518 



THE SENSE OF SIGHT. 



luminous objects, are ordinarily 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 



FIG. 187. 



A circle showing 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 cor- 

 responding 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 b3 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 or red is complemental of the middle tint 

 between green and blue. 



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 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 remains 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 di- 

 rectly by the object, but the opposite, or complemental color. 

 The spectrum of a red object is, therefore, green ; that of a 



