COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 897 



inconstant and not accurately determinable factor precludes making 

 direct quantitative comparison with the results obtained upon Vanessa 

 or other forms. If such a comparison were to be attempted, it would 

 probably be justifiable to leave out of account entirely the results on 

 the three specimens of Acris which appeared to be negative, since 

 Vanessa appears to be always positive, and the comparison would then 

 be between animals which were in similar constant phototropic states. 

 If such were done, the proportion of turnings to the larger light would 

 be greater for Acris (as based on Series D) than for Vanessa, and in so 

 far might perhaps be interpreted as indicating the formation of more 

 distinct and better images of the lights on the retinae of the former 

 than the latter. 



Parker (:03*) proved that li. ininens. is as a rule positively photo- 

 tropic to stimuli received by the skin, as well as to those received by the 

 eyes. The idea suggested itself of using Parker's methods and testing 

 in Acris the effect of the large and small lights when falling (a) upon 

 the eyes alone, the skin being protected from the light, and (b) upon the 

 skin after the optic nerves had been cut. If the persistent turning of 

 the normal frog toward the large area of light was to be attributed to 

 the ability of the eyes to form images of the lights, it would be expected 

 that frogs with the skin protected from the light would still react as 

 before, so long as vision with the eyes was unimpaired. On the other 

 hand, an eyeless frog — or what amounts to the same thing, one in 

 ^v•hich the optic nerves had been severed — might be compared directly 

 with an earthworm, so far as its light-perceiving abilities are concerned. 

 It has only the general integument for the reception of photic stimuli, 

 and while, as in the earthworm, this is undoubtedly sensitive to 

 differences of intensity, there is no more reason to suppose that it 

 would enable the animal to discriminate between different areas of 

 like intensity but unequal size than in the case of the worm. 



In making the test with the eyes exposed and the skin covered the 

 method employed by Parker was followed in detail. The skin was 

 removed in one piece from a large, dark-colored individual, turned 

 inside out, and drawn over a slightly smaller specimen which was 

 known to be active and strongly positive in its reaction to light. In 

 addition to the eyes, the snout and fore and hind feet were exposed to 

 the light. Only a single individual was used for this test, but the 

 nature of its responses was so decided that further experiments did not 

 seem necessary. When exposed as the normal frogs had been, to the 

 influence of the two lights, it turned toward the larger in 11 out of 14 

 trials, about as large a proportion as was obtained with normal animals. 

 The frog was much slower in its reactions than it had been previous to 



