ADAPTATION 289 



light and to its colour. Proof has been given that Grotthus' law 

 {q.v.) of photochemical action applies to this photolysis, and that 

 the amount of light absorbed when light of various frequencies 

 falls on the pigment is related quantitatively to the amount of 

 chemical action produced. That is, if we constructed two curves, 

 one showing the relative bleaching ])owers of light of various wave- 

 lengths and the other showing the amount of light absorbed by the 

 visual purple with light of the same wave-lengths, these curves 

 would run a similar course. Further, a third curve produced by 

 plotting wave-lengths against the intensity necessary to produce a 

 minimal peripheral sensation of grey-blue would be of the same 

 form as the other two. From this we infer that the amount of 

 visual purple bleached to produce similar sensations is the same 

 for all wave-lengths. 



Peripheral Vision. When light strikes the peripheral parts of 

 the retina it bleaches a certain amount of visual purple, producing 

 two substances, either or both of which stimulate the irritable 

 element. The general view is that the irritable elements concerned 

 are the rods. Some people, however, are of opinion with Edridge- 

 Green, that the rods merely produce the pigment and are necessary 

 for its restoration, but have nothing to do with actual perception. 

 That function they attribute exclusively to the cones which are 

 present in decreasing concentration as the distance from the fovea 

 increases. While admitting the validity of the evidence on which 

 this theory is founded, one may draw other conclusions from it. 

 It is difficult to shut one's eyes to the histological nature of the 

 nervous connections of the rods (q.v.). It has been suggested that 

 while the cones undoubtedly are stimulated by the bleaching of 

 the rhodopsin, the rods might be stimulated by the decrease of the 

 concentration of the pigment in their substances. 



Acidity. Dittler (1907) showed that the illuminated retina 

 became acid. This acidity decreased rapidly to the normal value 

 when the eye became dark-adapted, i.e. remained in the dark. 



Adaptation of Retina. The retinal mechanism has a rate of 

 adaptation to a constant stimulus that is similar to that of the 

 receptors for pressure, i.e. more rapid than that of the proprioceptors 

 in general, e.g. muscle-spindle, and a good deal less rapid than that 

 of touch, e.g. hair. A constant stimulus, therefore, ought to cease 

 to be effective in a few seconds. That is, after a latent period the 

 retina will send a burst of impulses to the cortex which will decline 

 rapidly at first and then gradually fade away. This appears to be 

 contradictory to our daily experience. But is it so ? If you fix 

 your eye steadily on this page and prevent your head and your 

 eyes from moving, you will find that the print becomes rapidly 



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