HERMANN VON HELMHOLTZ 



sensory surface, which again so affect the sensory 

 nerves as to give rise to differing states in the nerves 

 and nerve centres with differing concomitant sensa- 

 tions. The first line of thought is at the basis of 

 the hypothesis of Thomas Young. He supposed that 

 there are three fundamental colour sensations red, 

 green and violet by the combination of which all 

 other colours may be formed, and that there are in 

 the retina three kinds of nerve elements, each of 

 which is specially responsive to the stimulus of colour 

 of one wave-length, and much less so to the others. 

 If a pure red colour alone act on the retina, only the 

 corresponding nerve element for red sensation would 

 be excited, and so with green and violet. But suppose 

 the colour to be mixed, then the nerve elements will 

 be set in action in proportion to the amount of con- 

 stituent excitant rays in the colour. Thus, if all the 

 nerve elements be set in action, we shall have white 

 light ; if that corresponding to the red and green, the 

 resultant sensation will be orange or yellow ; if mainly 

 the green and violet, the sensation will be blue or 

 indigo, and the like. Helmholtz succinctly puts it as 

 follows : 



(i.) Red excites strongly the fibres sensitive to 

 red, and feebly the other two sensation, 

 red. 



(2.) Yellow excites moderately the fibres sensitive 

 to red and green, feebly the violet sensa- 

 tion, yellow. 



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