VISUAL SENSATIONS 647 



sensation of white. If the light be now removed, this breaking down 

 gives place to anabolism or building up of the white-black substance, 

 which excites the same nerve fibrils in a different way, giving rise to 

 a sensation of black. The white-black substance is affected not 

 only by white light but also by the colours red, green, yellow, blue, 

 and their mixtures. The other two visual substances are affected 

 only by red and green or by yellow and blue respectively. Hence 

 even the spectral colours do not give rise to pure sensations, there 

 being always some mixture of a sensation of white with the proper 

 colour-sensation. 



Most of the phenomena of colour-vision that we have mentioned 

 above can be equally well explained on either theory. Thus the 

 fact that blue and yellow together give rise to a sensation of while 

 may be explained on the Young-Helmholtz theory by saying that 

 the stimulation of all three sets of fibrils is equal, as will be seen by 

 adding together the ordinates of each curve in Fig. 294 at yellow 

 and at blue. 



Adopting Bering's hypothesis, we may say that, anabolism and 

 catabolism being equally excited in the yellow-blue substance, no 

 change in it takes place, and the sole sensation is that produced 

 by the stimulation of the white-black substance. The pairs of colours 

 that we have distinguished are therefore, according to this theory, 

 not in the strict sense of the word complementary, but antagonistic. 

 The fact that white light appears to us as a simple sensation and 

 gives us no suspicion of the coloured rays of which it may be com- 

 posed is in favour of Hering's theory. Cases of colour-blindness 

 would be reduced by Hering to two classes, viz. those in which the 

 red-green substance is lacking and those in which the yellow-blue 

 substance is lacking. Most of the data with regard to colour-blind- 

 ness have been worked out with reference to the Young-Helmholtz 

 theory, and have therefore been interpreted in accordance with this 

 hypothesis. 



It is very difficult, however, to harmonise the facts of colour- 

 blindness either with this or with Hering's hypothesis. It is better 

 therefore to abandon hypotheses altogether and to adopt a purely 

 empirical classification of colour-vision, as has been done by Edridge- 

 Green. This observer points out truly that very marked colour- 

 blindness may be present without any interference with the apprecia- 

 tion of the luminosity of any part of the spectrum. A person may 

 be able to see the spectrum up to its extreme red end, and yet dis- 

 tinguish in the spectrum only two colours, which we may call red 

 and violet. We may, in fact, regard discrimination of colour differ- 

 ence as superadded to and evolved later than the appreciation of 

 light. Discrimination may show various degrees of deficiency without 



