PHOTIC REACTIONS OF TADPOLES 



249 



From the apparent nature of the process during illumination, 

 the reaction-time in the table represents a duration of action 

 and the intensity represents the amount of energy which con- 

 trols the velocity of change in the receptors concerned in the 

 responses to light. In order that a swimming movement be 

 produced, it appears that a definite material change must be 

 produced in the sense organs as a stimulus. That the products 

 of the intensity of light and the time of action agree well with 

 each other for all intensities, can be little questioned from the 

 numerical values entered in the third column. It will be noticed, 

 however, that with the 20- and 30-candle-meter intensities, this 



TABLE 2 



Summary of results of the average reaction-times in seconds of tadpoles of Rana 

 claynitans to lights of eight intensities and the corresponding intensity 

 'Kreaction-time products 



product is somewhat large. As a matter of fact, this change in 

 the intensity-reaction-time product is not abrupt, but follows 

 a definite course. It is characterized by a gradual increase with 

 the increase in the degree of stimulation. These products with 

 the first five intensities are well within the range of the Bunsen- 

 Roscoe law. The deviation, which becomes so pronounced 

 after an intensity of illumination of 20 candle-meters is reached, 

 occurs in other experiments in similar manner, and will be dis- 

 cussed later in more detail. 



When we proceed to examine the results obtained and to plot 

 graphically the values of the intensities on the abscissa and the 

 values of the corresponding reaction-times on the ordinates, a 



