268 VASIL OBRESHKOVE 



Hecht ('18, '19 a, b) observed no such deviation as described 

 above. It is to be noticed, however, that the ratio of the range 

 intensities employed by him is about 1:20, while the ratio of 

 range of illuminations employed in the present study covers a 

 field of Ught intensities 1:500. With a range of intensities 1:20 

 my results are in accord with those obtained bj^ Hecht. 



VII. PHOTOSENSORY RECOVERY 



After animals which have been kept in the dark are exposed 

 to a continuous illumination of 10 candle-meters for an hour, 

 they show no sensitiveness to light. In terms of the changes 

 taking place in the photoreceptors, this may be said to be due to 

 a reduction of the photochemical substance to a minimum, and 

 the changes produced in the peripheral sense organs thereafter 

 to be of insufficient rate to act effectively as stimuli. As a 

 starting point, this enables us to study the nature of photosensory 

 recovery. 



The following method was employed in studying recovery. 

 The animal to be tested was placed in a white porcelain dish and 

 with an overhead light its sensitivity was determined. If the 

 animal responded with regularity, it was exposed to a continuous 

 illumination of 10-candle-meter intensity till no longer sensitive 

 to the light stimulus. The time necessary for this process varied 

 with different individuals from forty to sixty minutes, but for 

 uniformity of procedure it was decided to use one hour of con- 

 tinuous illumination. At the end of that period the animal 

 was tested several times, and if no response was noted within two 

 minutes from the time of the application of the stimulus, the 

 animal was left in the dark ten minutes and its sensitivity again 

 tested. After ten more minutes in the dark its reaction-time 

 was again tried. Four such experiments were made with four 

 animals, the summary results of which are given in table 7. 



From an inspection of the table it can be seen that in the 

 course of photosensory recovery, the reaction-time at first is 

 long, then gradually decreases, and finally becomes a constant 

 quantity. After this point, repeated stimulations at ten- 

 minute intervals in the dark revealed no very marked increase 



