242 NATURE OF PHOTOSENSORY PROCESS 



After some preliminary tests, four carefully controlled experiments 

 were made, several days apart. The results were so uniform that 

 further experimentation was deemed unnecessary. The data for the 

 individual experiments are given graphically in Fig. 6. It will be 

 seen that the effect of the temperature of the animal on the minimum 

 stimulating intensity is practically negligible. The temperature co- 

 efficients for 10°C. (15-25°), calculated in the ordinary way from 

 Fig. 6, are 1.04, 1.06, 1.07, and 1.06 respectively for Experiments 

 184, 185, 186, and 187. These values are so characteristic of endo- 

 energetic photochemical reactions, that, combined with the appli- 

 cability of the Bunsen-Roscoe law, they can lead to but one conclu- 

 sion. This is that the initial effect of the Hght in photic stimulation 

 is a purely photochemical phenomenon rather simple in nature. 



VI, 



As a result of these two sets of experiments we are justified in ac- 

 cepting the proposed hypothesis with a reasonable degree of confidence. 

 Of course the hypothesis is not final. At each step in its construction 

 I have usually suggested an alternative which is less simple than the 

 one eventually adopted. Therefore, it may be necessary to make 

 alterations in the details of the hypothetical chemical system as 

 further evidence accumulates. 



One consideration, however, remains of paramount significance and 

 must be the basis of any possible explanation of this kind of photosen- 

 sitivity. Tliis is that the mechanism of photoreception is not a single 

 process. Corresponding with the division of the reaction time into 

 an exposure or sensitization period and a latent period, there is a 

 fundamental division of the underlying machinery into an initial 

 photochemical reaction and a consequent ordinary chemical reaction. 

 This duaHty is patent in every experiment with Ciona and Mya. 



Whether the initial photochemical reaction is strictly reversible or 

 only pseudoreversible depends in a large measure on the relation be- 

 tween the primary and secondary reactions of photoreception. It is 

 often as compatible with the data to assume the primary reaction to 

 be 



