Quantum Theory of Vision. 299 



■"to the foregoing theory of colour vision must be limited. 

 The interpretation of colour is referred to the appreciation 

 by the nerve of the value of the quantum. A complete 

 •detailed analysis of the whole gamut of wave-lengths be- 

 tween red and violet on such a basis is, probably, unattainable, 

 even if it was any benefit to the organism. And if attain- 

 able it might result in badly differentiated colour sensations. 

 The evolutionary growth of three highly developed colour 

 sensations corresponding to the central and mean quanta of 

 the spectrum is the result. It is Nature's compromisegwith 

 her limitations. It is one which is, in part, cerebral in 

 character: the light-sensitive part of the brain accepting as 

 interpretive of the many separate frequencies a commingling 

 of sensations excited by the central and end frequencies. 

 These views do not preclude the possibility of more ihan 

 three primary colour sensations existing. I assume, how- 

 ever, that red, green, and violet are alone primary. If, 

 now, rays of the wave-length 5893 A.U., say, are received 

 -on the retina, no sensation special to this wave-length 

 arises, although electrons having velocities qnite peculiar to 

 it activate the nerve. It is more efficient for the organism 

 to develop special sensitivity towards three representative 

 stimuli, produced by widely differentiated quanta. Thns we 

 feel so much red sensation according to the proximity of 

 X 5893 to X 6563, and so much green sensation according 

 to its proximity to X 5161. The combined sensations we call 

 yellow, and yellow becomes a distinct sensation, although it 

 may be really compounded of two other sensations. It is a 

 sort of unconscious memory : the one stimulus " reminding: " 

 the colour-visual centre of the stimuli which evoke the 

 representative sensations, red and green. 



Of course it would be easy to imagine a purely objective 

 explanation of colour vision if rhodopsin exhibited appro- 

 priate absorption bands. But, on the contrary, its absorption 

 spectrum is remarkably uniform over the range of the visible 

 spectrum. 



(15) A colour-blind individual is one whose fovea! nerves 

 respond feebly to certain quanta. The same abnormality 

 affects the cones all over his retina. Thus if he is violet- 

 and green-blind, the quanta proper to E or to F produce 

 only a feeble stimulus. But those proper to and D are 

 fairly normal in the stimulus they excite. His brain has 

 developed no more than the one sensation, the maximum 

 luminosity of which lies between E and I). The abnormality 

 is fundamentally a physical deficiency ; and this leads to 

 mental deficiency,, as commonly happens in similar cases. 



