^ SELIG HECHT 243 



light 

 5 -» P + A 



"dark" 

 S <^ P + A' 



in which A' is an accessory precursor different from A, as it is to 



assxmie it to be 



light 

 S ^ P + A. 



"dark" 



I 



If, as we have postulated, the secondary is catalyzed by the products 

 of photolysis of the primary reaction, then the primary reaction is to 

 be considered as strictly reversible. If, however, the reaction be- 

 tween the two is conceived to be in the nature of a catenary chemical 

 interaction, then a pseudoreversible reaction is a more Hkely possibility 

 for the primary process than a strictly reversible one. Even in this 

 instance, a photochemical reaction is feasible, if the secondary process 

 itself is assumed to be reversible. 



None of these alternatives, however, appear very attractive because 

 of their innate complexity. This becomes especially true when it is 

 necessary to make a mathematical analysis of data in order to com- 

 pare theoretical expectation and actual performance. The proposed 

 hypothesis of photoreception is therefore to be preferred in its present 

 form, unless some glaring discrepancy arises. 



vn. 



Aside from their significance with regard to an hypothesis of photo- 

 reception, these experiments involve some general conceptions of 

 rather wide application. In the study of the responses of organisms 

 much agitation has resulted over the difference between those indi- 

 viduals which are sensitive to the condnuous action of light, and those 

 which are sensitive to a rapid change in the intensity of the light only. 

 The distinction as it is usually made rests on the assumption that for 

 the one group the effective stimulus is a definite quantity of light, 

 whereas for the other group the effective stimulus is the rate of change 

 of the intensity. The conception underlying this differentiation, how- 

 ever, is as fallacious as it has been common. I have no desire to enter 

 into a controversy which has been continued much too long. But the 



