CHAPTER X 



THE EFFECT OF EAPID CHANGES IN INTEN- 

 SITY OF LIGHT 



IT may prove necessary to make a similar assumption 

 for the effect of a constant illumination as was made by 

 Nernst for the theory of the action of galvanic currents, 

 namely that there are two antagonistic processes going 

 on, one being the photochemical effect of light and the 

 second either a process of diffusion of the substances 

 formed or a chemical reaction of the opposite character 

 as that caused by the action of the light. Many animals 

 which are oriented by constant illumination react by a 

 quick, jerky movement when the intensity of light is 

 either rapidly increased or diminished. In this case the 

 effect is determined by the rapidity of the change in the 

 intensity, ^ , and not by the product of intensity into dura- 

 tion of illumination, it. 291 These twitching or jerking 

 effects caused by a rapidly changing intensity of light 

 are comparable to the twitching brought about in a muscle 

 by a rapid increase or decrease in the intensity of a cur- 

 rent. The writer described such reactions first for tube 

 worms like Serpula, which withdraws suddenly into its 

 tube when a shadow passes over it or when the intensity 

 of light is suddenly diminished in some other way. The 

 anthropomorphists, of course, declare this reaction to be 

 induced by the instinctive fear of an enemy, oblivious of 

 the fact that if they were consistent they would have to 

 give the same explanation for the twitching of a muscle 

 upon rapid changes in the intensity of a current. The 



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