COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 401 



orient them with the hand. Care had to be taken to avoid a one- 

 sided turning due to circus compensation or thigmotactic stimuli 

 received by this act, and in order to guard against such results the 

 animal was left sitting screened from the lights for a period of 15 

 seconds to 80 seconds after it had been placed in orientation ; if 

 it had not moved by the end of that time the screens were removed, 

 exposing it to the lights from the two fields. 



Rana clamata was found to be much slower in its reactions than 

 Acris. It was usually given 5 minutes in which to react, and if at 

 the end of that period it had not moved, it was induced to do so by 

 means of an electrical stimulus applied to its lower side. This was 

 accomplished by having the board on which the fi"og was placed 

 crossed by fine parellel copper wires 1 cm. apart, alternate wires being 

 connected respectively to the opposite poles of a single " Columbia 

 Dry Cell No. 6." By means of a simple " key " placed in the circuit 

 the frog sitting on the wires was stimulated by the " make " and 

 " break " of the current whenever the key was pressed down and re- 

 leased. This method was used rather than simply touching the ani- 

 mal from behind, as it was believed that there was less chance of the 

 stimulus being one-sided, and because no movement of the operator 

 was necessary in the field of vision of the animal. On the whole it 

 was found to work very satisfactorily. 



The results need not be given in detail, but may be stated briefly 

 as follows : 



1. As stated by Torelle (: 03), Baiia clamata is positively photo- 

 tropic in nearly all cases at the ordinary temperature of the room 

 (about 20° C). 



2. Such positive individuals, when exposed to the two light areas 

 of different size, turned in the great majority of cases toward the 

 larger light. This result agrees with that obtained with Acris, but as 

 would be expected from an animal which responds less quickly and 

 definitely to a single light, the proportion of the turnings to the large 

 light was not so large. 



3. At low temperatures (6° C. to 10° C.) the frogs were usually 

 negative in their responses to a single light, or at least were indifferent 

 or inconstant in their reactions ; and in accordance with this 



4. Such individuals turned toward the smaller of the two lights, or 

 were inconstant and gave inditferent results under these conditions as 

 with the single light. This last result was first obtained from a speci- 

 men which was brought into the experimenting room from a tank in 

 which the water registered 8° C. and was at once tested with the two 

 lights. Its first 8 reactions were all toward the small light, after which 



VOL. XLIl. — 20 



