PHOTIC REACTIONS OF TADPOLES 



247 



between the constants of two uncorrelated series is 



\[e 



+ El, 



where Ei and E^ are the constants of the two series to be com- 

 pared. By this means it was found that the probable error of 

 the difference between the means of the reaction-times at 5 and 

 10 candle-meters is 0.74. Since the difference between the 

 average reaction times at these two intensities is 12, which is 

 more than 16 times the probable error 0.74, and since a similar 

 condition is true for the other higher and more effective inten- 

 sities of illuminations, it is obvious that the values obtained are 

 of significance in a discussion of the photodynamic nature of 

 the responses. 



Between 0.71 candle-meter and 0.3 candle-meter-f inter- 

 mediate intensities were used which do not appear in any of the 

 tables, but it was noticed that in the lower range a point was 

 reached where the animal no longer responded to the stimulus. 

 This point in tadpoles of Rana clamitans was found to be 0.3 

 candle-meter. This with the other facts presented leads to the 

 conclusion that in this animal there is an optimum intensity to 



