PHOTIC REACTIONS OF TADPOLES 245 



varies very little with the rise and fall of temperature. Not- 

 withstanding this fact, after some preliminary work, the tempera- 

 ture during the process of experimentation was regulated to 

 approximately 19°C. 



With a constant intensity of 10 candle-meters, the reaction- 

 time was considerably reduced. From the general course of 

 the readings it became evident that the stronger light was more 

 effective, for the animals responded to it with a greater degree of 

 regularity. The reaction-time, which was 24.1 seconds at 5 

 candle-meters, being in each case the average of about 90 read- 

 ings, was now reduced to 12.1 seconds. This definite relation be- 

 tween the strength of stimulus and the effect produced was re- 

 tained with an intensity of 30 candle-meters. The average 

 reaction-time with this intensity for ninety-three readings was 

 6.1 seconds. The actual values obtained with these three in- 

 tensities are given in table 1 with their average reaction-times, 

 standard deviation, and probable error. Firom an inspection 

 of the table, it is evident that a stronger stimulus requires less 

 time to produce the same effect, namely, a simple swimming 

 movement, than a weaker one. 



In addition to the values obtained with 5, 10, and 30 candle- 

 meters other intensities were used. These were 0.3, 0.7, 1.2, 

 15, and 20 candle-meters and with each intensity about 100 

 readings were secured. In all the experiments in which the 

 relation of the reaction-time to the intensity was studied, this 

 relation between stimulus and effect produced was retained. 

 The weaker the light, the longer was the period required to pro- 

 duce a reaction. With the higher intensities more uniformity 

 of responses was obtained, showing that the energy is more 

 effective as a stimulus, judging from the gradual decrease in the 

 standard deviation and probable error (table 1). The probable 

 errors with the more effective intensities are small. In order to 

 determine whether the differences between the average reaction- 

 times for the various intensities are of .sufficient magnitude for 

 statistical value, the probable errors of these differences were 

 computed on the basis that the probable error of the difference 



