INORGANIC RESPONSE 151 



I have already advocated the use of the term 

 positive for currents which flow towards the stimulated, 

 and negative for those whose flow is away from the 

 stimulated. If such a convention be adopted, no con- 

 fusion can arise, even when, as in the given cases, the 

 currents of injury undergo a change of direction. 



Normal response positive. The normal effect of light 

 on the retina, as noticed by all the observers already 

 mentioned, is a positive variation, during exposure to 

 light of not too long duration. Cessation of light is 

 followed by recovery. On these points there is general 

 agreement amongst investigators. Deviations are re- 

 garded as due to abnormal conditions of the eye, owing 

 to rough usage, or to the rapid approach of death. 

 For just as in the dying plant we found occasional 

 reversals from negative to positive response, so in the 

 dying retina the response may undergo changes from 

 the normal positive to negative. 



The sign of response, as we have already seen in 

 numerous cases, depends very much on the molecular 

 condition of the sensitive substance, and if this condi- 

 tion be in any way changed, it is not surprising that the 

 character of the response should also undergo alteration. 



Unlike muscle in this, successive retinal responses 

 exhibit little change, for, generally speaking, fatigue is 

 very slight, the retina recovering quickly even under 

 strong light if the exposure be not too long. In 

 exceptional cases, however, fatigue, or its converse, the 

 staircase effect, may be observed. 



Inorganic response under the stimulus of light. 

 It may now be asked whether such a complex vital 



