164 The Animal Mind 



light (196). The nature of the " dermal light sensation " 

 remains a mystery. It can hardly, in frogs, be a painful 

 irritation, since it produces a positive response; and it is 

 not due to heat rays, for it occurs when these are inter- 

 cepted by passing the light through water. As Parker 

 says, radiant heat and light, "distinct as they seem to our 

 senses, are members of one physical series in that they are 

 both ether vibrations, varying only in wave length" (538). 

 While, then, the nerve endings in human skin are sensi- 

 tive only to the slower of these vibrations, the heat rays, 

 those in the skin of the frog may respond to the whole series, 

 with what accompanying sensation qualities we cannot 

 say. It is interesting to note that Pearse (566), working 

 with frogs and salamanders, normal and blinded, finds that 

 red light, which stands nearest to heat in vibration fre- 

 quency, is most effective for the blinded animals, blue light 

 for the normal ones. In the young of frogs and salaman- 

 ders it has been shown that the skin nerves are the source 

 of dermal reactions to light. 



The frog's eye is sensitive to light rays from all the 

 spectral regions visible to man ; the distribution of bright- 

 nesses in the spectrum is like that of normal human vision, 

 and the dark-adapted eye shows a shift of the brightness 

 values to correspond with those of the dark-adapted hu- 

 man eye. One method by which these results were ob- 

 tained was that of testing the electric effect (action cur- 

 rents) of stimulating frogs' eyes with light of different colors : 

 the maximum effect for the light-adapted eye was in the 

 yellow green, that for the dark-adapted eye was in the yel- 

 low (324 a). Another method was to illuminate food with 

 light of different colors and to observe in what lights it 

 was most readily seized. From the results Hess (305) 

 concludes that amphibian vision is qualitatively like that 



