PHOTOKINETIC REACTIONS OF FROG TADPOLES 363 



movements, was used for the periods of 'darkness.' With the 

 aid of a dark box and suitable black cloth screens all other light 

 was prevented from entering the jar. 



In order that the animals might become adapted to the new 

 conditions they were placed in the jar singly at least an hour 

 previous to experimentation. The temperatures of the water 

 in the aquaria and in the jar were the same. The animals were 

 observed for periods of fifteen minutes each, in 'darkness,' fol- 

 lowed by illumination from one of three intensities of light, except 

 in the last few trials where this order was reversed. A trial, 

 therefore, consisted in observing an animal under two or m.ore 

 conditions of illumination. The time that the animal was in 

 motion was recorded and the per cent of time in activity deter- 

 mined. The per cent of photokinesis, then, is the difference be- 

 tween the per cents of activity of the same animal in light and in 

 'darkness.' An observation period of fifteen minutes was plenty 

 long enough, since a response to photic stimulation was always 

 noticed within at least four minutes. During the first series of 

 trials normal animals were used; in the second series, animals 

 adapted to darkness; and in the third, animals adapted to light. 

 The dark adapted animals had been inclosed in a light-proof 

 box for periods varying from two to thirty-two days, while the 

 light adapted animals had been subjected to the illumination of 

 a 32 candle power light for periods varying from 78 to 148' 

 hours. During all the experiments, care was taken to prevent 

 any mechanical stimulation. 



OBSERVATIONS 



1. Photokinesis. The majority of animals were found to be 

 decidedly photokinetic. Although the per cent of photokinesis 

 is small the activity observed in strong light was distinctly dif- 

 ferent from that observed in 'darkness.' The movements in 

 'darkness' were slow, vshort, indefinite as to direction, and many 

 times were only tail movements; as compared with those when 

 the animal was subjected to a beam of light. In the latter 

 conditions the swimming was rapid, of longer duration, and in 

 many directions, apparently caused by the presence of light. 



