COLOR-VISION AND COLOR-BLINDNESS. 693 



perceptive — resulting sensation, violet. When all three sets of fibers 

 are stimulated at ouce the resulting sensation is white, and when a 

 normal eye is directed to the spectrum the region of greatest luminos- 

 ity is in the middle- of the yellow ; because, while here both the green- 

 perceptive and the red-perceptive fibers 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-preceptive fibers are wanting or paralyzed, has only two 

 fundamental colors in the spectrum instead of three. Spectral red 

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

 preceptive fibers, and hence appears as a saturated green of feeble 

 luminosity; saturated, because it scarcely at all excites the violet- 

 preceptive fibers. The brightest part of the spectrum instead of being 

 in the yellow is in the blue-green, because here both sets of sensitive 

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

 fibers preceptive of green are supposed to be wanting or paralyzed, the 

 only stimulation produced by spectral green is that of the red-precep- 

 tive and of the violet-perceptive fibers; and where these are equally stim- 

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

 is a sort of rose color, 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 con- 

 sider the facts we shall see that spectral red, which somewhat feebly 

 stimulates the green-perceptive fibers of the normal eye, and spectral 

 green, which somewhat feebly stimulates the red-perceptive fibers of 

 the normal and also of the green-blind eye, must appear to the green- 

 blind to be one and the same color, differing only in luminosity, and 

 that in an opposite sense to the preception of the red-blind. In other 

 words, red and green are undistinguishable from each other as colors 

 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 color. The conditions of violet-blindness are analo- 

 gous, 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 color-blindness, the blindness to red and to 

 green, exist in about 4 per cent, of the male population and in perhaps 

 1 per thousand of females. Among the rest there are slight differences 

 of color-sense, partly due to differences of habit and training, but of 

 little or no practical 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 pro- 

 portions in which the mixture should be made differ within certain 

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



