272 HUMAN PHYSIOLOGY 



by a sodium light, because the extracted visual purple is bleached 

 by daylight, not by red or yellow light. The composition of visual 

 purple is not known. 



2. In the dark the pigment of the epithelial layer in a frog 

 collects in a cell body, while, in the light, it moves along 

 the processes between the cones and rods. 



The pigment is not directly necessary for the perception of light, 

 for individuals in whom pigments are lacking (albino) are able to 

 see. 



3. In frogs and fishes the inner processes of the cones 

 become shorter and thicker in the light. 



4. In the illuminated retina certain electrical phenomena, 

 connected with the irritation, have been observed, but their 

 cause and significance are not known. 



II. Visual sensation. 



A. The intensity of visual sensation. The intensity of 

 visual sensation is dependent upon : 



1. The intensity of the stimulus. The stronger the light 

 the greater the intensity of the sensation. The sensibility 

 for perceiving the difference in the intensity of different lights 

 follows Weber's law (page 256). 



2. The size of the illuminated portion of the retina. A 

 small light object may appear darker than a larger but less 

 luminous object. 



3. The duration of the action of the light. 



(a) The rise of visual sensation. A considerable length 

 of time, about o. 16 second, elapses between the beginning 

 of the action of the light and the time that the sensation of 

 light has reached its greatest intensity. Hence a bright 

 light acting for a very short time may appear darker than a 

 less bright light acting for a longer time. 



(/>) Disappearance of the visual sensation; positive after- 

 image. If the light disappears suddenly, the visual sensa- 

 tion remains for a short time. This is called the positive 

 after-image. Upon this depends the well-known phenome- 

 non that if, in the dark, a glowing coal is moved forward and 

 backward, the coal does not appear as a luminous point at 



