Report of the Committee on Colour- Vision. 287 



In considering the question as to how far red-green blindness 

 can be regarded as a mere deficiency in colour-perception, it is 

 important to bear in mind that, according to recent observation, 

 considerable deviations from the normal type may occur without 

 any approach to colour-blindness. If we imagine a di-chromatic 

 system be derived from an abnormal tri-chromatic system by the 

 suppression of one sensation, it will differ from a di-chromatic 

 system similarly derived from a normal system of colour-vision. 



Blindness to violet, and shortening of the violet end of the Violet colour- 

 spectrum, have been described, but the instances are very few. blindness. 

 One case of apparent violet-blindness of which the Committee 

 have cognizance answers accurately to the Young-Helmholtz 

 theory, on the supposition that the violet sensation is absent 

 {see No. 4, Plate I). 



Three other cases of congenital colour-blindness investigated 

 by the Committee deserve special mention; two (brothers) in 

 which there was but one sensation, answering probably to the 

 violet sensation of the Young-Helmholtz theory, and the third in 

 which the principal sensation was a pure green with perception 

 of white and probably a slight trace of red. As these were 

 all cases of congenital colour-blindness, they are mentioned as 

 in some measure confirming the theory in question {see Note a). 



Another theor3 T , that of Hering, starts from the observation that Hering's 

 when we examine our own sensations of light we find that theory of 

 certain of these seem to be quite distinct in nature from each colour- vision, 

 other, so that each is something sui generis, whereas we 

 easily recognise all other colour sensations as various mixtures 

 of these. Thus, the sensation of red and the sensation of 

 yellow are to us quite distinct: we do not recognise anything 

 common to the two ; but orange is obviously a mixture of red 

 and yellow. Green and blue are equally distinct from each 

 other and from red and yellow, but in violet and purple we recog- 

 nise a mixture of red and blue. White again is quite distinct 

 from all the colours in the narrower sense of that word, and 

 black which we must accept as a sensation, as an affection of 

 consciousness, even if we regard it as the absence of sensation 

 from the field of vision, is again distinct from everything else. 

 Hence the sensations, caused by different kinds of light or by the 

 absence of light, which thus appear to us distinct, and which we 

 may speak of as " native" or " fundamental " sensations, are white, 

 black, red, yellow, green, blue. Each of these seems to us to 

 have nothing in common with any of the others, whereas in all 

 other colours we can recognise a mixture of two or more of these. 



This result of common experience suggests the idea that these 

 fundamental sensations are the primary sensations, concerning 

 which we are inquiring. And Hering's theory attempts to 

 reconcile, in some such way as follows, the various facts of 

 colour-vision with the supposition that we possess these six 

 fundamental sensations. The six sensations readily fall into 

 three pairs, the members of each pair having analogous relations 



