104 COLOUR-BLINDNESS. [BOOK in. 



impulses. In the second class, the yellow-blue substance has 

 undergone an expansion similar to but probably greater than 

 that which obtains in the yellow-sighted but otherwise normal 

 eyes mentioned above, it is sensitive to even the rays at the red 

 end of the spectrum; hence the spectrum to eyes of this class 

 seems of the ordinary visible length. 



765. So far then both theories may be made to explain the 

 ordinary phenomena of colour-blindness ; but it is obvious that 

 the subjective condition of the colour-blind must be different 

 according to one theory from what it is according to the other. 

 According to the Young-Helmholtz theory the red-blind person 

 does not experience in any degree the sensation of either red or 

 yellow ; from the green of the spectrum to the red end he only 

 sees some sort of green. Indeed along the whole spectrum, the 

 sensations which he experiences are only various kinds of green 

 and blue, with various amounts of the sensation whatever it be, 

 whether white or simply green-blue, or some other sensation 

 unknown to the normal eye, which results from the mixture of 

 the green and blue sensations. The green-blind person, accord- 

 ing to the same theory, has only the sensations of red and blue, 

 with the sensation whatever it may be derived from the mixture 

 of these two, he never has the sensation of either green or yellow. 

 Obviously the sensations of the two classes ought to differ very 

 widely. 



According to Hering's theory, both classes agree 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. 



We cannot of course tell what are the actual sensations of the 

 colour-blind ; no man can tell what are the sensations of his fellow 

 man; but a person who having normal vision in one eye was 

 colour-blind in the other eye could act as an interpreter. Such 

 cases are recorded ; and of them it is said (the cases were of the 

 red-blind class) that the sensation which they experienced in 

 their colour-blind eye from the red side of the spectrum 

 resembled not the green but the yellow of their normal eye. 

 Moreover intelligent colour-blind persons who have studied their 

 own cases are confident that something corresponding to the 

 yellow sensation of the normal eye enters largely into their 

 vision, and is not, as according to the Young-Helmholtz theory 

 ought to be the case, wholly absent. No great stress of course 

 can be laid on this, but as far as it goes it supports the conclusion 

 that what we can learn about actual subjective conditions is in 

 favour of Hering's theory. We may add that according to 

 Hering's theory we should expect the sensations of the so-called 

 red-blind and green-blind not to be wholly unlike since they 

 differ only in the amplitude of their sensations of yellow, whereas 



