664 LATENT PERIOD OF MYA 



This means an upsetting of the balance in the reversible reaction 



system 



S^P + A 



previously established for the photic response of Mya (Hecht, 1919, a). 

 Such a difficulty, however, might be surmounted by assuming the 

 photochemical reaction to be pseudoreversible like the reduction of 

 ferric oxalate (Sheppard, 1914, p. 213). As a result, the formation 

 of an additional substance would have to be postulated in order to 

 replace the one used up in the latent period reaction. Matters 

 would thus become increasingly complicated, and the value of the 

 hypothesis considerably lessened as a means of explanation. 



All the data may, however, be accounted for very simply on the 

 basis of the third possibility. This assumes that the latent period 

 represents the duration of an independent chemical reaction which 

 is catalyzed by the presence of one or both of the precursor sub- 

 stances formed by the light. In this manner none of the precursor 

 material is used up, and the balance of the reversible photochemical 

 reaction is maintained. The reaction of the latent period may be 

 considered to be arranged in the dark, in that the reacting substances 

 are all present. It requires merely the action of the light in the 

 production of precursor materials to cause the reaction to proceed 

 with an effective velocity due to the catalytic influence of the fresh 

 precursor materials. 



The two characteristics of the latent period find their explanation 

 in this way, in harmony with the previously developed concepts of 

 the mechanism of photosensitivity. The latent period is a chemical 

 reaction; this agrees with the observed effect of the temperature 

 (Hecht, 1919, b). The rate of the latent period process is a linear 

 function of the amount of precursor substances formed during the 

 exposure period. In explanation of this it will be remembered that 

 it is a common characteristic of catalyzed reactions for the velocity 

 of the reaction to be a linear function of the concentration of catalyst 

 (Euler, 1912, p. 132). 



