122 Mr, B. Brudenell Carter [May 9, 



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- j)erceptive 

 and the red-perceptive fibres are stimulated in a high degree, the 

 violet-perceptive 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 paralysed, 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 

 paralysed, the only stimulation produced by spectral green is that 

 of the red-j)erceptive and of the violet-perceptive 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 [sliown]. In like manner, the white of the red-blind is a mixture 

 of green and violet \s}ioii-n\ ; 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-perceptive 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 percef)tiou 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 4 per cent, of the male population, and 

 in perhaps one per thousand of females ; while among the rest there 

 are slight differences of colour-sense, partly due to diiferences of habit 

 and training, but of little or no practical importance. One such 

 difference, to which Lord Eayleigh 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 projDortious in which the mixture should 

 be made difier within certain limits for difierent people. The difier- 

 ences must, I think, depend ujDon differences in the pigmentation of 

 the yellow spot, rather than upon any defect in the nervous apparatus 



