HISTORICAL REVIEW OF MODERN THEORIES 201 



our consciousness as mixtures of these sensations. They may be divided 

 into two groups, the tone-free or colourless (black and white) and the 

 toned or coloured. The tone-free are simple sensations, neither contains 

 any element of the other. The toned are also, inter se, simple sensations 

 in the same meaning of the term. Both groups may be divided into 

 three opponent pairs, black and white, red and green, yellow and blue. 

 So much for the fundamental psychological conceptions. 



From the physiological point of view these opponent sensations can 

 be hypothetically brought into line with other physiological processes. 

 We can imagine three substances of unstable chemical constitution : 

 when white is seen the black-white substance undergoes dissimilation 

 (D) or katabolic change, when black the same substance undergoes 

 assimilation (A) or anabolic change. The sensation is dependent upon 

 the ratio of D : A, not upon the absolute intensity of either. Similarly 

 red is seen when the red-green substance undergoes the Z)-process, 

 green when it undergoes the ^4-process ; and yellow when the yellow- 

 blue substance undergoes the D-process, blue when it undergoes the 

 ^4-process. 



Several difficulties are at once apparent. First, while we readily 

 accept the proposition that katabolic changes arouse sensations it will 

 not be so readily admitted that anabolic changes can produce opponent 

 sensations, or indeed any sensations at all. Though this may affect 

 the physiological analogies of the theory it is not of fundamental 

 importance to the theory as a theory of colour vision. Opponent 

 chemical processes on the analogy of oxidation and reduction, or 

 electrical on the analogy of positive and negative electricity, and so on, 

 would serve the purpose equally well and avoid the difficulty. 



Second, what must, however, be conceded as a great difficulty of 

 all theories of colour vision, black and white occupy a peculiar relation- 

 ship as compared with the others. Black is on this theory the result 

 of a physiological process excited by an internal stimulus and not the 

 result of mere absence of stimulation. Moreover, whereas we can pass 

 continuously from black to white, or vice versa, through innumerable 

 gradations of grey, there is no such uniform passage from red to green 

 or from yellow to blue. We can indeed pass by innumerable gradations 

 from red to yellow, yellow to green, green to blue, and blue to red, 

 respectively, but there are no intermediaries between red and green, 

 and yellow and blue, corresponding to grey between black and white. 

 Further, the fundamental toned colours differ in brightness and darkness, 

 and this may be regarded as an inherent black-white element in the 



