SELIG HECHT 343 



latent period. The physical connection between these two is, how- 

 ever, not direct. It is made by the mediation of the freshly formed 

 precursor substances. Strictly speaking then, equation (4) is subject 

 to two interpretations. It may represent either the photochemical 

 effect of the light or the catalytic effect of the precursors. 



Actually, however, the latter interpretation is excluded. We have 

 assumed that the relation between the concentration of precursors 

 and the velocity of the latent period reaction is linear. "Such pro- 

 portionality between concentration of the catalyst and the velocity of 

 reaction is found to hold in numerous enzyme reactions within quite 

 wide limits of concentration" (Euler, 1912, p. 132). Equation (4) 

 because of its logarithmic nature cannot therefore represent the 

 catalytic effect of the freshly formed precursors on the reaction of the 

 latent period. It must consequently express the photochemical ac- 

 tion of the light and should then be written 



E = ln I. (5) 



Here E means photochemical effect as measured by the decomposi- 

 tion of the photosensitive substance S into its precursors P and A. 

 If it were possible E would be written in grams of precursors formed 

 by the light. As it is, it must be expressed in terms of the velocity 

 of the latent period reaction, which is directly proportional to the 

 concentration of precursor substances. 



V. 



Equation (5) as it stands is simple and clearly expresses the facts as 

 we found them experimentally. The intensity / is the independent 

 variable and the photochemical effect E is the dependent variable. 

 The facts may, however, be stated in the reverse manner by saying 

 that the intensity is an exponential function of its photochemical 

 effect. Equation (5) then becomes 



I =* e^ (6) 



all the terms possessing their previous significance. 

 The differential of the last equation (6) states that 



1-^ (" 



