COLOUR VISION. 



173 



ever, can not be got by using any one pair; to get the whole range 

 of possible colour sensations you must take at least three spectral 

 colours and vary these. There are many combinations of three 

 which can be taken, we have chosen those most generally used. 

 Experiment 69, Effect of Simultaneous Action on the 

 Retina of Three Primary Colours. Put into the rotator a 

 red, a green, and a violet disk. Keep on altering the propor- 

 tions of each until you have got (a) a colourless light, (b) each 

 of the colours of the spectrum and (c) a purple. 

 As you have seen, all the various colour sensations which we 

 are capable of receiving can be produced by combined action on 

 the retina of three simple stimuli. To put the matter differently, 

 the whole range of the response of the retina to light can be ex- 

 pressed as a function of three independent variables. 



R. 



ft O C^ -B V- 



FIG. 48. To illustrate the Young theory of colour vision. 



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YOUNG-HELMHOLTZ THEORY OF COLOUR VISION. 'Any theory which is to be 

 advanced in explanation of colour vision must be based on the fundamental facts 

 which we have just described. The earliest of the modern theories, and one 

 which has given rise to a great deal of experimental work, was first suggested by 

 Young and later elaborated by Helmholtz. According to this there are three 

 different components in the retina, whether anatomical units, or chemical sub- 

 stances, is not stated. Each of these is supposed to be most effectively acted 

 upon by light waves of a quite limited part of the spectrum, and those waves 

 which influence one of the substances most have very little effect on either of the 

 other two. For the sake of illustration, red, green and violet, were selected 

 as the 'primary colours.' One component is supposed to be sensitive to red 

 waves, less so to orange and responsive in a gradually decreasing degree to the rest 

 of the spectrum. The green component is affected most by green, to a less extent 

 by yellow or blue, and little by the colours at either end. The third component, 



