THE SENSE OF VISION. 343 



stimulate not only the red-perceiving elements, but to a slight extent also (to 

 use the language of the Helmholtz theory) the green- and violet-perceiving 

 elements of the retina. The eifect of this will be that of mixing a small 

 amount of white with a large amount of red light — i. e. it will produce the 

 sensation of incompletely saturated red light. This dilution of the sensation 

 can be avoided only by previously exhausting the blue- and green-perceiving 

 elements of the retina in a manner which will be explained in connection with 

 the phenomena of after-images. 



Retinal Stimulation. — Whenever by a stimulus applied to an irritable 

 substance the potential energy there stored up is liberated the following phe- 

 nomena may be observed : 1. A so-called latent period of variable duration 

 during which no effects of stimulation are manifest ; 2. A very brief period 

 during which the effect of the stimulation reaches a maximum ; 3. A period 

 of continued stimulation during which the effect diminishes in consequence of 

 the using up of the substance containing the potential energy — i. e. a period 

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

 slowly passes away. 



Fig. 153.— Diagram showing the effect of stimulation of an irritable substance. 



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



153, 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 1) 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 



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

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



