252 VASIL OBRESHKOVE 



V. PHOTOSENSITIVITY OF THE SKIN AS COMPARED WITH THAT 



OF THE EYE 



1. The skin 



In terms of the Bunsen-Roscoe law, which was demonstrated 

 as being valid in the photosensitivity of R. clamitans tadpoles, 

 it is easy to see that the sensitivity of one animal as compared 

 with another, or of two types of photoreceptors in the same ani- 

 mal, is dependent upon the rapidity with which changes of 

 sufficient strength are produced in the sense organs. Photo- 

 receptors, therefore, are comparable to photographic plates, 

 whose relative sensitivity is determined by the intensity of light 

 and time of exposure necessary to produce the same degree of 

 change on the various plates. In this the Bunsen-Roscoe law 

 finds its first applicability in making possible the classification of 

 degrees of sensitivity in terms of their corresponding intensity- 

 reaction-time products. 



When tadpoles with eye and skin intact were subjected to 

 light, it was at first supposed that the responses were those due 

 to stimuli received through the eye. The next problem that 

 naturally suggested itself was to determine the sensitivity of 

 the skin as compared with that of the eye. 



In reviewing the extensive literature on the responses of 

 amphibians to light, it is found to be the general opinion that 

 responses may be brought about by stimulating either the eye or 

 the skin. Among some of the workers advocating this view are 

 Graber ('84), Dubois ('90), Parker ('03), Reese ('06), Pearse 

 ('10), Laurens ('14), and more recently Cole and Dean ('17). 

 All of this work has been purely qualitative and in no case has 

 an attempt been made to determine quantitatively the amount 

 of energy necessary to stimulate, 1) the skin alone, 2) the eye 

 alone, and, 3) both eye and skin. The only exception to this 

 is found in the observations of Reese and of Pearse, that when 

 the eyes of Necturus were removed, a greater period elapsed 

 before orientation than normally. The short reaction-time 

 that occurs when the eyes are present is ascribed by these workers 

 to stimulations received through those organs. 



