1240 COLOUR-BLINDNESS. [BOOK in. 



sensations, would lead to a general deficiency of vision; for the 

 lack of one-third of visual sensations would be equivalent to a 

 diminution of the total illumination of external objects to the 

 extent of one-third, and this, unless we suppose that the normal 

 eye lives in a superfluity of light must, especially in feeble light, 

 lead to dim vision ; moreover a vision which has to trust to two- 

 fold differences must be less sure than one based on three-fold 

 differences. But this does not necessarily follow; the two 

 remaining sensations might become more highly developed, might 

 so to speak expand in the absence of the third. And as a 

 matter of fact the general vision of colour-blind people seems to 

 be as good as that of normal eyes ; moreover, within the range of 

 the colours which they can see, colour-blind people are if any- 

 thing more acute than most people ; though they regard as more 

 or less alike two colours which seem to the normal eye wholly 

 unlike, they can more easily detect minute differences such as 

 those of shade or bone, within each of the two colours. 



764. The phenomena, however, of these two classes of colour- 

 blind eyes can also be interpreted on Hering's theory. In both 

 of them the red-green substance may be supposed to be 

 missing, and their dichromic vision to be made up exclusively of 

 changes in the yellow-blue and white-black substances. Since 

 they are thus supposed to have neither red nor green sensations, 

 they must necessarily confound red and green; and the smaller 

 differences, which, as we have seen, divide into two classes all those 

 which confound red with green may be explained as follows. 



Even in eyes which may 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 arrange- 

 ment 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 sensation of yellow ; 

 according to the Young- Helmholtz theory yellow 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 mixture seems of the same hue 

 and the same brightness as the yellow, not shewing either a 

 reddish or a greenish tone. When this is done it is found that 

 different people differ very materially 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 



