SELIG HECHT 137 



chemical effect. For the fovea (cones), however, a similar inter- 

 pretation depends on a linear relation between intensity and photo- 

 chemical effect. What constitutes the basis of this difference between 

 rod and cone will be a nice point if it is ever established. I have 

 already suggested (Hecht, 1919-20, b) that the logarithmic relation may 

 hinge on some absorption phenomenon associated with the sensory 

 mechanism. However, a detailed treatment of this and other possi- 

 bilities is not appropriate at this juncture, and will be reserved for a 

 later occasion. 



The general idea which I have suggested as underlying all these 

 phenomena of photic sensitivity has an immediate use. In the case of 

 Mya it has served as a means of suggesting fresh and crucial 

 experiments that have already contributed materially to its support. 

 In venturing to suggest a similar mechanism for the eye, I hope that 

 it may help to point the direction in which further work may be done. 

 It would seem that by such a method progress may be made in the 

 rather chaotic field of visual physiology. A concrete picture, though 

 it limits itself to the initial events in vision, should serve better than 

 the vague, sweeping theories of which there are already far too many. 



SUMMARY. 



1 . After a discussion of the sources of error involved in the study of 

 dark adaptation, an apparatus and a procedure are described which 

 avoid these errors. The method includes a control of the initial light 

 adaptation, a record of the exact beginning of dark adaptation, and an 

 accurate means of measuring the threshold of the fovea after different 

 intervals in the dark. 



2. The results show that dark adaptation of the eye as measured by 

 foveal vision proceeds at a very precipitous rate during the first few 

 seconds, that most of the adaptation takes place during the first 30 

 seconds, and that the process practically ceases after 10 minutes. 

 These findings explain much of the irregularity of the older data. 



3. The changes which correspond to those in the fovea alone are 

 secured by correcting the above results in terms of the movements of 

 the pupil during dark adaptation. 



4. On the assumption that the photochemical effect of the light is 

 a linear function of the intensity, it is shown that the chirk adaptation 



