PROPERTIES OF THE RETINA. 349 



wave-lengths represented by the complementary colors setting up 

 by their combined action the same photochemical processes that 

 normally are induced by the sunlight. 



After-images. — As the name implies, this term refers to images 

 that remain in consciousness after the objective stimulus has ceased 

 to act upon the retina. They are due doubtless to the fact that the 

 changes set up in the retina by the visual stimulus continue, with 

 or without modification, after the stimulus is withdrawn. After- 

 images are of two kinds: positive and negative. In the positive 

 3fter-images the visual sensation retains its normal colors. If one 

 looks at an mcandescent electric light tor a tew seconds and then 

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

 siderable time in its normal colors. Objects of much less inten- 

 sity of illumination may give positive after-images, especialy 

 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. 147). 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 tliis 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 »re 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. 147 one moves the white paper used 

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

 varies proportionally to its distance. 



