MODERN OPTICS AND PAINTING. 



573 



kind has actually reached the eye. I think you will admit that the 

 theory is modest in demanding only three sets of nerves, for in the 

 ear, as it seems, there are three thousand nerve-fibrils for the percep- 

 tion of the separate notes. In the eye it would not have been practi- 

 cable to have employed a separate nerve-fibril for each different tint, 

 for a reason which a moment's thought will render manifest. 



But to resume : according to our theory, the first set of nerves re- 

 sponds powerfully to the action of the longer waves, or to that kind 

 of light which we call red ; the second set is arranged for waves of 

 medium length, it is strongly set in action by what we call green 

 light ; and, finally, the third set is stimulated into action by the short- 

 est waves, or by violet light. Let us for the present call them the 

 red, green, and violet nerves. This diagram shows their relation to 

 the colors of the spectrum (see Fig. 1). As I have just intimated, 



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these nerves can be set into action by electricity or pressure, and other 

 causes besides light. Taking this into consideration, the next point 

 in the theory will not seem so singular to you : it is, that each set of 

 nerves is capable of being acted on, to a lesser extent, by waves of 

 light not properly belonging to it ; so, for example, the set adapted 

 for green light can, to some extent, be stimulated by red light. In a 

 case like this, the sensation will still remain that which we call green, 

 though actually produced by red light. The theory demands this, 

 and the results of experiments on persons who are color-blind to red 

 light are in accordance with it, and presently I hope to give some ex- 

 perimental illustrations of it. The red and violet nerves also have 

 this property, and can be partially set into action by light which does 

 not belong to them, but in each case the sensation remains the one 

 that properly appertains to them. 



The last point of the theory is, that, when by any cause all three 

 sets of nerves are excited into action with about the same intensity, 

 the resulting sensation is that which we call white. 



We are now in a condition to take up the explanation of the 

 sensations which' we call yellow, orange, and blue. Let us suppose 

 for a moment that the eye is acted upon by waves of light shorter 



