Dr. W. Pole on Colon ^Blindness. 



53 



? ex pede Herculem" and I may claim a more extensive 

 discipleship under the great master in another branch of his 

 labours. 



The description of colour-blindness which Helmholtz gave 

 in the old edition is so well known that it is only necessary 

 to refer to a few passages bearing on our comparisons with 

 the new one. The author stated (p. 294) that the patients 

 saw in the spectrum only two colours, which they for the 

 most part called blue and yellow, and he mentioned experi- 

 ments on a patient tried by himself, which proved that for 

 this person's eyesight all colours could be compounded by 

 mixtures of yellow and blue. He then explained how, on 

 Grassmann's principles of colour mixtures, " for an eye which 

 acknowledges trichromic matches, but which confuses red 

 with green, it follows that all the hues which it especially 

 distinguishes may be compounded of two other colours some- 

 thing like (etwa) yellow and blue." 



The author, however, in further explanation, pointed out 

 why he believed that yellow and blue might not be the 

 colour-sensations really experienced. He said, p. 297 : — 



In the Young hypothesis the colour invisible to the colour- 

 blind eye could naturally be only one of the fundamental colours, 

 . . . Bed-blindness would thus be explained on Young's hypo- 

 thesis by a paralysis of the red-perceiviug nerves and it 



follows from this that the red-blind perceive only green, violet, 

 and their mixture, blue. 



He also (p. 299) mentioned another class, in regard to 

 whom " it might be conjectured (kann man vermuthen) that 

 their defect lay in an insensitiveness of the green-perceiving 

 nerves." These passages, it will be seen, contain the essence 

 of the original application of Young's theory to dichromic 

 vision, which became so popular. 



We now turn to the new edition ; and as it is matter of 

 common knowledge that, in the interval of time since the 

 previous publication, the applicability of the hypothesis re- 

 ferred to has been seriously questioned, we look with interest 

 to what is said now on the matter. 



We may take it up at page 3(52, where we find a mathe- 

 matical investigation by geometry and algebra, to show that 

 if two colours, R and G, appear alike to the colour-blind, 

 there may be found another colour which, though visible to 

 the normal eye, is invisible to them. All this is just as it 

 stands in the old edition ; but now we come to the first 

 change of importance. The author, at the end of the investi- 

 gation, adds a new passage as follows : 



