910 COLOUR-BLINDNESS. [Book hi. 



Even in eyes which ma)- be considered normal as regards 

 colour vision, eyes which certainly cannot be called colour- 

 blind, considerable differences will, on closer examination, be 

 found in regard to sensations of yellow. If by means of a 

 special arrangement we bring a certain amount of the red part 

 of the spectrum and a certain amount of the green part of the 

 spectrum on to the eye at the same time, the result is a sensa- 

 tion of yellow ; according to the Young-Helmholtz theory yel- 

 low is a mixture of red and green. By the same arrangement 

 we can bring on to the eye at the same time a certain amount 

 of the actual yellow of the spectrum. In this way we can 

 make a match between a mixture of spectral red and green on 

 the one hand, and spectral yellow on the other, comparing the 

 mixed sensation derived from two parts of the spectrum with 

 the sensation derived from a single (yellow) part. We have to 

 adjust the quantities of red light and green light until the mix- 

 ture seems of the same hue and the same brightness as the yel- 

 low, not shewing either a reddish or a greenish tone. When 

 this is done it is found that different people differ very materi- 

 ally as to the proportion of red and green, the proportion of the 

 intensities of the two sensations, necessary to make the match 

 with yellow ; with the same quantity of red some need more 

 green, others less green, to make the match. This, on the 

 Young-Helmholtz 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 fundamental sensation, 

 it may be interpreted as meaning that in passing along the spec- 

 trum towards the red end, the point at which the yellow-blue 

 substance ceases to be affected by tajs 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. 151 (in way of anabolism), 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 ana- 

 bolic 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. 151, 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 pro- 



