THE SENSE OF VISION. 



789 



of fatigue ; 4. A period after the stimulation has ceased in which the effect 

 slowly passes away. 



FIG. 241. Diagram showing the effect of stimulation of an irritable substance. 



The curve drawn by a muscle in tetanic contraction, as shown in Figure 



241, illustrates this phenomenon. Thus, if A D represents the duration of the 

 stimulation, A B indicates the latent period, B C the period of contraction, 

 C D the period of fatigue under stimulation, and D E the after-effect of 

 stimulation showing itself as a slow relaxation. When light falls upon the 

 retina corresponding phenomena are to be observed. 



Latent Period. That there is a period of latent sensation in the retina 

 (i. e. an interval between the falling of light on the retina and the beginning 

 of the sensation) is, judging from the analogy of other parts of the nervous 

 system, quite probable, though its existence has not been demonstrated. 



Rise to Maximum of Sensation. The rapidity with which the sensation of 

 light reaches its maximum increases with the intensity of the light and varies 

 with its color, red light producing its maximum sensation sooner than green 

 and blue. Consequently, when the image of a white object is moved across 

 the retina it will appear bordered by colored fringes, since the various con- 

 stituents of white light do not produce their maximum effects at the same 

 time. This phenomena can be readily observed when a disk on which a 

 black and a white spiral band alternate with each other (as shown in Figure 



242, A) is rotated before the eyes. The white band as its image moves out- 



A B 



FIG. 242. Disks to illustrate the varying rate at which colors rise to their maximum of sensation. 



ward or inward over the retinal surface appears bordered with colors which 

 vary with the rate of rotation of the disk and with the amount of exhaustion 

 of the retina. Chromatic effects due to a similar cause are also to be seen 

 when a disk, such as is shown in Figure 242, B (known as Benham's spectrum 



