COLORED SPECTRA. 467 



after-image left is white. This difference in the appearance of the 

 spectrum, as compared with the object, is thus explained. In the first 

 experiment, the portion of the retina on which the image of the white 

 spot has been received, being exhausted, is less susceptible of the ac- 

 tion of a given quantity of light, than the surrounding unexhausted 

 portion which corresponded to the black ground, and, therefore, though 

 receiving white light, it is less acted on by it than the rest of the 

 retina. On the other hand, in the second experiment, the portion of 

 the retina which has received the image of the black spot, is unex- 

 hausted, and is therefore more susceptible than the rest, of the action 

 of a given quantity of light. For the same reasons, the spectrum, 

 produced by gazing at the sun, and then turning the eyes from it, is 

 dark, if the eyes be turned towards a white surface, though it is lumi- 

 nous, if the eyes be closed, or directed towards a dark surface. The 

 spectra, which result from the impressions of colorless objects, are, as 

 a rule, themselves colorless. But when luminous rays of great inten- 

 sity fall upon the retina, different phenomena ensue. Thus if the eyes 

 be turned towards the sun when shining brightly, and then be covered, 

 the spectrum, at first, is of the same color as the sun itself, but rapidly 

 assumes different colors in regular succession, before it vanishes ; it 

 first becomes yellow, then orange, red, green, violet, and black. When 

 the eyes, instead of being covered, are turned towards a white surface, 

 the after-image passes through the same series of colors, the order, 

 however, being reversed. In any case, these colors are due to certain 

 states of the retina, and are called subjective, accidental, or physiolog- 

 ical colors. The spectra thus formed, move with the eyes ; their size 

 increases with the distance of the surface on which they are projected ; 

 their vividness and duration are proportional to the strength and dura- 

 tion of the primary impression ; and they fade away gradually, with 

 successive changes of color. These after-images are weaker in the sides 

 of the eyes, than in the centres or points of distinct vision ; they may be 

 produced by objects to which the attention has not been directed ; and 

 there are persons who have a singular power of retaining or reviving 

 them. 



The most remarkable spectra are those produced by the impressions 

 of distinctly colored objects. The color of the spectrum, in these 

 cases, is always complementary to that of the object ; thus, if after 

 looking steadily at a red object, the eye be turned on to a sheet of 

 white paper, the spectrum is green ; the spectrum of a green object, 

 in the same manner, is red ; that of a blue object is orange. The 

 explanation of this, is, that the retina is so exhausted as regards the 

 color first looked at, that it is no longer so readily excited by the cor- 

 responding colored rays of the white light, which pass from the surface 

 of the paper, but only by the complementary rays. After looking at 

 a given color, and then turning the eye to the complementary color, 

 the latter appears brighter and more intense than natural. Primary 

 colors are more exciting to the retina than secondary and tertiary 

 colors ; and of the three primary colors, red is the most exciting, and 

 blue the least so. Contrasted colors, and contrasts of light and shade, 

 heighten their separate effects. Thus, the depth of shadows is always 



