no6 



VISION. 



chemical ; that light acts primarily on certain visual substances ; and that, 

 further, these substances are limited in number corresponding to the com- 

 ponents of sensation. The behaviour of visual purple and the analogy of 

 photography have probably largely contributed to the general acceptance 

 of this view, and certain of the facts of colour-vision, and especially those 

 of persistence and induction, naturally suggest a process of this kind. 



The two chief theories of colour -vision are component theories, 1 and 

 assume the existence of a limited number of visual substances. There is 

 no direct evidence of the existence of such substances. They are purely 

 hypothetical factors, which enable the facts of light and colour-vision to 

 be expressed simply and consistently. 



The chief theory at present advocated which rejects the idea of com- 

 ponents is that of Wundt, who holds that the series of colour sensations 

 which pass into one another by insensible gradations have their basis in 

 a series of changes in one . complex substance, which similarly pass 

 gradually into one another. 



Young-Helmholtz theory. — The theory of Thomas Young, adopted 

 by Helmholtz and Maxwell, is a component theory, and is closely 



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C " O E 6 r G H 



Fig. 396. 



-Konig and Dieterici. 



connected with the doctrine of specific nervous energy. In its earlier 

 form it was supposed that there were three kinds of nerve fibre in the 

 retina corresponding to three primary colours. In its modern form 

 these have been replaced by three photochemical substances, with three 

 corresponding fundamental sensations. Each of the three substances 

 is acted on by light of all wave-lengths, but on one the action of the 

 long-wave end of the spectrum greatly predominates, on another the 

 middle, and on the third the short-wave end. The action of the 

 spectrum on each of the three substances is usually represented by 

 curves such as those in Fig. 396. 



1 G. E. Miiller lias recently used the term "component theory" in another sense. A 

 component theory is, according to him, one in which the sensation white is regarded as a 

 mixture of component chromatic sensations. Used in this sense, the Young-Helmholtz 

 is a component theory, while Hering's theory is not. 



