288 OUTPOSTS OF THE INTELLIGENCE SERVICE 



we remember that the retina is primarily part of the nervous 

 system, and that the electrical variations found are actually those 

 accompanying the nervous impulse, and do not differ materially 

 from those found by Adrian in the optic nerve itself. The change 

 in E.M.F. does not occur exactly at the moment when the stimu- 

 lating light falls on the retina. There is a definite latent period 

 during which, it is presumed, some photochemical change takes 

 place. In the case of peripheral vision, where rods are the main 

 elements present, this latent period is occupied by the bleaching 

 of the visual purple and the liberation of whatever substance is 

 responsible for stimulating the rods, but in the fovea there is no 

 visual purple to bleach. We can only assume at present that some 

 substance is formed or altered by the incidence of light on the 

 fovea, which in turn stimulates the cones. Edridge-Green has 

 brought forward evidence that visual purple may diffuse from the 

 surrounding rods into the fovea, be bleached by light there, and so 

 stimulate the cones, Hecht and others have studied the relation- 

 ship between the intensity and colour of the incident light on the 

 one hand and the })hotochemico-clectrical response on the other. 

 With monochromatic light a geometric rise in the intensity of the 

 light causes an arithmetic increase in the E.M.F. produced, i.e. 

 if we increase the illumination from 60 candles to 3,600 candles 

 we would double the potential developed. With coloured lights of 

 apparently equal intensities, the yellows are more effective as 

 stimulants when the general intensity is fairly high, and the greens 

 when the intensity is low. 



The following table from Frohlich (1013) shows that, in order to 

 produce the same E.M.F., it is necessary to have a greater intensity 

 of blue light than of white light, and a much greater intensity of 

 light when it is red than when it is blue. 



TABLE XL 



(2) Chemical Changes. We have seen that rhodopsin is bleached 

 by incident light, with a velocity related to the intensity of the 



