August ii, 1892] 



NATURE 



345 



spectrum ; purple must be added to fill in the gap between the 

 red and violet. Helmholz found no constant ratios between 

 the wave lengths of homogeneous complementaries ; and it is a 

 striking fact that, while a mixture of the green and red, or of 

 the green and violet undulations gives rise to a sensation such 

 as could be produced by rays of intermediate wave length, no 

 such effect follows the mingling of rays from opposite halves of 

 the spectrum. Pure green, with a wave length of 527 millionths 

 of a millimetre, marks the division between the right and left 

 halves. The mixture of blue from the right and yellow from 

 the left side does not produce the intermediate green, but a 

 sensation of white. A mixture of blue or violet and red pro- 

 duces not green, but its complementary — purple. On the tri- 

 chromic theory, the sense of white produced by the mingling 

 of any of these two colours is simply regarded as the result of 

 a balanced stimulation of the red, green, and violet terminals. 



But Young's theory is beset with serious difficulties. It im- 

 plies the existence of three sets of terminals in the retina, and 

 these must all be found in the central part of the yellow spot 

 where cones alone are present. Three sets of cones there would 

 be necessary to respond to the red, green, and violet light, and 

 a colourless pencil of light could not be seen uncoloured, unless 

 it falls on three cones, which we know from astronomical obser- 

 vations is not the case. Therefore, if there are three different 

 terminals, it seems necessary, in the human retina at all events, 

 that they should be found in every single cone in the yellow 

 spot. But I cannot believe it possible that within a single cone 

 there can be three sets of fibrils capable of simultaneous stimu- 

 lation in different degrees, and of ultimately transmitting im- 

 pulses through three different fibres to three different cells in 

 the brain. That would imply a greater number of fibres in the 

 optic nerve, than of terminals in the retina, and we know that 

 precisely the reverse is the case. The anatomical difficulty is 

 therefore great, and I am unable to see how it can be sur- 

 mounted. 



The phenomena of colour-blindness also offer great difficulty. 

 In several cases of apoplectic seizure it has happened that the 

 centre for vision on both sides of the brain has been completely 

 or partially paralyzed by the extravasated blood. In such cases 

 the sense of colour may be entirely lost either for a time or per- 

 manently, while the sense of light and form remain — although 

 impaired. The loss of colour sense in some cases has been 

 found complete in both eyes ; in most of the recorded cases the 

 loss of colour sense was limited to the right or left halves of 

 both eyes ; that is, if the lesion affected the vision centre on the 

 right side of the brain, the right halves of both eyes were blind 

 to all colours. That illustrates the fact that a sense of light 

 does not necessarily imply a sense of colour. The colour 

 sense probably involves a more highly refined action of the 

 sensory cell than the mere sense of light and form, and is on 

 that account more liable to be lost when the nutrition of the 

 sensory cell is interfered with. In the normal eye the peripheral 

 zone of the retina is totally blind to colour. If you turn the 

 right eye outwards, close the left, and then move a strip of 

 coloured paper from the left to the right in front of the nose, 

 the image of the paper will first fall on the peripheral zone of 

 the retina, and its form will be seen, though indistinctly, but not 

 its colour. It is difficult to say in that case whether the colour- 

 blindness is due to the state of the retina or to that portion of 

 the vision centre in the brain associated with it. The absence 

 of cones from the peripheral part of the retina has been assigned 

 as the cause, but it is much more probable that the portion of 

 the vision centre associated with the periphery of the retina, 

 being comparatively little used, is less highly developed for 

 form sensation, and not at all for colour sense. It is evident 

 that the production of a sense of white or grey in the absence 

 of all colour sense is not to be explained on the theory that it 

 results from a balanced stimulation of red, green, and violet 

 nerve terminals. 



I need scarcely say that colour-blindness has attracted a large 

 share of attention, not only because of its scientific interest, but 

 still more on account of its practical importance in relation to 

 the correct observation of coloured signals. In 1855 the late 

 Prof. George Wilson,^ of this city, called attention to the grow- 

 ing importance of the subject. Some years ago Prof. Holmgren 

 made an elaborate statistical inquiry regarding it at the instance 

 of the Swedish Government, and lately it has been investigated 



' Wilson, "Researches on Colour-Blindness," Edinburgh, i«55. 



