238 



NATURE OF PHOTOSENSORY PROCESS 



the last readings were repeated a few times. An example of an exper- 

 iment which will indicate the procedure is given in Table II. The 

 entire series of experiments took 6 days, the same animals being used 

 throughout. They were in good condition after the experiments, 

 and remained alive in the laboratory for many days afterwards. 



From the determination of the individual minimum distance, it is 

 simple to calculate the minimum intensity by the inverse square law. 

 This was done for each animal for every exposure before an average 



TABLE II. 



Determination of Minimum Stimidating Distance. 



Animal 120, July 18, 1919, exposure, 0.053 second. 



R. = Response elicited. 



N. R. = No response elicited. 



value was made. A summary of the experiments, giving the average 

 figures, is shown in Table I and graphically in Fig. 5. The third 

 column of Table I shows clearly enough that the Reciprocity Law of 

 Bunsen and Roscoe holds true for this form of photic sensitivity. 

 The curve drawn in Fig. 5 is a theoretical one on the assumption that 

 the photochemical effect (E) is a function of the intensity (/) and the 

 exposure (t), so that 



Ek = rt = 5.62 



