274 VASIL OBRESHKOVE 



absorbed (Grotthus, 1819; Draper, '41), and this varies very- 

 little with a change of temperature. 



Henri et Henri ('12) find that the excitability of Cyclops by 

 ultraviolet light is independent of temperature. Hecht ('19), 

 in studying the photodynamic nature of Mya with respect to 

 the amount of energy received and the length of the reaction- 

 time, assigns a value for the temperature coefficient of 1.1 for 

 a change of 10°C. These two studies are the only ones to my 

 knowledge on the temperature coefficient of animals responsive 

 to light. The magnitude of the temperature coefficient is in 

 close accord wdth those of chemical reactions due to light, thus 

 affording strong evidence that the sensitivity of animals to 

 light is of a photochemical kind. Osterhout ('17) points 

 out that a process which has a temperature coefficient as high 

 as 2 cannot be considered of physical nature. For photosynthe- 

 sis, he assigns a value of 1.7 for the temperature coefficient. This 

 slightly higher value, he holds, is evidence that the process of 

 photosynthesis is of a combined nature, photochemical and 

 chemical. 



In the light of the observations made in the study of the 

 photodynamic nature of the responses of Rana clamitans tad- 

 poles, namely, that, with a constant intensity of illumination at 

 one-minute intervals in the dark, the reaction-time shows no 

 marked variation from a value which is also constant, we have 

 a suggestion that the changes in the receptors are of an orderly 

 type. Whatever these changes are, they must involve a photo- 

 chemical substance, normally present in the receptors, a part 

 of which undergoes a change to a different state under the in- 

 fluence of light. For simplicity, at this point of the discussion 

 this may be expressed by assuming a change from A to B, where 

 A is chemically different from B and only in this last state 

 capable of acting as a stimulus for the nerve endings with which 

 it is in immediate contact. Such a hypothetical representation 

 is usually employed for the measurement of photochemical 

 changes in chemistry, and it is variously modified by Weigert 

 ('11) and Sheppard ('14) to meet the requirements of the order 

 of photochemical reactions which they have classified. Two 



