vii KETINAL EXCITATION 381 



other chromatic substances. This explains why coloured sensa- 

 tions only occur under special favourable conditions, and are 

 usually associated witli simultaneous colourless sensations which 

 diminish their saturation. 



All rays of the visible spectrum excite dissimilation in the 

 white-black substance, but in a different degree according to their 

 wave-lengths. In the yellow-blue and red-green- substances, on 

 the contrary, some rays excite dissimilation, others assimilation, 

 others again produce no effect. 



The extent of the katabolic or anabolic change in each of the 

 three visual substances depends not only on the intensity of the 

 stimulus, but also on the excitability of the substances. This 

 explains why the same objective mixture of light-rays may appear 

 lighter or darker, coloured or colourless, according to the " tuning " 

 (Stimmung) or functional state of the retina. 



Hering only admits two forms of partial colour-blindness 

 red -green and yellow -blue blindness, according as one or other 

 of the chromatic visual substances is defective. If both are 

 absent, the colour-blindness is total. 



This theory, which Hering terms that of the "opponent 

 colours," has elucidated many facts that are of value in the 

 interpretation of colour-vision, but when critically examined it 

 leaves a number of obscure points unexplained. Von Kries, for 

 instance, pointed out the existence of two distinct types of 

 red -green blindness which cannot be explained either by the 

 Young -Helmholtz or by the Hering theory. There are also 

 different types of total colour-blindness which cannot be adequately 

 interpreted on these theories. 



Schenck accordingly propounded another hypothesis which 

 combines certain points of the theories of Young-Helmholtz and 

 Hering, while it provides a physiological explanation of the 



(various forms of colour-blindness whether partial (dyschromat- 

 opsia) or total (achromatopsia). 



XII. Schenck's theory of colour -sensations and colour- 

 blindness is, as he expressly states, a revival, development, and 

 elaboration of the theory of colour-perception in indirect vision 

 put forward by his master, A. Tick (1879-1900). It may be 

 termed the " developmental theory " of colour-vision. 



Schenck accepted the three primary colours of Young- 

 Helmholtz as adequate for fully-developed colour -vision. Like 

 Tick, he substituted blue for violet, since it is possible with 

 appropriate mixtures of red, green, and blue to obtain any other 

 quality of colour -sensation. Three specific visual substances 

 correspond to the three fundamental sensations, and are excitable, 

 respectively, by the vibrations of long, medium, and short wave- 

 length, as was assumed by Helmholtz. 



Schenck adopts Tschermak's division of each of the three visual 



