CHAP, in.] SIGHT. 103 



match. This, on the Young-Helinholtz theory, is interpreted as 

 meaning that the development of the red and green primary 

 sensations differs even in people whose colour vision is considered 

 normal. But on Hering's theory, in which yellow is a funda- 

 mental sensation, it may be interpreted as meaning that in 

 passing along the spectrum toward the red-end, the point at 

 which the yellow-blue substance ceases to be affected by rays of 

 light, is placed much nearer the red end in some people than in 

 others. By Hering's hypothesis the green of the spectrum affects 

 not only the red-green substance, cf. Fig. 147 (in way of ana- 

 bolism), but also to some extent the yellow-blue substance (in way 

 of katabolism) ; the red rays on the other hand affect the yellow- 

 blue substance very slightly, while the (pure) yellow rays are 

 neutral to the red-green substance producing neither katabolic 

 red nor anabolic green, but simply yellow by katabolic action 

 on the yellow-blue substance. This at least represents the 

 condition of the majority of eyes. If, however, we suppose that 

 in other eyes 'the yellow-blue substance is considerably affected 

 by red rays, if in Fig. 147, we suppose the curve representing the 

 yellow sensation to be considerably extended towards the red end, 

 in these eyes the red rays would give rise to a sensation of yellow 

 at the same time that they excited a sensation of red, the red 

 would be mixed with yellow ; hence in such eyes a certain amount 

 of red being already mixed with yellow would need less green 

 (with its necessarily accompanying yellow) to produce a certain 

 amount of yellow as the result of the mutual neutralisation 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, 

 show that in different eyes the yellow sensation (in Hering'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 effected 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 

 shortened. The feeble white visual impulses excited are in- 

 sufficient to affect consciousness unless supported by red visual 



