vii RETINAL EXCITATION 377 



Thomas Young (1807), which was taken up and perfected by 

 Helmholtz (1852), of Wundt (1874-1902), and of Hering (1878). 

 Schenck more recently (1900-8) formulated a new theory in 

 which he attempted to supplement and reconcile those of his 

 predecessors. 



Young regarded the colours of the solar spectrum as the 

 .summation of different mixtures of three simple or fundamental 

 colours red and violet, the two extremes, and green, the middle 

 hue of the spectrum. " It is certain," he wrote, " that the perfect 

 sensations of yellow and of blue are produced respectively by 

 mixtures of red and green and of green and violet lights, and 

 there is reason to suspect that these sensations are always com- 

 pounded of the separate sensations combined : at least this 

 supposition simplifies the theory of colours : it may therefore be 

 adopted with advantage, until it be found inconsistent with any 

 of the phenomena ; and we may consider white light as composed 

 of a mixture of red, green, and violet only." l 



On Young's original theory all colour -sensations are com- 

 pounded from three fundamental sensations which are qualitatively 

 constant, and only vary in intensity. To conform to the law of 

 specific energy Young assumed as a subsidiary hypothesis that 

 three kinds of receptor nerve-fibres are present in the retina at 

 every point that can be stimulated by the three colours, to which 

 three kinds of perceptor elements for red, green, and violet corre- 

 spond in the brain-centres. 



Against Young's theory is the fact that it is not possible by 

 the mixture of these three colours to reproduce all the tones and 

 saturations of the colours of the spectrum mixed colours being 

 always less saturated than their components. 



Helmholtz easily disposed of this and other difficulties by 

 assuming that each fundamental colour is capable of stimulating 

 the three hypothetical receptor elements, but in different degrees, 

 according to the difference of wave-lengths. Light of longer 

 wave-length chiefly excites the elements that are sensitive to red ; 

 that of intermediate wave-length, the elements sensitive to green ; 

 that of short wave-length, the elements sensitive to violet. 



If the colours of the spectrum from red to violet are arranged 

 in horizontal series, the three curves (Fig. 181), according to 

 Helmholtz, approximately represent the excitability of Young's 

 three specific nerve-elements. The red rays (J?) stimulate the red 

 elements strongly, the other two elements weakly ; the same 

 applies to the green (6-') and violet ( F) rays ; and this accounts for 

 the sensations of red, green, or violet. 



On this presumption the sensation produced by the red of the 

 spectrum must include that of white, which results from weak, 



1 Lectures on Natural Philosophy, by Thomas Young, 1807. XXXVII. "On 

 Physical Optics," p. 439. 



