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MANUAL OF PHYSIOLOGY. 



If colors which lie nearer to each other in the spectrum than 

 these complementary colors be mixed, the result is some color 

 which is to be found in the spectrum between the two mixed. 



The perception of the vast variety of shades of color that we 

 can distinguish can only be explained by means of this color- 

 mixing. We assume that there are three primary colors which 

 overlap one another in the spectrum so as to produce all the va- 

 rious tints. These are red, green, and violet ; the arrangement of 

 which may be thus diagrammatically explained (Fig. 232). 



We must further assume that there are in the retina three 

 special sets of nerve terminals, each of which can only be stimu- 

 lated by red, green, or violet respectively, and the innumerable 



FIG. 232. 



Diagram of the three Primary Sensations: 1 =red; 2 = green; 3= vi- 

 olet. The letters below are the initials of the colors of the spectrum. The 

 height of the shaded part gives extent to which the several primary sensa- 

 tions are excited by different kinds of light in the spectrum. 



shades of color, we see, depend upon mixtures of different strengths 

 of these primary colors, so as to produce different degrees of stimu- 

 lation of each set of nerve terminals. 



The view that such special nerve apparatus do exist for red, 

 green, and violet, is supported by the fact that the most anterior 

 or marginal part of the retina is incapable of being stimulated 

 by red objects which, therefore, look black when only seen by 

 this part of the retina. This inability to see red may extend 

 over the whole retina, as is found in some persons who may be 



