THE SENSES. 



735 



primary color sensations, of three pair of antagonistic or Complemental 

 colors, black and white, red and green, and yellow and blue, and that 

 these are produced by the changes either of disintegration or of assimu- 

 lation taking place in certain substances, somewhat it may be supposed of 

 the nature of the visual purple, which (the theory supposes to) exist in 

 the retina. Each of the substances corresponding to a pair of colors, 

 being capable of undergoing two changes, one of construction and the 

 other of disintegration, with the result of producing one or other color. 

 For instance, in the white-black substance, when disintegration is in 

 excess of construction or assimilation, the sensation is white, and when 

 assimilation is in excess of disintegration the reverse is the case; and 

 similarly with the red-green substance, and with the yellow-blue sub- 

 stance. When the repair and disintegration are equal with the first 



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Fig. 434. 



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Fig. 434. Diagram of the three primary color sensations. (Young-Helmholtz theory ) 1, is 

 the red; 2, green, and 3, violet, primary color sensations. The lettering indicates the colors of 

 the spectrum. The diagram indicates by the height of the curve to what extent the several 

 primary sensations of color are excited by vibrations of different wave lengths. 



Fig. 435. Diagram of the various simple and compound colors of light, and those which are 

 complemental of each other, i.e., which, when mixed, produce a neutral gray tint. The three 

 simple colors, red, yellow, and blue, are placed at the angles of an equilateral triangle, which 

 are connected together by means of a circle ; the mixed colors, green, orange, and violet, are 

 placed intermediate between the corresponding simple or homogeneous colors ; and the com- 

 plemental colors, of which the pigments, when mixed, would constitute a gray, and of which the 

 prismatic spectra would together produce a white light, will be found to be placed in each case 

 opposite to each other, but connected by a line passing through the centre of the circle. The fig- 

 ure is also useful in showing the further shades of color which are complementary of each 

 other. If the circle be supposed to contain every transition of color between the six marked 

 down, those which, when united, yield a white or gray color, will always be found directly op- 

 posite to each other; thus, for example, the intermediate tint between orange and red is com- 

 plemental of the middle tint between green and blue. 



substance, the visual sensation is gray; but in the other pairs when this 

 is the case, no sensation occurs. The rays of the spectrum to the left 

 produce changes in the red-green substance only, with a resulting sensa- 

 tion of red, while the (orange) rays further to the right affect both the 

 red-green and the yellow-blue substances; blue rays cause constructive 

 changes in the yellow-blue substances but none in the red-green and so 

 on. These changes produced in the visual substances in the retina are 

 perceived by the brain as sensations of color. 



The spectra left by the images of white or luminous objects are 

 ordinarily white or luminous ; those left by dark objects are dark. Some- 

 times, however, the relation of the light and dark parts in the image 



