KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 465 



Rose, 1860 and 1865. 



The dissatisfaction with the explanation of colour-blindness on Young's plan began 

 early. The first to express it prominently was Edmund Rose, who, after the careful 

 examination of a large number of patients, pronounced the theory irreconcilable with the 

 facts observed. 



Helmholtz, in an Appendix to the first edition of his work (1867), noticed Rose's 

 experiments, which, however, he considered " altogether insufficient to shake the validity 

 of Young's theory," but he admitted (p. 848) that, "in the case of congenital colour- 

 blindness, 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 arise." 



This was clearly a new idea ; but the first person to give it a practical form was 

 Mr John Aitken, F.R.S., of Falkirk, who in a paper on Colour Sensation, read before 

 the Scottish Society of Arts, 9th July 1872, suggested that "the nerves might be so 

 constructed that the red nerves might be sensitive to all the rays to which the green 

 nerves are sensitive, and the green nerves sensitive to all the rays to which the red 

 nerves are sensitive ; " — so that, both nerves being excited by either red or green rays, 

 " the sensation produced would be what we call yellow." 



Leber, 1873. 



A reproduction of this suggestion was (probably quite independently) published 

 immediately afterwards in an article by Dr Th. Leber, " On the Theory of Colour- 

 Blindness, and on the manner in which certain objections to the Young-Helmholtz 

 theory, which have arisen from the examination of colour-blind persons, may be 

 reconciled with it." 



He noticed the difficulty raised by Rose and others, that the warm colour seen by 

 the colour-blind was not green but yellow ; and he suggested that this difficulty was 

 merely due to a misapplication of Young's theory. It was a mistake, he said, to suppose 

 that one of the three sets of sensitive fibres had fallen out of use ; he preferred to assume 

 that they were all three still active, but that their degrees of excitability for certain 

 wave-lengths had become changed. He remarked that the sense of yellow arises from 

 the equal excitement of the red and the green ; and the simpler explanation was, that 

 the appearance of yellow in the dichromic spectrum might be caused by the excitement 

 of the red and green fibres being equally strong. 



Although Leber was not the originator of this explanation, he was the first to give it 

 prominence in the discussion of colour-blindness, and it usually bears his name. 



VOL. XXXVII. PART II. (NO. 22). 4 B 



