230 NATURE OF PHOTOSENSORY PROCESS 



The evidence which has already been published has established 

 different phases of this hypothesis. (1) Dark adaptation depends on 

 the regular decrease in the concentration of the residual precursors 

 present in the sensory system. This disappearance of precursors is 

 independent of light, proceeds according to the dynamics of a bimolec- 

 ular reaction (Hecht, 1918-19, &), and most probably results in the 

 reformation of photosensitive material. (2) This "dark" reaction 

 has a temperature coeflEicient of 2.4 for 10°C., similar to those usually 

 found for chemical processes. (3) It is quite characteristic of cata- 

 lyzed reactions that the velocity of the reaction is a hnear function 

 of the concentration of catalyst. Similarly we find that the velocity 

 of the second reaction, L — > T, is a Hnear function of the concentration 

 of freshly formed precursor catalysts (Hecht, 1918-19, c). (4) The 

 simple chemical nature of the catalyzed reaction, L — >• T, is evidenced 

 by its quantitative behavior in relation to the temperature. It fol- 

 lows the theoretical expectation according to the Arrhenius equation 





in which the velocity constants (k) vary with the absolute tempera- 

 tures (r). In this instance the constant jj. = 19,680, a value charac- 

 teristic of simple processes like hydrolyses and saponifications (Hecht, 

 1918-19, d). 



In spite of this array of evidence, there is one significant portion 

 of the hypothetical chemical system, the vaHdity of which still re- 

 mains to be demonstrated. This is the assumption that the reaction 

 S -^ P + A is really photochemical in nature. In other words, it 

 has still to be proved that the action of the light on the sensory process 

 possesses the ordinarily well demonstrated characteristics of photo- 

 chemical reactions. The present investigation has therefore been 

 concerned with precisely this aspect of the matter; and it is the object 

 of this paper to show that the initial effect of the stimulus is indeed 

 photochemical in nature. 



n. 



The experiments were performed with the clam, Mya arenaria, 

 which is so common at Woods Hole, Mass. An idea of the appear- 

 ance of this animal may be obtained from Fig, 1, which is made from 



