360 HENRY LAURENS AND HENRY D. HOOKER, JR. 



ping its forward movement and hanging apparently motionless 

 or swimming slowly upward. 



The investigation of the question as to whether the rate of loco- 

 motion, particularly in the lights of high stimulating effect, is 

 uniform throughout the course or whether there are variations 

 in the rate of locomotion which can be correlated with the dis- 

 tance already gone, the length of exposure, the relative proximity 

 to the source (intensity), etc., is one which is reserved for future 

 investigation. Some interesting data may thus be obtained, 

 e.g., from the comparison between the average rate for the whole 

 distance, or period, with the rates at different parts. In other 

 words, the determination as to whether the rate is constant for 

 the whole period or variable, and thus whether the curve ex- 

 pressing the average rate is a straight line or a curve. 



DISCUSSION 



There is no doubt that light stimulates nerve-endings through 

 a photochemical reaction, the stimulation being mediated by 

 photoreceptors, not necessarily structurally defined, but sensi- 

 tive to photochemical change in the substance with which they 

 are in contact. The effect of light is therefore due not to light 

 directly, but to the chemical changes which it causes, these 

 changes involving the formation of a substance or of substances 

 which, according to mass action and reaction velocity, act as 

 'inner stimuli. ' 



Although the effect of light is due to the mass action of the 

 chemical compounds which it produces (the photochemical re- 

 action product), it is not assumed that the rates of the photochem- 

 ical reactions themselves follow the simple law of mass action, 

 since the rate is controlled by the amount of the light energy 

 absorbed per unit time, and not by the actual number of molecules 

 present (Sheppard, pp. 211, 217 ff). 



The photochemical process is fundamentally an electric one, 

 in that there is a raising of potential, due to ionization, as the 

 immediate effect of light (electrolytic dissociation is governed by 

 the law of mass action). An increase in permeability, resulting 



