PERCEPTION OF COLORS. 



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of the triangle weights corresponding to the intensities of the primary colors, so 

 that the point of the color-chart, consequently the mixed color sought, is the 

 center of gravity of the triangle, with its angles thus weighted. In the production 

 of the mixed color, the intensity of the three primary colors must be represented 

 in the same proportion as the weights. 



Various theories have been suggested to explain color-perception: 



1. According to one theory, the elements of the retina, while uniform in type, 

 are affected in different ways by the variously colored lights (vibrations of the 

 ether of different wave-length, rapidity of vibration, and refractive exponent). 



2. The theory of Thomas Young and Hermann v. Helmholtz assumes the 

 existence of three different terminal elements in the retina, corresponding to 

 the primary colors: Stimulation of the first kind produces the sensation of red, 

 of the second green, and of the third violet. The red-perceiving elements are 

 affected most strongly by the light of greatest wave-length (red rays) , the green- 

 perceiving by the light of medium wave-length (green rays) , the violet-perceiving 

 by the light of shortest wave-length (violet rays). For the explanation of many 

 phenomena it must be assumed that each spectral color excites all forms of fibers, 

 some slightly, and others strongly. If it be conceived that the spectral colors 

 placed in their natural order in a horizontal direction in Fig. 301 (from red to 

 violet), then the three curves shown may represent the amount of excitation of 

 the three kinds of retinal elements: the continuous curved line that of the 

 red-perceiving, the dotted line that of the green-perceiving, and the interrupted 



FIG. 301. Diagrammatic Representation of the Young-Helmholtz Color Theory. 



line that of the violet-perceiving. Pure red excites the red-perceiving elements 

 strongly, but the other two forms slightly (expressed by the heights of the ordinates 

 erected at R), and the sensation of red results. Pure yellow excites the red- 

 perceiving and the green-perceiving elements with moderate activity, the 

 violet elements less actively, and the sensation of yellow results. Pure" green 

 excites the green-perceiving elements strongly, much less so the two other forms, 

 and the sensation of green results. Pure blue excites the green and violet ele- 

 ments with moderate activity, the red element slightly, and the sensation of blue 

 results. Violet excites the corresponding elements strongly, the others slightly, 

 and the sensation of violet results. Stimulation of any two elements produces 

 the impression of a mixed color; while an equal stimulation of all gives rise to 

 the sensation of white. This hypothesis of the Young-Helmholtz theory, affords, 

 in fact, a simple and clear survey and explanation of the phenomena of the physio- 

 logical doctrine of color. The theory is a development of the doctrine of Joh. 

 Miiller as to the specific energy of the nerve-fibers. The findings in the structure 

 of the retina, moreover, have been adapted to the theory. Accordingly, the 

 cones alone are supposed to be the terminal apparatus tor color-perception. In 

 cases of congenital color-blindness the cones appear to be absent in the peripheral 

 portions of the retina. The presence of longitudinal striations in their outer 

 segments is regarded as proving that they represent multiple terminal end-organs. 

 The degree of sensitiveness of any part of the retina to color is proportionate to 

 the number of cones. It is most developed in the macula lutea, which has only 

 cones; much less so further away from the macula, being lost, finally, in the 

 periphery. The rods are supposed to be concerned only with the power of dis- 

 tinguishing between quantitative sensations of light. According to v. Kries, 

 they are especially adapted for vision in poor illumination. 



3. In explanation of visual perception, Ewald Hering proceeds upon the 

 proposition that what reaches consciousness as a visual perception is the psychic 

 expression of the metabolic change in the visual substance, that is in those nervous 

 elements that are concerned in the act of vision. This substance, like every 

 other organic substance, undergoes decomposition or dissimilation during the 



