Quantum Theory of Vision. 301 



within the nerve would be distributed at radial distances 

 from the surface proportional to the frequency o£ the light 

 which gave rise to them. The " red " electrons (i. e. those 

 exciting pre-eminently the red sensation) outermost, then the 

 " green 9 ' and lastly the " violet." But in the case of very 

 intense light stimulus the disturbance due to the passage of 

 very many electrons might result in permitting only very few 

 of the red and green, but allowing abundant violet, to collect 

 in this manner ; the latter attaining the outer limits of the 

 field of disturbance. Now in the colour succession of after- 

 images there is found evidence for both these modes of 

 distribution. We must suppose that when the light stimulus 

 is withdrawn the electrostatic field gradually breaks down. 

 We have the red electrons going first ; for they are the most 

 strongly attracted ; the green following, and finally the 

 violet. We assume that in the act of reverting to the 

 sensitising molecule the electron creates fresh colour sen- 

 sation. But the energy available must be less than that 

 originally possessed by the electron when entering the 

 nerve. Einstein's explanation of the law of Stokes respecting 

 fluorescence (Allen, loc. cit. p. 190) maybe invoked. In the 

 present case there should be a diminution of the intensity 

 of the stimulus ; in other words a shift of the sensation 

 towards the red end. 



Now it is agreed by many observers that amidst many 

 variants the after-images appear very generally in the 

 order red, green, blue (Parsons, loc. cit. pp. Ill, 261). 

 This is for moderate to bright light. For long continued 

 excitation by more intense light,- blue takes precedence of 

 all. There is also evidence for a lowering of the spectral 

 sensation. Thus McDougall writes : " An important feature 

 of the after-images of bright white light is that, after a 

 first short period in which two colours fuse to give yellow, 

 or, as is the case after the brighest lights, all three fuse 

 to give white, the colours that in turn occupy the area of 

 the after-image, alone and unchanging for considerable 

 periods, are red green and blue only. The red is a rich 

 crimson red, decidedly less orange than the red of the solar 

 spectrum, the blue is a rich ultramarine, and the green a 

 pure green having no inclination towards blue or yellow." 

 He goes on to describe the high saturation of these colours. 

 As regards the repetitional effect generally observed, that is 

 the recurrence of the three colours in the like order, we 

 again find a photographic counterpart in recurrent reversal. 

 This is, according to the electronic theory of photography, 

 due to the break-down of a succession of latent images ; 



