No. 1072, Vol. 42] 



NATURE 



57 



entitled to a monopoly of an appellation which is con- 

 veniently applied also to others. The totally colour-blind 

 would see a coloured picture as if it were an engraving, 

 or a drawing in black and white, and would perceive 

 differences between its parts only in the degree in which 

 they differed in brightness. 



A more common condition is the existence, in the 

 centre of the retina, of a kind of vision like that which 

 normally exists in the zone next surrounding it — that is, a 

 blindness to green. Persons who are blind to green 

 appear to see violet and yellow much as these are seen 

 by the normal-sighted ; and they can see red, but they 

 cannot distinguish it from green. Others, and this form 

 is more common than the preceding, are blind to red ; 

 and a very small number of persons are blind to violet. 

 Such blindness to one of the fundamental colours may be 

 either complete or incomplete — that is to say, the power 

 of the colour in question to excite its proper sensation 

 may be either absent or feeble. In some cases, the defect 

 is so moderate in degree as to be adequately described 

 by the phrase "defective colour-sense." 



The experiments of Helmholtz upon colour led him to 

 supplement the original hypothesis of Young by the 

 supposition that the special nerve elements excited by 

 any one colour are also excited in some degree by each 

 of the other two, but that they respond by the sensation 

 appropriate to themselves, and not by that appropriate to 

 the colour by which they are thus feebly excited. This, 

 which is often called the Young-Helmholtz hypothesis, 

 assumes that the pure red of the spectrum, while it mainly 

 stimulates the fibres sensitive to red, stimulates in a less 

 degree those which are sensitive to green, and in a still 

 less degree those which are sensitive to violet, the result- 

 ing sensation being red. Pure green stimulates strongly 

 the green-perceptive fibres, and stimulates slightly both 

 the red-perceptive and the violet-perceptive — resulting 

 sensation, green. Pure violet stimulates strongly the 

 violet-perceptive fibres, less strongly the green-perceptive, 

 least strongly the red-perceptive — resulting sensation, 

 \iolet. When all three sets of fibres are stimulated at 

 once, the resulting sensation is white ; and when a normal 

 eye is directed to the spectrum, the region of greatest 

 luminosity is in the middle of the yellow ; because, while 

 here both the green-perceptive and the red-perceptive 

 fibres are stimulated in a high degree, the violet-percep- 

 tive are also stimulated in some degree. 



According to this view of the case, the person who is 

 red-blind, or in whom the red-perceptive fibres are wanting 

 or paralyzed, has only two fundamental colours in the 

 spectrum instead of three. Spectral red, nevertheless, is 

 not invisible to him, because it feebly excites his green- 

 perceptive fibres, and hence appears as a saturated green 

 of feeble luminosity ; saturated, because it scarcely at all 

 excites the violet-perceptive fibres. The brightest part 

 of the spectrum, instead of being in the yellow, is in the 

 blue-green, because here both sets of sensitive fibres are 

 stimulated. In the case of the green-blind, in whom the 

 fibres perceptive of green are supposed to be wanting or 

 paralyzed, the only stimulation produced by spectral 

 \ green is that of the red-perceptive and of the violet-per- 

 ceptive fibres : and where these are equally stimulated, 

 we obtain the white of the green-blind, which, to ordinary 

 eyes, is a sort of rose-colour, a mixture of red and violet. 

 In like manner, the white of the red-blind is a mixture of 

 green and violet ; and, if we consider the facts, we shall 

 see that spectral red, which somewhat feebly stimulates 

 , the green-perceptive fibres of the normal eye, and spectral 

 , green, which somewhat feebly stimulates the red-percep- 

 tive fibres of the normal, and also of the green-blind eye, 

 must appear to the green-blind to be one and the same 

 colour, differing only in luminosity, and that in an opposite 

 sense to the perception of the red-blind. In other words, 

 red and green are undistinguishable from each other, as 

 colours, alike to the red-blind and to the green-blind ; but 



to the former the red, and to the latter the green, appears, 

 as compared with the other, to be of feeble luminosity. 

 In either case, the two are only lighter and darker shades 

 of the same colour. The conditions of violet-blindness 

 are analogous, but the defect itself is very rare ; and, as 

 it is of small industrial importance, it has attracted but a 

 small degree of attention. 



Very extensive investigations, conducted during the 

 last few years both in Europe and in America, have shown 

 that those which may be called the common forms of 

 colour-blindness, the blindness to red and to green, exist 

 in about four per cent, of the male population, and in 

 perhaps one per thousand of females. Among the rest, 

 there are slight differences of colour-sense, partly due to 

 differences of habit and training, but of little or no prac- 

 tical importance. One such difference, to which Lord 

 Rayleigh was the first to direct attention, has reference to 

 yellow. The pure yellow of the spectrum may, as is 

 generally known, be precisely matched by a mixture of 

 spectral red with spectral green ; but the proportions in 

 which the mixture should be made differ within certain 

 limits for different people. The difference must, I think, 

 depend upon differences in the pigmentation of the yellow 

 spot, rather than upon any defect in the nervous apparatus 

 of the colour-sense. There is a very ingenious instru- 

 ment, invented by Mr. Lovibond, and called by him the 

 " tintometer," which allows the colour of any object to be 

 accurately matched by combinations of coloured glass, and 

 to be expressed in terms of the combination. In using 

 this instrument, we not onlv find slight differences in the 

 combinations required by different people, but also in the 

 combinations required by the two eyes of the same person. 

 Here, again, I think the differences must be due either to 

 differences in the pigmentation of the yellow spot, or pos- 

 sibly also to differences in the colour of the internal 

 lenses of the several eyes, the lens, as is well known, 

 being usually somewhat yellow after middle age. The 

 differences are plainly manifest in comparing persons 

 all of whom possess tri-chromatic vision, and are not 

 sufficient in degree to be of any practical importance. 



Taking the ordinary case of a red-blind or of a green- 

 blind person, it is interesting to speculate upon the ap- 

 pearance which the world must present to them. Being 

 insensible to one of the fundamental colours of the spec- 

 trum, they must lose, roughly speaking, one-third of the 

 luminosity of Nature; unless, as is possible, the deficiency 

 is made good to them by increased acuteness of perception 

 to the colours which they see. Whether they see white 

 as we see it, or as we see the mixtures of red and violet, 

 or of green and violet, which they make to match with it, 

 we can only conjecture, on account of the inadequacy of 

 language to convey any accurate idea of sensation. We 

 have all heard of the blind maji who concluded, from the 

 attempts made to describe scarlet to him, that it was like 

 the sound of a trumpet. If we take a heap of coloured 

 wools, and look at them first through a glass of peacock- 

 blue, by which the red rays are filtered out, and next 

 through a purple glass, by which a large proportion of 

 the green will be filtered out, we may presume that, under 

 the first condition, the wools will appear much as they 

 would do to the red-blind ; and, under the second, much 

 as they would do to the green-blind. It will be observed 

 that the appearances differ in the two conditions, but that, 

 in both, red and green are practically undistinguishable 

 from each other, and appear as the same colour, but of 

 different luminosity. 



Prior to reflection, and still more, prior to experience, 

 we should be apt to conjecture that the existence of 

 colour-blindness in any individual could not remain con- 

 cealed, either from himself or from those around him ; 

 but such a conjecture would be directly at variance with 

 the truth. Just as it was reserved for Mariotte, in the 

 reign of Charles II., to discover that there is, in the field 

 of vision of every eye, a lacuna or blind spot, correspond- 



