THEORIES OF COLOUR-VISION. 



1109 



highly saturated carmine-red, bluer than spectral red ; a green, lying between 

 540 and 560 in the yellow-green; and a blue, corresponding to ultramarine; 

 all three fundamental colours being much more saturated than the colours of 

 the spectrum. Their relation is shown in Fig. 399, in which the curved 

 lines represent the spectrum, x, y, and z the three fundamentals, the distance 

 of the spectral curve from the sides of the triangle representing the difference 

 of saturation. This difference in saturation is held to show that light of any 

 wave-length acts on all three photochemical substances ; the different colours 

 depending simply on the quantitative relations of the three processes. 



It will be noticed that the red and blue fundamentals of Helmholtz corre- 

 spond with those of Kb'nig deduced by a different method, and also, as will be 

 seen later, with the fundamental red and blue of Hering arrived at in a 



FIG, 399. Helmholtz. 



wholly different way. The greens of Helmholtz and Konig, on the other 

 hand, differ considerably, 505 A (K) and 540 to 560 A (H) ; both, however, 

 lie between the fundamental yellow and green of Hering (see p. 1114). 1 



Adaptation and induction. Of all the facts of light and colour- 

 vision, the most difficult to explain by the Young-Helmholtz theory are 

 adaptation and induction. Adaptation was almost completely neglected 

 by Helmholtz, and he apparently did not regard it as anything 

 requiring special explanation. He explained after-image phenomena by 

 fatigue, supplemented when necessary by psychological explanations. 

 In addition to the arguments already brought against the fatigue 

 explanation in general, there are other special objections to it when 

 combined with the hypothesis that the sensation white is a complex of 

 chromatic sensations. It is a necessary consequence of this hypothesis 



1 For criticism of Konig's methods by Hering, see Arch. f. d. ges. PhysioL, Bonn, 1893, 

 Bd. xlvii. S. 310 ; and 1895, vol. Ix. S. 540. 



