616 HANDBOOK OF PHYSIOLOGY. 



sensation is gray; but in the other pairs when this is the case, no sensa- 

 tion occurs. The rays of the spectrum to the left produce changes in the 

 red-green substance only, with a resulting sensation of red, whilst the 

 (orange) rays further to the right affect both the red-green and the 

 yellow-blue substances; blue rays cause constructive changes in the yel- 

 low-blue substance, 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 ordi- 

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

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

 under certain circumstances, be reversed in the spectrum; what was 

 bright may be dark, and what was dark may appear light. This occurs 

 whenever the eye, which is the seat of the spectrum of a luminous object, 

 is not closed, but fixed upon another bright or white surface, as a white 

 wall, or a sheet of white paper. Hence the spectrum of the sun, which, 



/Mue 



Fm 415 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 complemental 

 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 figure 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 opposite to each other; thus, for 

 example, the intermediate tint between orange and red is complementary of the middle tint between 

 green and blue. 



while light is excluded from the eye is luminous, appears black or gray 

 when the eye is directed upon a white surface. The explanation of this 

 is, that the part of the retina which has received the luminous image re- 

 mains for a certain period afterwards in an exhausted or less sensitive 

 state, while that which has received a dark image is in an unexhausted, 

 and therefore much more excitable condition. 



The ocular spectra which remain after the impression of colored 

 objects upon the retina are always colored; and their color is not that of 

 the object, or of the image produced directly by the object, but the 

 opposite, or complemental color. The spectrum of a red object is, there- 

 fore, green; that of a green object, red; that of violet, yellow; that of 



