PHOTIC REACTIONS OF TADPOLES 241 



observed in the dark with the aid of a constant dim light, to 

 determine the frequency of the spontaneous movements. When 

 their state of activity was such as to leave no doubt that their 

 movements were due to the applied stimulus of light, the experi- 

 mental work was begun. This usually took place when the 

 time of rest in the dark was several times longer than the time 

 required to produce a reaction after the stimulus had been 

 applied. Then any movement after the stimulus was applied 

 could not be questioned as having been the result of the effect 

 of light. 



2. Apparatus 



The apparatus for the first set of experiments (fig. 1) con- 

 sisted of a small rectangular table (a, 6), on which was placed a 

 bicycle tire (c) and then a rectangular board (n) . On this rested 

 a round white porcelain dish (v) of the size already mentioned, 

 containing a number of tadpoles of equal photosensitivity as 

 previously determined. A short distance in front of the table 

 and slightly above the level of the vessel was a dim light of 1 

 candlepower inclosed in a box (q). The front of the box was 

 made of a number of layers of thin translucent paper, which 

 allowed just sufficient light to escape for the observation of the 

 animals. The legs of the table rested upon individual mats 

 (m) each formed, from the floor up, of, 1) a rectangular rubber 

 plate 3.5 cm. in thickness; 2) a tile of the same size and thick- 

 ness; 3) a pad of excelsior, and, 4) a small square board. It 

 was believed that such an arrangement would absorb any vibra- 

 tion which might occur during the experimentation. The 

 light (s) was from above. By using a flash-light bulb of 0.5 

 candlepower and Mazda tungsten electric bulbs of 10 and 40 

 watts and by varying the distance of the bulb from the tadpole, 

 a series of intensities was secured ranging from 0.3 candle-meter 

 to 500 candle-meters. The amount of light which these bulbs 

 produced was accurately measured by means of a Lummer- 

 Brodhun photometer. 



