CHAP. HI.] SIGHT. 911 



duce a certain amount of yellow as the result of the mutual 

 neutralization of the red and green. In such cases we may 

 suppose not that the whole relation of the yellow-blue substance 

 to wave-lengths is altered, but merely that the sensitiveness to 

 long wave-length is increased ; the curve of the yellow-blue is 

 not shifted bodily along the spectrum, but the form of the curve 

 is altered so that, the maximum of yellow remaining the same, 

 the yellow end of the curve extends further into the red. Not 

 only this match of red and green with yellow, but other matches 

 of a similar nature, shew that in different eyes the yellow sen- 

 sation (in Bering's sense) is more prominent in some people 

 than in others, that some people so to speak are more yellow 

 sighted than others. 



The application of this fact to the colour-blind cases is obvious. 

 In the one class, the red-blind of the Young-Helmholtz theory, 

 the relations of the primary sensations, the distribution along the 

 spectrum of the visual substances are the same as in the normal 

 eye save that the red-green substance and the corresponding 

 sensations are missing; and since the visibility of the red end of 

 the spectrum is chiefly affected by the red sensation, the white- 

 black substance being as compared with the red-green substance 

 but slightly sensitive to the extreme rays, the spectrum is short- 

 ened. The feeble white visual impulses excited are insufficient 

 to affect consciousness unless supported by red visual impulses. 

 In the second class, the yellow-blue substance has undergone an 

 expansion similar to but probably greater than that which obtains 

 in the yellow-sighted but otherwise normal eyes mentioned above, 

 it is sensitive to even the rays at the red end of the spectrum ; 

 hence the spectrum to^eyes of this class seems of the ordinary 

 visible length. 



568. So far then both theories may be made to explain 

 the ordinary phenomena of colour-blindness ; but it is obvious 

 that the subjective condition of the colour-blind must be different 

 according to one theory from what it is according 1 to the other. 

 According to the Young-Helmholtz theory the red-blind person 

 does not experience in any degree the sensation of either red or 

 yellow ; from the green of the spectrum to the red end he only 

 sees some sort of green. Indeed along the whole spectrum, the 

 sensations which he experiences are only various kinds of green 

 and blue, with various amounts of the sensation whatever it be, 

 whether white or simply green-blue, or some other sensation 

 unknown to the normal eye, which results from the mixture of 

 the green and blue sensations. The green-blind person, accord- 

 ing to the same theory, has only the sensations of red and blue, 

 with the sensation whatever it may be derived from the mixture 

 of these two, he never has the sensation of either green or yellow. 

 Obviously the sensations of the two classes ought to differ very 

 widely. According to Hering's theory, both classes agree in 



