882 THE SENSE OF SIGHT 



mixed, while the spectral orange possesses a wave length of 608 /z/i. 

 Colors which are more widely separated than the complementary colors, 

 produce a sensation of purple which is not a spectral color at all but 

 may be obtained by combining red with violet, the two spectral ex- 

 tremes. If one or the other of a pair of complementary colors is added 

 in excess, the resultant sensation is a color similar to the one present 

 in excess with more white mixed in with it. Supposing that we em- 

 ploy orange and blue, with the blue present in greater amount than is 

 necessary to produce white, the result is an unsaturated blue, i.e., 

 pale blue. 



Visual After-effects. The fusion of the colors described in the 

 preceding paragraphs, depends upon the persistance of the individual 

 stimulations, a second color being thrown upon the retina before the 

 first sensation has had sufficient time to disappear. This is really true 

 of all visual impressions, because they invariably last longer than the 

 stimulus. Everything else remaining equal, these after-images depend 

 in a large measure upon the intensity of the primary stimulus, i.e., 

 upon its strength and duration. Thus, an electric spark generally 

 leaves a very decided impression in consciousness, because it is intense 

 although of very brief duration. Quite similarly, if one looks at the 

 light of a candle, and then closes his eyes, this image persists for some 

 time thereafter in its natural colors. It then fades away, meanwhile 

 undergoing certain changes from greenish blue to indigo, violet, rose 

 and pale orange. But this phenomenon is not restricted to mere 

 white-black impressions, but also to specific colors. In any case, we 

 designate them as positive after-images, because they do not change 

 their original character. Negative after-images, on the other hand, do 

 not retain their character, but assume colors complementary to those 

 of the object producing them. White becomes black, red a bluish 

 green, yellow an indigo blue, and so on. These images are obtained 

 more frequently than those of the positive kind and may be produced 

 in the following way. If we gaze intently for a few moments at a red 

 disc upon a white surface and then at a uniform white background, 

 an after-image of this disc is obtained which, however, appears green, 

 while the background assumes a reddish shade. This phenomenon is 

 usually explained upon the basis of fatigue of the retina toward this 

 particular color, although it is difficult to reconcile this hypothesis 

 with all the facts. Nevertheless, it is easy to understand that the 

 after-image must appear in the complementary color, because the reti- 

 nal component producing the sensation, say of red, has been considerably 

 reduced by the exposure, while its greenish-blue element is still present 

 in normal amounts and is, therefore, still able to produce its character- 

 istic effect. 



Contrast. If we place a small white disc upon a larger black field, 

 the former appears whiter than it would if not contrasted in this way. 

 Quite similarly, a small black dot adjusted upon a white general field 

 appears much darker in color than one resting upon a background of 



