THE SENSE OF VISION. 791 



The rapidity with which the retina becomes fatigued varies with the color 

 of the light. Hence when intense white light falls upon the retina, as when 

 we look at the setting sun, its disk seems to undergo changes of color as one 

 or another of the constituents of its light becomes, through fatigue, less and 

 less conspicuous in the combination of rays which produces the sensation of 

 white. 



The After-effect of Stimulation. The persistence of the sensation after the 

 stimulus has ceased causes very brief illuminations (e. g. by an electric spark) to 

 produce distinct effects. On this phenomenon depends also the above-described 

 method of mixing colors on a revolving disk, since a second color is thrown 

 upon the retina before the impression produced by the first color has had time 

 enough to become sensibly diminished. The interval at which successive stim- 

 ulations must follow each other in order to pro- 

 duce a uniform sensation (a process analogous 

 to the tetanic stimulation of a muscle) may be 

 determined by rotating a disk, such as repre- 

 sented in Figure 244, and ascertaining at what 

 speed the various rings produce a uniform sen- 

 sation of gray. The interval varies with the 

 intensity of the illumination from 0.1 " to 

 0.033". The duration of the after-effect de- 

 pends also upon the length of the stimulation 

 and upon the color of the light producing it, 

 the most persistent effect being produced by the FIG. 244,-Disk to illustrate the persistence 



r T . . 3 . . . of retinal sensation (Helmholtz). 



red rays. In this connection it is interesting to 



note that while with the rapidly vibrating blue rays a less intense illumination 

 suffices to stimulate the eye, the slowly vibrating red rays produce the more 

 permanent impression. 



After-images. When the object looked at is very brightly illuminated the 

 impression upon the retina may be so persistent that the form and color of the 

 object are distinctly visible for a considerable time after the stimulus has ceased 

 to act. This appearance is known as a " positive after-image/' and can be best 

 observed when we close the eyes after looking at the sun or other bright source 

 of light. Under these circumstances we perceive a brilliant spot of light which, 

 owing to the above-mentioned difference in the persistence of the impressions 

 produced by the various colored rays, rapidly changes its color, passing gen- 

 erally through bluish green, blue, violet, purple, and red, and then disappear- 

 ing. This phenomenon is apt to be associated with or followed by another 

 effect known as a " negative after-image." This form of after-image is much 

 more readily observed than the positive variety, and. seems to depend upon the 

 fatigue of the retina. It is distinguished from the positive after-image by the 

 fact that its color is always complementary to that of the object causing it. In 

 the experiment to demonstrate the fatigue of the retina, described on p. 790, 

 the white spot which appears after the black disk is withdrawn is the " nega- 

 tive after-image" of the disk, white being complementary to black. If a 



