192 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In fact, Loeb ('88) has stated as a general law, that the more primitive 

 the photoreceptor, the greater is its sensitiveness to the rays toward the 

 violet end of the spectrum, as compared to those toward the opposite 

 end. Graber ('83, p. 225) stated that in the phototropic responses of 

 Triturus the rays became more and more like darkness in their effects 

 as the red end of the spectrum was approached ; and that this was true 

 of eyeless individuals as well as those in normal condition. Dubois 

 ('90, p. 358) observed that blue was more effective than red in produc- 

 ing responses from a blinded Proteus when only the tail was illumi- 

 nated. Opposed to these observations are those of Kuhne ('78 b , 

 p. 119), who found that, while normal frogs rested in green when there 

 was equal opportunity to rest in blue, blinded individuals showed no 

 such reactions. The results described in the present paper agree with 

 those of Kuhne, and it seems to be evident that the photoreceptors in 

 the skin of the frog and toad have little or no sensitiveness to color 

 differences, as such. 



H. Comparison of the Reactions of Eyeless Toads to Heat 

 and to Light. 



It has long been known that the skin of amphibians could be stimu- 

 lated by heat, and the opinion has been expressed that there are recep- 

 tors which are open to stimulation by either heat or light. Koranyi 

 ('93) showed that heat, as well as light, might produce motor reactions 

 when it was applied to the skin of a frog. Parker (:03 b , p. 34) says : 

 " It is conceivable that in the lower vertebrates, like the frog, the end 

 organs of the skin are stimulated by radiant energy of a wide range, 

 including what is for us both radiant heat and light, and that the de- 

 scendants of these organs in the skins of higher vertebrates are more 

 restricted in function and are ordinarily sensitive to radiant heat and 

 its effects." Washburn (:08, p. 142) also says, " While, then, the nerve 

 endings in the human skin are sensitive only to the slowest of these 

 vibrations, the heat rays, those in the skin of the frog, may respond to 

 the whole series." 



During the experiments with eyeless toads the question arose as to 

 whether the supposed photic reactions might not, after all, be due to 

 the influence of heat. And, although a heat screen containing water 

 was used in all experiments, there was a possibility that the light was 

 converted into heat as it was absorbed by the skin, and that the sensi- 

 tiveness was to heat rather than light. Furthermore, the part of the 

 apparatus containing the lamp was warmed somewhat during a series 

 of experiments and gave off a small amount of heat. A crude test as 



