VISUAL PURPLE 



659 



segments of the cones. These colored bodies are said to be oil globules of 

 various colors red, green, and yellow called chromophanes, and are found 

 only in the retinae of animals other than mammals. The rhodopsin at any 

 rate appears to be derived in some way from the retinal pigment, since the 

 color is not renewed after bleaching if the retina be detached from its pig- 

 ment layer. The second change produced by the action of light upon the 

 retina is the movement of the pigment cells. On the stimulation by light 

 the granules of pigment in the cells which overlie the outer part of the rod 

 and cone layer of the retina become diffused into the parts of the cells be- 

 tween the rods and cones, the melanin granules, as they are called, passing 



FIG. 472. Sections of Frog's Retina Showing the Action of Light upon the Pigment Cells and 

 upon the Rods and Cones, (von Gendesen-Stort.) A, From a frog which had been kept in the 

 dark for some hours before death; B, from a frog which had been exposed to light just before being 

 killed. Three pigment cells are shown in each section. In A the pigment is collected toward the 

 nucleated part of the cell, in B it extends nearly to the basis of the rods. In A the rods, outer 

 segments, were colored red (the detached one green) ; in B they had become bleached. In A the 

 cones, which in the frog are much smaller than the rods, are mostly elongated ; in B they are all con- 

 tracted. 



down into tne processes of the pigment cells. A movement of the cones and 

 possibly of the rods is also said to occur, as has been already incidentally 

 mentioned. Under the influence of the stimulus of light the outer parts of 

 the cones, which in an eye protected from light extend to the pigment layer, 

 are retracted. It is even thought by some that the contraction is under 

 the control of the nervous system. Finally, according to the careful researches 

 of Dewar and McKendrick, and of Holmgren, it appears that the stimulus of 

 light is able to produce an action current in the retina. McKendrick believes 

 that this is the electrical expression of those chemical changes in the retina 

 of which we have already spoken. 



Color Sensations. When a ray of sunlight enters the eye it pro- 



