THE SENSES. 225 



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 sensa- 

 tions 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. Sometimes, 

 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, while light is 

 excluded from the eye, is luminous, appears black or grey 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 remains for a certain 

 period afterward 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 ob- 

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



Ttil 



'blue 



green. 



FIG. 384. Diagram of the various simple and compound colors of light, and those which are com- 

 plemental of each other, i.e., which, when mixed, produce a neutral grey 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 grey, and of which the prismatic spectra would to- 

 gether produce a white light, will be found to be placed in each case opposite to each other, but con- 

 nected 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 

 grey color, will always be found directly opposite to each other; thus, for example, the intermediate 

 tint between orange and red is complemental of the middle tint between green and blue. 



or complemental color. The spectrum of a red object is, therefore, green; 

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

 and so on. The reason of this is obvious. The part of the retina which 

 receives, say, a red image, is wearied by that particular color, but remains 

 sensitive to the other rays which with red make up white light; and, 

 therefore, these by themselves reflected from a white object produce a 

 green hue. If, on the other hand, the first object looked at be green, 

 the retina being tired of green rays, receives a red image when the eye is 

 VOL. II. 15. 



