate after, say, six correct answers, makes a 
 mistake ; the examiner says, '* Are you sure ? " 
 
ON COLOUR-BLINDNESS 65 
 
 Then the candidate knows at once that he has 
 made a mistake, and makes a guess, very pro- 
 bably 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 call- 
 ing 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 accord- 
 ance 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 w^ill guess 
 when examined in this way. The examiner must 
 receive the examinee with a smiling face and 
 courteous manner, and appear pleased and satis- 
 fied 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-blind- 
 ness. 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 
 
06 THE HUNTERIAN LECTURES 
 
 in the case of visual acuity. Further details will 
 be found in my book on Colour-Blindness.^ 
 
 Summary of method of examination. — (i) Show 
 all the colours on one disc with the largest aper- 
 ture. (2) Show the reds, greens, and yellow modi- 
 fied 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 com- 
 bining blue or purple with the signal green. (8) 
 Show a colour, for instance, green, and then com- 
 bine 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 Classi- 
 fication 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. 
 
 I. The Classification Test. — (a) Description. — 
 This test consists of 4 test colours and 180 con- 
 fusion colours; 150 coloured wools, 10 skeins of 
 silk, 10 small squares of coloured cardboard, and 10 
 
 ^ International Scientific Series. Kegan Paul & Co., 1909. 
 
ON COLOUR-BLINDNESS 67 
 
 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 re- 
 spectively. 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 mis- 
 taken 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.) 
 
 (6) 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 
 
68 THE HUNTERIAN LECTURES 
 
 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 
 
 Fig. 5. 
 
 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 with- 
 out 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. 
 
ON COLOUR-BLINDNESS 69 
 
 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. 
 
70 THE HUNTERIAN LECTURES 
 
 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 
 
 ) O 
 
 ill 
 
 em 
 fill 
 
 Hi 
 
 m 
 
 Hi 
 
 10 
 HI 
 
 m 
 11 
 
 ® © 
 
 Fig. 6. 
 
 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 '' With- 
 out Red " and two special cards marked '' With- 
 out Green." (See Fig. 6.) 
 
 Many normal-sighted persons might object to the 
 
ON COLOUR-BLINDNESS 71 
 
 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.T 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 penta- 
 chromic 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. Ill, 
 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 Classifica- 
 tion 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 tests colours are to be found, and which 
 contain none of the test colour. 
 
 The examiner should continually change the 
 
72 THE HUNTERIAN LECTURES 
 
 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 : (i) 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.'^ — (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 adjust- 
 able shutters with vertical edges ; the shutters can 
 be moved into the field from right and left respec- 
 tively, 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 
 
 ^ Made by A. Hilger, 75a, Camden Road, London, N.W. 
 
ON COLOUR-BLINDNESS 
 
 73 
 
 the left drum-head is 5320 and that on the right 
 drum-head is 5920, the region of the spectrum from 
 
 Fig. 7. 
 
 Fig. 8. Fig. 9. 
 
 wave-length 5320 to wave-length 5920 is exposed 
 to view in the eyepiece. 
 
74 THE HUNTERIAN LECTURES 
 
 (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 previ- 
 ously 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 inten- 
 sities 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 
 
ON COLOUR-BLINDNESS 75 
 
 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, Pro- 
 fessor 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 otir largest railway com- 
 panies 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 
 
76 THE HUNTERIAN LECTURES 
 
 day, with NageFs 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, Ui 
 
 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, x j 
 
 Cavendish Square, London, W, 
 
 Telephone : 447 Mayfair. 
 
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