THE SPECIAL SENSES. 



closes his eyes he continues to see the luminous object for a con- 

 siderable time in its normal colors. Objects of much less intensity 

 of illumination give positive after-images, especially when the 

 eyes have been kept closed for some time, as, for instance, upon 

 waking in the morning. In negative after-images the colors are all 

 reversed, that is, they take on the complementary qualities (see 

 Fig. 140). White becomes black, red a bluish green, and vice versa. 

 Negative after images are produced very easily by fixing the eyes 

 steadily upon a given object for an interval of twenty seconds or 

 more and then closing them. In the case of colored objects the 

 after-image is shown better, perhaps, by turning the eyes upon a 

 white surface after the period of fixation is over. After-images 

 produced in this way often appear and disappear a number of 

 times before ceasing entirely, and, although the color at first is the 

 complementary of that of the object looked at, it may change before 

 its final disappearance. Anyone who has gazed for even a brief 

 interval at the setting sun will remember the number of colored and 

 changing after-images seen for a time when the eye is turned to 

 another portion of the sky. That several different after-images 

 are seen in this case is due to the fact that the eyes are not kept 

 fixed under the dazzling light of the sun, and a number of different 

 images are formed, therefore, upon the retina. 



After-images may be used in a very instructive way to show that 

 our estimates of the size of a retinal image vary with the distance to 

 which we project it, that is, with the distance at which we suppose 

 we see it. Once the image is, so to speak, branded on the retina, 

 its actual size, of course, does not vary, but our judgment of its size 

 may be made to vary rapidly by projecting the image upon screens at 

 different distances. If, for instance, in obtaining the after-image 

 of the strips shown in Fig. 140 one moves the white paper used 

 to catch the image toward and away from the eye, the apparent size 

 varies proportionally to its distance. 



Color Contrasts. By color contrast is meant the influence 

 that one color field has upon a contiguous one. If, for instance, a 

 piece of blue paper is laid upon a larger yellow square, the color 

 of each of them is heightened by contrast. A piece of blue 

 paper on a blue background does not appear so saturated as when 

 placed against a yellow background. The influences of contrast 

 may be shown in a great variety of ways.* For instance, if a disc 

 like that in the illustration, Fig. 142A , is rotated rapidly, it should 

 give circles of gray, the darkest at the middle; but each circle should 

 be uniform as it is made by the fusion of a definite amount of white 

 and black. On the contrary, the appearance obtained is that repre 

 sented in Fig. 1425. Each circle appears darker on its outer edge 



*Rood, "Modern Chromatics/' " International Scientific Series." 



