244 VASIL OBRESHKOVE 



If animals that were to be repeatedly stimulated were allowed 

 one minute of rest in the dark before stimulation was renewed, 

 it was found possible to secure from fifteen to twenty readings 

 from the same individual without any evidence of exhaustion. 

 Now and then in the course of the experiment inconsistencies 

 occurred. These were of the type of premature response or 

 delayed response in the midst of a series of readings otherwise 

 characterized by marked regularity. These irregular responses 

 were recorded, however, and it will be evident from an inspec- 

 tion of the results that they were not numerous enough to throw 

 doubt on the numerical values obtained. An insight into the 

 cause of these occasional irregularities is difficult to obtain. 

 Premature responses were associated at times with discharges 

 from the intestine. 



IV. DYNAMICS OF SENSITIVITY TO LIGHT 



When tadpoles of Rana clamitans are subjected to a stimulus 

 of 5 candle-meters, the light being applied from above, the period 

 which elapsed between the time of the application of the stimulus 

 and that of the response had approximately a fixed value. Re- 

 peated stimulations at one-minute intervals revealed only 

 slight changes in their period. These slight deviations from the 

 mean of the reaction-time for this and other intensities were 

 only such as were to be expected from normal law of error, as 

 was demonstrated by a study of frequency curves. The reac- 

 tion time for a set of ninety-eight readings at 5 candle-meters 

 showed an average of 24.1 seconds. 



Since not all of the readings were taken during the same day, 

 and since this part of the work was done during the winter 

 months, the variations in temperatures were recorded. Obser- 

 vations based on approximately 2,000 readings with various 

 intensities of light showed no evidence that a temperature varia- 

 tion between 16°C. and 19°C. has an observable effect on the 

 reaction-time. This is not surprising in view of the fact that the 

 velocity of reaction in photic operations is dependent upon the 

 actual amount of light absorbed per unit of time and this amount 



