INORGANIC RESPONSE 167 



6 When diffuse light is allowed to impinge on the eye of 

 the frog, after it has arrived at a tolerably stable 

 condition, the natural E.M.F. is in the first place 

 increased, then diminished ; during the continuance of 

 light it is still slowly diminished to a point where it 

 remains tolerably constant, and on th,e removal of light 

 there is a sudden increase of the E.M. power nearly up 

 to its original position.' 1 



(3) I have sometimes obtained the following curious 

 result. On the incidence of light there is a response, 

 say, upward. On the continuation of light the response 

 declines to zero and remains at the zero position, there 

 being no further action during the continuation of 

 stimulus. But on the cessation or ' break ' of light 

 stimulus, there is a response downwards, followed by 

 the usual recovery. This reminds us of a somewhat 

 similar responsive action produced by constant electric 

 current on the muscle. At the moment of ' make ' 

 there is a responsive twitch, but afterwards the muscle 

 remains quiescent during the passage of the current, but 

 on breaking the current there is seen a second respon- 

 sive twitch. 



Resume. So we see that the response of the 

 sensitive inorganic cell, to the stimulus of light, is in 

 every way similar to that of the retina. In both we have, 

 under normal conditions, a positive variation ; in both 

 the intensity of response up to a certain limit increases 

 with the duration of illumination ; it is affected, in both 

 alike, by temperature ; in both there is comparatively 

 little fatigue ; the increase of response with intensity of 



1 Proc. Hoy. Svc. Edin. 1873, p. 153. 



