276 VASIL OBRESHKOVE 



light intensity, the appreciation of a shadow may serve as a 

 warning of the approach of an enemy. In species capable of 

 responding to an increase of intensity, the appreciation of the 

 change of illumination must also be of value. It is evident, 

 therefore, that animals better equipped for judging light and 

 darkness and for responding to such stimuli, are better adapted 

 for their life struggle. The more sensitive the receptors are, 

 the more value they must have for the individuals. 



In reflexes due to light we recognize two important periods. 

 These are a sensitization period, during which energy is received 

 for the chemical processes necessary in the sense organs, and a 

 secondary period w^hich is occupied by such processes as the 

 diffusion of chemical products to the nerve endings, transmission 

 of the impulse, and a contraction of the muscles of the organ 

 involved in the response. When the time taken up by the 

 secondary processes is considerable, it must be deducted from 

 the reaction-time in determining the actual duration of the 

 preceding chemical process. 



In judging the relative photosensitivity of animals, a separa- 

 tion of the reaction-time into its two phases, with the assignment 

 of a value for each, is important. Animals, however, whose 

 photoreceptors are characterized by the quickness with which 

 they can execute the chemical processes necessary for the stimu- 

 lation of nerve endings, but also by slowness in the secondary 

 processes, gain little from the quickness with which the first 

 process is executed. In such animals the duration of both 

 processes must be considered in judging photosensitivity. 



The Bunsen-Roscoe law furnishes us with an important quan- 

 titative means in testing the photosensitivity of animals. By 

 extending this type of experimentation to a large number of 

 animals, it will be possible to gain important insight into the 

 nervous mechanism of these forms. The intensity-time prod- 

 ucts of three such animals have already been recorded : Ciona 

 intestinalis (Hecht, '18), Mya arenaria (Hecht, '19 a, b, '20), 

 and Rana clamitans. It is hoped that this work will be further 

 extended. 



