THE SENSES. 



615 



chemical rays/ which have a powerful chemical action. The rays which 

 can be perceived by the brain, i. e., the colored rays, must stimulate the 

 retina in some special manner in order that colored vision may result, 

 and two chief explanations of the method of stimulation have been 

 suggested. 



(1.) The one, originated by Young and elaborated by Helmholtz, 

 holds that there are three primary colors, viz., red, green, and violet, 

 and that in the retina are contained rods or cones which answer to each 

 of these primary colors, whereas the innumerable intermediate shades of 

 color are produced by stimulation of the three primary and color termi- 

 nals in different degrees, the sensation of white is produced at the same 

 time when the three elements are equally excited. Thus if the retina be 

 stimulated by rays of certain wave length, at the red end of the spec- 

 trum, the terminals of the other colors, green and violet, are hardly 

 stimulated at all, but the red terminals are strongly stimulated, the re- 

 sulting sensation being red. The orange rays excite the red terminals 



Fio. 413. 



Fia. 414. 



FIG. 414. 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 9f 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, 



considerably, the green rather more, and the violet slightly, the resulting 

 sensation being that of orange, and so on. 



(2 ) The second theory of color (Hering's) supposes that there are six 

 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 assimila- 

 tion 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, be- 

 ing 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 assimi- 

 lation is in excess of disintegration the reverse is the case; and similarly 

 with the red-green substance, and with the yellow-blue substance. When 

 the repair and disintegration are equal with the first substance, the visual 



