THE DARK ADAPTATION OF THE HUMAN EYE. 



By SELIG HECHT. 



{From the Physiological Laboratory, College of Medicine, Creighton University, 



Omaha.) 



(Received for publication, February 10, 1920.) 



The phenomenon of retinal adaptation is one of the most familiar 

 facts of sensory physiology. Not only is the mere adaptability of the 

 human eye well known, but, since Aubert's (1865) first measurements, 

 there has grown up a body of quantitative data describing the course 

 of adaptation (Nagel, 1911). This is especially true of the adaptation 

 of the eye to dim lights. 



All the data on dark adaptation show that on entering a dark room 

 after a stay in the outside daylight the eye at once begins to increase 

 in sensitivity. At first this increase appears to be slow; but after 5 

 minutes the increase is quite rapid, the eye acquiring a sensitivity 

 several hundred times its initial value. After 30 minutes sojourn in 

 the dark the sensitivity still increases, but more slowly than before; 

 and after 45 minutes or an hour the maximum sensitivity is reached. 

 The final sensitivity varies slightly with different people, but in the 

 fully adapted condition the eye is easily 5,000 or 10,000 times more 

 sensitive than it was at the beginning. 



In Fig. 1 is given the record of a dark adaptation experiment made 

 by Piper (1903). The results are representative of the numerous, 

 published experiments. Piper points out that, although earlier 

 workers {e.g. Aubert, 1865) believed the rate of adaptation to be great- 

 est at first, the greatest increase in sensitivity really occurs in the 

 middle of the course of adaptation. The curve of sensitivity according 

 to Piper shows three parts: an initial slow phase, an intermediate 

 rapid one, and a final phase ending in a maximum. 



In spite of our familiarity with the phenomena of dark adaptation, 

 and the trustworthiness of the measurements describing them, their 



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