236 VASIL OBRESHKOVE 



various intensities of illumination, the time required was in- 

 versely proportional to the intensity of the light. The inten- 

 sity of light and the time of action varied in such a manner in 

 the production of the same effect that the product of these two 

 variables was always constant. This can be expressed: 



I XT =K, 



where I stands for the intensity of light, T the time of action, 

 and K the constant effect produced. 



Much has been written regarding the responses of animals 

 to light, and it is now beheved that light acts as a stimulus to 

 the photoreceptors through a change in a chemical substance 

 with which the nerve endings are in immediate contact. Parker 

 and his students have described a number of fishes and amphib- 

 bians possessing photoreceptors in the skin. This work has been 

 recently extended to animals belonging to other groups. It 

 seems advisable, therefore, to undertake a study with the aim 

 of determining (1) to what extent the Bunsen-Roscoe law is 

 appUcable to animals which respond to light; (2) the dynamic 

 nature of the responses, and, (3) the extent to which chemical 

 changes produced in receptors are analogous to those in general 

 chemistr3^ 



In sensory reflexes, however, the apparent effect produced 

 — let us say as a change in a specific peripheral receptor — is pre- 

 ceded by several other steps, namely: 1) a diffusion of the sub- 

 stance newly formed in the receptor to the specific nerve endings; 

 2) transmission of the impulse from the nerve endings to the 

 adjuster and thus to the muscles, and, 3) contraction of the 

 muscles. When in the test of the validity of the transference 

 of energy, these intervening steps occupy a relatively long period, 

 they must be separated from the energy-receiving period, 

 analyzed, and appropriate values given to them. 



We have reason to believe, however, that a species whose 

 responses are characterized by rapid changes in the receptors 

 after stimulation and a very short secondary period is an ani- 

 mal better adapted to receive and respond to a specific stimulus 

 than one in which these changes are slow. When the specific 



