128 THE NATURE OF FOVEAL DARK ADAPTATION 



Moreover, the experiments which I have just reviewed, though 

 differing in accuracy from mine, are indeed roughly comparable to 

 them when properly evaluated, as I have pointed out above. They 

 show, for example, that foveal adaptation practically ceases after 10 

 minutes in the dark. In addition, if the value of the Hght-adapted 

 eye as a standard be tripled or quadrupled, as it must at least be 

 because of the initial speed of adaptation, the final degree of dark 

 adaptation represents about 1 per cent of the condition of the light- 

 adapted eye. This is practically the same value which is found in 

 the present experiments. CalUng the initial threshold 454 X 10~* ml., 

 the final threshold of 3.4 X 10"^ ml. represents a drop to 0.8 per cent. 

 Therefore the results with red light show the same course of adaptation 

 as experiments with any other light. A careful analysis of our present 

 data will thus be of significance for the general problem of adaptation 

 and foveal vision. 



VI. 



The data as they stand include the results of two phenomena: the 

 movement of the iris, and the change in the fovea. In order to isolate 

 the effects due to the foveal changes alone, it will be necessary to 

 correct the data for variation in the pupil size. If the pupil area 

 did not vary, the energy received by the retina would depend on the 

 intensity of the illumination. On the other hand, if the intensity 

 were to remain constant, the energy transmitted to the retina would 

 be directly proportional to the var3dng area of the pupil. It then 

 follows that since both intensity and pupil area vary, the energy 

 received by the retina is proportional to the product of the intensity 

 and the area. 



We have measured the intensities; it is therefore necessary to 

 introduce the data for the area of the pupil. These can be supplied 

 from the experiments made by Reeves (1918). The experimental 

 situations in the two cases are comparable. Reeves determined the 

 changes in the pupillary diameter in darkness following adaptation to 

 artificial illumination of about 100 ml. In my experiments the fight 

 adaptation field had a brightness of 90 ml. These illuminations 

 result in practically the maximum effect in constricting the pupil. 



