202 PROCEEDINGS OF THE AMERICAN ACADEMY. 



laterally, so that there is probably no overlapping in the fields. Nor is 

 any definite image formed, as Beer ('98) has shown that the eye cannot 

 be accommodated to any extent, and amphibians therefore depend 

 upon motion rather than the form of objects to warn them of danger or 

 to enable them to capture food. A frog or toad will allow a worm to 

 lie in full view as long as it is quiet, but as soon as the worm moves 

 it is devoured. The vision of amphibians is therefore limited to rather 

 ill-defined outlines of the surrounding objects, and the comparative 

 brightness or dulness, or possibly the colors, of objects will have con- 

 siderable importance in determining the nature of the responses of an 

 individual. The reactions brought about when the eyes alone are 

 illuminated are similar to those which take place when such stimula- 

 tion affects both the skin and eyes. When only one eye is stimulated 

 by light coming from in front of a toad, the individual usually does 

 not go toward the light but turns toward the stimulated side. These 

 facts indicate that the eyes in their relations to objects in the field of 

 vision serve more as direction eyes than as camera eyes. Cole has 

 recently given additional support to this view by showing that am- 

 phibians placed between two lights of equal intensity but of different 

 areas go toward the larger area ; thus demonstrating that the size of 

 the area illuminated is of importance in the visual processes. Kiihne 

 ('78°) has shown that the eye of the frog is sensitive to light rays from 

 the whole range of the visible spectrum, and the results described in 

 the present paper, as well as those of other observers (p. 165), indicate 

 that the rays toward the violet end are most effective in producing 

 photic responses. These apparent differences in sensitiveness to what 

 appear to the human eye as colors may, however, be only differences in 

 intensity when received by the frog's eye. 



The skin is known to act as a photoreceptor in ten representative 

 species of amphibians, and individuals show tropic reactions which are 

 like those of animals in normal condition after their eyes have been 

 excised. There is no great differentiation shown in the structure of 

 the nerve endings in amphibians' skins, and Parker (:03 b , p. 34) has al- 

 ready been quoted as saying, " it is conceivable that in the lower verte- 

 brates, like the frog, the end organs of the skin are stimulated by 

 radiant energy of wide range, including what is for us both heat and 

 light." There seems to be no doubt, however, that the amphibian 

 skin is sensitive to light as such, and no tropic responses are induced 

 by radiant heat having the same energy value as the light which does 

 induce marked tropic reactions. Our knowledge of the comparative 

 sensitiveness of the skin in different regions of the body is rather 

 limited, but it shows that there is no uniformity among different am- 



