240 



NATURE OF PHOTOSENSORY PROCESS 



On the basis of these results, there seems good reason to believe 

 that the initial action of the Hght on the sense organ is photochemical 

 in nature. This is precisely what we have assumed in our hypothesis. 

 However, there is still another test possible in this connection, and it 

 was also apphed in the study of the effect of the Hght on the photo- 

 sensory process. 



It is almost axiomatic to say that photochemical reactions possess 

 low temperature coefficients, very near 1.00 for 10°C. (Sheppard, 

 1914, p. 304). The obvious thing, therefore, is to determine the ef- 

 fect of the temperature on photoreception. 



TABLE III. 

 Minimum Stimulating Distance at Different Temperatures. 

 Animal 186, August 20, 1919, exposure, 0.016 second. 



R. = Response elicited. 



N. R. = No response elicited. 



Here again it is of the utmost importance to distinguish between 

 the two periods of the reaction time. The reaction time as a whole 

 possesses a high temperature coefficient. For this, however, the effect 

 of temperature on the latent period is entirely responsible (Hecht, 

 1918-19, d). Therefore, if our results are to be of the desired signifi- 

 cance, we must study the relation between the temperature and the 

 occurrences in the exposure period only. For the same reasons as 

 before, this resolves itself into a determination of the minimum inten- 

 sity necessary to elicit a response at different temperatures with a 

 given exposure. 



