270 VASIL OBRESHKOVE 



illustrates the velocity of the process of recovery and the nature 

 of the chemical changes in the receptors. In the beginning the 

 process moves with extreme rapidity. The velocity of change 

 in the sense organs then gradually decreases in rate, as seen in 

 the diminution of the slope in the curve, and finally the ciirve in 

 its lower course is almost parallel to the abscissa, thus indi- 

 cating that the process of photosensory recovery has reached a 

 state of equilibrium. 



From the nature of the experimental results obtained, it is 

 evident that the reaction-time at any moment in the course of 

 recovery is an expression of the amount of substance present in 

 the system at that moment. The relation is inverse, for the 

 greater the amount the shorter is the period required to produce 

 a definite change. 



VIII. THE PROCESS OF FATIGUE 



When tadpoles are subjected to continuous illumination for 

 a considerable time they become no longer sensitive to light. 

 The course of the process of fatigue was studied, the method 

 employed being similar to that used in studying the process of 

 photosensory recovery. The individuals to be tested were 

 placed in a porcelain dish with the source of illumination above 

 them; whereupon their photosensitivity was tested. Only those 

 tadpoles were used which had been kept previously in the dark 

 and which showed normal behavior. Usually two individuals 

 at a time were placed in the vessel. In such cases only the 

 reaction-time of the individual to respond first was recorded. A 

 light intensity of 10 candle-meters was employed. The individ- 

 uals were exposed for ten minutes to continuous illumination, 

 and at the end of that period, after one minute of darkness, the 

 light was again applied and the reaction-time of the first individual 

 to move was recorded. After another ten minutes of continuous 

 illumination, the process was repeated. Only one reading was 

 taken after the application of the stimulus. Now and then in 

 the course of the experiments irregularities occurred in the 

 midst of readings which otherwise showed uniformity of re- 

 sponse. To check the significance of an occasional deviation 

 another record was taken immediately after the diverse reading, 



