COLOUR-VISION 433 



every colour has some other colour which is comple- 

 mentary to it. If instead of mixing the colours in 

 pairs we mix them in threes, then it Ijucomes still more 

 easy to produce a resultant white. Thus by mixing red, 

 green and blue, with due regard to the relative amount 

 and intensity of each, an excellent white is readily 

 obtained. But these three colours enable us to do more 

 than merely produce white. By properly adjusting the 

 proportions of each on the disc of the colour-top we can 

 easily produce an orange and a yellow, as also a violet. 

 In other words these three colours and their mixtures 

 give rise to all the several kinds of colour-sensation 

 which we derive from a spectrum. Further, by suitable 

 mixture of these colours, together with white or black, 

 we can produce the other colours which we see in natural 

 objects around us but which are wanting in the spectrum. 

 Thus purple is extremely common in the world and can 

 be made at once by mixing red and blue. Hence these 

 three colours have come to be regarded as primary 

 colours, and we may speak of the sensations to which 

 they give rise as primary sensations. 



The foregoing considerations lead at once to the view 

 that all our sensations of colour may be regarded as the 

 outcome of a very limited number (three) of simple or 

 primary sensations corresponding to red, green and 

 blue. In accordance with this fact a theory has been 

 put forward ^ that there are in the visual apparatus three 

 kinds of nervous structure of which each corresponds to 

 one of the primary colours and is most easily set in action 

 by one of these colours. Thus the stimulation of one 

 of them gives rise to one of the primary sensations, the 

 simultaneous stimulation of all three to the same extent 

 gives rise to the sensation of white and their simultaneous 

 stimulation to varying degrees gives rise to all the other 



1 This theory was first propounded by an Englishman, Dr. Thomas 

 Young, the originator of the uniulatory theory of light. In later times 

 it was adopted and amplified hiy Helmholtz, and is therefore known 

 as the Young-Helmholtz theory. 



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