

Dr. W. Pole on Colour-BUndness. 55 



the customary expression of painters, the colour corresponding to 

 the red' or less refrangible half of the spectrum should be called 

 the warm colour, and that of the blue half the cold colour ; and we 

 will do so in what follows. 



At pages 367-8 he remarks on the distinction of the 

 classes of " red-blind " and " green-blind," but more positive 

 opinions are given later. 



At pages 368 and 369 we come upon the " new J ' explana- 

 tion of colour-blindness already alluded to. The idea of this 

 appears to have arisen with Helmholtz himself at a very 

 early period. In 1860 Edmund Rose pointed out the diffi- 

 culty of Young's original explanation, stating that after 

 careful examination of 59 colour-blind patients, he found it 

 irreconcilable with the facts observed. In 1867, when the 

 last portions of Helmholtz's work appeared, he noticed Rose's 

 experiments, admitting as an alternative suggestion (p. 848), 

 that " in the case of the congenital colour-blind it might well 

 be imagined that the activity of the nerve-fibres might not 

 be removed, but that the intensity curves of the three kinds 

 of light-sensitive elements might change, whereby a much 

 greater variability in the effect of objective colours on the 

 eye might enter" *. 



Leber, in 1873, expanded the idea and put it into a practical 

 form. The great difficulty had been the continually increasing 

 evidence that the warm sensation of the colour-blind corre- 

 sponded with the normal yellow, and not with red or green 

 as the Young explanation would require ; and Leber pointed 

 out that if, instead of abolishing the reel or the green 

 element, the two w r ere assumed, by changes in their intensity 

 curves, to coincide, forming yellow r , the whole difficulty 

 would vanish. This explanation was afterwards repeated by 

 Fick, and at a later time, 1886, reproduced by Konig in his 

 communication to the British Association. 



Helmholtz takes this up in his new edition apparently with 

 approval, and much matter will be found bearing on it in 

 pages 368 and following, and again at page 376, He thinks 

 that if the green curve has gone over to the red, it will 

 produce the sensations of the " green-blind "; but if the red 

 curve has gone over to the green, it will produce those of 

 the " red-blind," there being, however, less frequent inter- 

 mediate degrees between the two extremes. 



In pages 372 to 371 he applies the same explanation (as 

 Fick had done) to the colour phenomena of the normal 



* The new explanation is always ascribed to Leber; this clear antici- 

 pation of it by Helmholtz seems hitherto to have escaped attention, oven 

 by Dr. Konig-. 



