504 DARK ADAPTATION OF EYE 



Unfortunately all the investigators following and including Piper 

 have published their results as Empfindlichkeit, without giving the 

 minimum intensities as they found them. This has been the most 

 obvious impediment in the way of a proper treatment of their data. 

 The undue emphasis on the distorted results has detracted from the 

 regularities evidenced by the actual data themselves. It is, however, 

 simple in all cases to compute backwards and to find in this way 

 what the original facts are. By taking the reciprocal of the pubhshed 

 values of the Empfindlichkeit I have calculated the corresponding 

 minimum intensities for most of the pubhshed experiments. It is not 

 possible to state what the unit of intensity is in Piper's experiments. 

 I cannot find its definition anywhere in his article. By inference 

 from the work of other investigators I judge it to be about 4 X 10~^ 

 meter candles. The unit of sensitivity in Nagel's adaptometer is 

 definitely stated as the reciprocal of the intensity as measured in 

 meter candles. However, in order to make these data comparable to 

 Piper's, as well as to avoid the use of long decimals, I have multi- 

 plied the minimum intensity by 10^, thus making the unit of minimum 

 intensity in Nagel's data as 1 X 10~^ meter candles. 



The results as we find them now are represented by Fig. 2, which is 

 the same experiment as Fig. 1. It is not possible in a single drawing 

 to show how the intensity varies throughout the test. I have there- 

 fore redrawn the lower part of the curve in Fig. 2 using a magnified 

 scale of ordinates to show the changes which take place after the 

 first 10 minutes. It is obvious, as Aubert originally maintained, 

 that the process of adaptation begins immediately, and that the 

 minimum intensity decreases enormously during the first few min- 

 utes in the dark. 



With the data in their present form we may now proceed to deter- 

 mine what the peculiarities inherent in them are which prevent their 

 ready interpretation. 



III. 



The experiments on retinal adaptation are a series of determina- 

 tions of the visual threshold in dim light. Each test is a measurement 

 of the minimum energy for a sensory effect. The interpretation of 

 the findings must then hinge to a large extent on the phenomena that 



