502 



DARK ADAPTATION OF EYE 



From Fig, 1 it might really be supposed that practically no change 

 occurs during the first 5 minutes of dark adaptation. Nothing is 

 farther from actual fact. During this interval the minimum inten- 

 sity necessary for a visual stimulus, — the visual threshold, — drops 

 to less than half its initial value. In the experiment of Fig. 1 this 



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Fig. 1. The course of dark adaptation expressed in terms of the increase in 

 sensitivity. This experiment is typical of the many which have been pubhshed. 

 The ordinates are a million times the reciprocal of the minimum intensity; the 

 abscissae the time in the dark. 



