SELIG HECHT 345 



larly over the surface, and its maximum concentration is near the tip 

 within the siphon, where the light reaches it only after it has passed 

 through the sensitive surface . Moreover, individuals vary widely in the 

 amount of pigment they display, some being practically free from it. 



Whether our results are ultimately due to the property of an ab- 

 sorbing medium in the sense organ, or whether the equation repre- 

 sents a basic photochemical phenomenon cannot therefore be decided 

 at present. Certain it is that some purely photochemical effects 

 present a similar condition. For example during the period of normal 

 exposure of a photographic plate, the photochemical effect is a logarith- 

 mic function of the intensity (Weigert, 1911, p. 86). The final mean- 

 ing of our results will therefore await the elucidation of similar data 

 in photochemistry proper. 



It may be pointed out that the logarithmic relation between the 

 intensity of the light and its effect in photoreception agrees with the 

 general idea expressed in the Weber-Fechner law. This agreement, 

 however, is more apparent than fundamental. The Weber-Fechner 

 law is itself merely a psychophysiological statement of the general 

 "compound interest" principle which our results also follow. The 

 comparison does bring out the fact that a logarithmic relation is not 

 pecuHarly a biological phenomenon, as much of the discussion of the 

 Weber-Fechner law may lead one to suppose. It is well known in 

 physical chemistry, and depends on the change in a function proceed- 

 ing at a rate proportional to its own magnitude. 



VI. 



Before concluding this paper it will be of interest to synthesize the 

 knowledge that we have so far gained of the energy relations in pho- 

 toreception. The two components of light are its intensity and the 

 time of its action. The photochemical effect of each of these compon- 

 ents has now been investigated, and quantitative expressions have 

 been deduced for them. In addition we have studied the relation of 

 these two variables to each other. Are the various findings consistent 

 with one another, and can any additional information be gained by 

 their combination? 



We learned that for the minimum energy necessary to elicit a re- 

 sponse the time and the intensity follow the Reciprocity Law of Bun- 