NO. 1 189, VOL. 46] 



by a committee of the Royal Society of London, who have quite 

 recently published their report. ^ 



Although colourblindness occasionally results from disease of 

 the brain, retina, or optic nerve, it is usually congenital. Total 

 colour-blindness is extremely rare, but partial colour-blindness 

 is not uncommon. It occurs in about 4 per cent, of males, but 

 in less than i per 1000 of females. Its most common form is 

 termed red-green blindness, in which red and green sensations 

 appear to be absent. So far as I can find, the first full and re- 

 liable account of the state of vision in red-green blindness is 

 that given in 1859 by Mr. Pole,^ of London, from an examin- 

 ation of his own case, which appears to be a typical one. The 

 state of his vision is dichromic ; his two-colour sensations are 

 yellow and blue. The red, orange, and yellowish-green parts 

 of the spectrum appear to him yellow of different shades. 

 Greenish-blue and violet appear blue, and between the yellow 

 and blue portions of the spectrum, as it appears to him, there is 

 a colourless grey band in the position of the full green of the 

 ordinary spectrum. This neutral band is seen in the spectrum 

 in all cases of dichromic vision. It may appear white or grey 

 according to the intensity of the light, and it apparently results 

 from an equilibrium of the two sensations ; no such band is seen 

 in the spectrum by a normal eye. Mr. Pole, in the account of 

 his case given now three and thirty years ago, considered it im- 

 possible to explain his dichromatic vision on the commonly re- 

 ceived theory that his sense of red is alone defective, and that 

 his sense of yellow is a compound of blue and green. He be- 

 lieved his green quite as defective as his red sensation, and that 

 yellow and blue are quite as much entitled to be considered 

 fundamental sensations as red and green. He suggested that in 

 normal colour vision there are at least four primary sensations — 

 red and green, yellow and blue. Prof. Hering is commonly 

 accredited with the four-colour theory, but it was previously 

 suggested by Pole.^ 



A year after Pole's paper appeared, Clerk-Maxwell ■• published 

 his celebrated paper on the theory of compound colours, to which 

 he appended an account of his observations on a case of what 

 he believed to be red-blindness, but which we now know must 

 have been red-green blindness. The spectrum appeared dichro- 

 mic, its only colours being yellow and blue. His description 

 of the case does not materially differ from that given by Pole ; 

 but Clerk- Maxwell believed in the trichromic theory of nor- 

 mal vision, and that red-green and blue are the three primary 

 sensations ; consequently he supposed that the yellow sensation 

 of a red blind person is not pure yellow, but green. 



It is evident that much depends on the question, " Is the yel- 

 low sensation of a red-green blind person the same as that of 

 normal vision?" For many years it was impossible to give a 

 definite answer to that question, but the answer can now be 

 given, as we shall immediately see. Colour-blindness is frequently 

 hereditary, and two or three cases are known in which the de- 

 fective colour sense was limited to one eye, while in the other 

 eye colour vision was normal. In such a case observed 

 by Prof. Ilippel, of Giessen, there was red-green blindness in 

 one eye. Holmgren, who examined Hippel's case, has pub- 

 lished an account of it.'"' With one eye all the colours of the 

 spectrum were seen, but to the other eye the spectrum had only 

 two colours with a narrow grey band between them at the 

 junction of the blue and yellow. The yellow seen by the eye 

 with the red-green defect had a greenish tinge like that of a 

 lemon, but in other respects the observations confirmed Pole's 

 account of his own case. 



Hippel's case seems to me important for another reason. By 

 some it is believed that congenital colour defect is due to the 

 brain. If there had been defective colour sense on one side of 

 the brain, it would not have implicated the whole of one eye, 

 but the half of each eye. Its limitation to one eye, therefore, 

 seems to me to suggest that the fault was in the eye rather than 

 in the brain. 



Another interesting fact in this relation is that in every normal 

 eye, just behind the peripheral zone of total colour-blindness, to 



' " Report of the Committee on Colour Vision," Proc. Roy. Soc. Lond., 

 July 1892. 



2 W. Pole, " On Colour-Blindness," Phil. Trans., 1859, vol. cxlix. 

 P- 323- 



^ Ibid.t p. 331. 



•» Clerk-Maxwell. *' On the Theory of Compound Colours," &c, Phil. 

 Trans , i860, vol. cl., p. 57. 



5 F. Holmgren, •' How do the Colour-Blind see the Different Colours?" 

 Proc. Rov. Soc. Lond., 1881, vol. xxxi. p. 302. 



