272 VASIL OBRESHKOVE 



in the light. From the table it is seen that the reaction-time 

 is at first normal and then gradually becomes longer with the 

 increase of continuous illumination. After thirty minutes, 

 although the individuals were still responsive, they showed a 

 reaction-time suggesting that they were nearing exhaustion. 

 With only one exception, complete fatigue occurred after forty 

 minutes of continuous illumination. From the curve in figure 

 9 it is seen that the process of fatigue is characterized at first 

 by a slow velocity of change, then by a gradual increase, then 

 by a decrease, and finally by a sudden break, where the animals 

 no longer responded. This is in marked contrast with the 

 velocity of change during photosensory recovery where the 

 rate of change in the receptors at first increases rapidly, then 

 gradually decreases, and finally becomes constant — a pro- 

 cess characteristic of monomolecular reaction. The process of 

 fatigue, however, is not a simple process progressing only in 

 one direction, but it is being opposed also by the return of a 

 certain amount of the decomposed material to its original state, 

 at a rate independent of the intensity of illumination. This 

 was shown in the study of photosensory recovery. Because of 

 this fact, therefore, the observed results furnish us with only 

 a limited quantitative criterion for an analysis of the chemical 

 process in the sense organs during illumination. 



The dependence of the velocity of change upon the active 

 masses finds close analogy in the retinal electrical changes due 

 to illumination. Dewar and McKendrick (74) showed that the 

 effect of continuous action of light upon the retina of the frog is 

 marked at first by an increase in the electrical change, then by 

 a gradual diminution, and finally by constancy. Similar phenom- 

 ena in the electrical changes of the retina have been shown by 

 Kuehne und Steiner ('80), Waller ('00), and Einthoven and 

 Jolly ('08), who demonstrated that, whether the currents are 

 positive or negative, the retina becomes less and less sensitive 

 to the light after it has been subjected to continuous illumination. 



Hecht ('18) showed that Ciona intestinalis is fatigued by 

 light when the individuals are repeatedly stimulated after one- 

 minute intervals in the dark. Rana clamitans tadpoles, re- 



