912 COLOUR-BLINDNESS. [BOOK in. 



seeing neither red nor green, all their sensations are made up of 

 yellow and blue, with white and black, and the only difference 

 between the two classes is that the one, the green-blind of the 

 other theory, see more yellow than do the other. 



569. We have treated of the colour-blind as if they were 

 confined to those who confounded red with green. According 

 to the Young-Helmholtz theory, another class of colour-blind is 

 possible, the violet- or blue-blind, those who while possessing red 

 and green sensations lack the third, violet or blue sensation. 

 Such a kind of vision is impossible on Hering's theory ; accord- 

 ing to that theory, if a person fails to see blue, he must also fail 

 to see yellow. 



Lastly we may remark that absolute colour-blindness, a con- 

 dition in which shades of black and white alone indicate the 

 features of external objects, while possible on Hering's theory, 

 is impossible on the Young-Helmholtz theory. According to 

 the latter a person reduced to one primary sensation must see 

 either red, green, or blue ; this one sensation is excited in him 

 both by objects which we called coloured and by objects which 

 we call white. He would probably call it white ; but it would 

 be either red, green, or blue. According to Hering's theory 

 he might still see white and black in the total absence of both 

 the red-green and the yellow-blue substance. Now, cases do 

 undoubtedly occur, though they are relatively rare, of marked 

 colour-blindness, which are neither red-blind nor green-blind. 

 Cases further occur which may be described as cases of total 

 colourless blindness ; the subject recognizes only differences of 

 luminosity. Cases, moreover, occur in which one eye only is 

 affected, the other being normal, so that the subject can describe 

 the sensations of the affected eye by help of those of the normal 

 eye ; and a not small number of cases occur in which the defi- 

 ciency of colour sensation is not congenital, affecting the whole 

 of the eye, but the result of disease, and limited to a part of 

 the retina, the rest of which may be normal. And it might be 

 thought that by an examination of these several cases it would 

 be easy to decide definitely between the two theories. But 

 when this examination is carried out, many difficulties arise in 

 the way of reaching such a decision ; and indeed some of the 

 facts observed seem compatible with neither theory. A full 

 discussion of these difficulties and only a full discussion 

 would be satisfactory is impossible here. We may simply 

 remark that when we said that absolute colour-blindness, the 

 limitation of visual sensations to those of black and white, was 

 impossible on the Young-Helmholtz theory, we should have 

 added, unless it be supposed that in such cases all the three 

 primary sensations coincide, none of them being absent, much 

 in the same way that two may be supposed to coincide in red or 

 green blindness. 



