248 VASIL OBRESHKOVE 



light for the most prompt form of response. As the Hght in- 

 tensities fall below this optimum, the effectiveness of the stimulus 

 diminishes to cease at a certain point regardless of the length 

 of time of exposure. Within the range of effectiveness, however, 

 the principle of transference of energy was found to hold, for the 

 action of light upon the photoreceptors produced an effect. This 

 effect was equivalent to the amount of energy received, and it 

 was measured in terms of the time necessary at a definite inten- 

 sity to produce a response. 



From the data thus far given, we have no way of studying the 

 exact changes which occur in the sense organs. It seems obvious, 

 however, that light must be absorbed in order to act as a stimulus 

 and that some energy must be used up in order to initiate a 

 change in the receptors. This in fact is a law governing photo- 

 chemical phenomena. This law was first demonstrated by 

 Grotthus (1819). It was later confirmed by others (Draper, '41). 

 Lasareff ('07) showed that the Grotthus law is a quantitative 

 photochemical law and that there is a definite relation between 

 the amount of light absorbed per unit of time and the velocity 

 of chemical change produced. 



In the light of the observations made thus far and on a basis 

 of our knowledge of the mechanism of other types of receptors 

 better known to us, we are forced to assume that the changes 

 which occur in the sense organs in tadpoles of Rana clamitans 

 during illumination are of a chemical kind. The reason for 

 this assumption will become clearer when additional data are 

 presented. 



For a discussion, then, of the dynamic process in the photo- 

 receptors we must consider more critically the relation of the 

 reaction-time to the intensity of light. On the supposition 

 that the relation is dependent on the velocity of a chemical 

 change and assuming the validity of the Bunsen-Roscoe law, 

 the product of these two variables should be a constant quantity 

 for all intensities. This leads us to a discussion of table 2, 

 which contains a summary of the results entered in table 1. 

 In table 2 each figure in the second column represents the average 

 of about ninety readings, and in the third column are given the 

 intensity-reaction-time products. 



