SELIG HECHT 131 



pupil diameter. These results are plotted graphically in Fig. 6, and 

 their values are indicative of the changes which take place in the 

 fovea independent of the movements of the iris. It is to the inter- 

 pretation of these data that we shall now turn our attention. 



VII. 



In considering the action of light in vision, it is hardly an assumption 

 to suppose that the first effect of the light is the photochemical decom- 

 position of a sensitive substance. Our measurements represent this 

 effect in terms of the minimum intensity of illumination required 

 to produce a visual sensation at different times in the dark. What 

 is the relation between the intensity of the stimulating light and 

 its objective photochemical effect? In the analysis of peripheral 

 adaptation (Hecht, 1919-20, b) I assumed that the photochemical 

 effect is proportional to the logarithm of the intensity. The reasons 

 for this assmnption were first, that such a relationship had been found 

 in the clam, My a; and second, that the gamut of change undergone 

 during peripheral adaptation was so enormous that a logarithmic 

 relation is the only one that would bring the data into a form amenable 

 to ordinary comprehension and analysis. These reasons are not 

 imperative. They derive their final justification on the pragmatic 

 ground that the assumption of a logarithmic relation results in the 

 formulation of a simple chemical picture which accounts for the data 

 satisfactorily. 



The range of dark adaptation in the fovea is not a tithe of the 

 extent encountered in the periphery. The older data indicate a change 

 from 10,000 to 1 in the minimum intensity necessary for a peripheral 

 effect. Cobb's (1919) more recent experiments, as well as my own 

 unpublished ones, indicate even a greater change than this. The 

 older measurements were faulty in a number of ways, particularly in 

 their neglect of the first few seconds of adaptation. Compared to 

 these changes, those presented here for the fovea are really small in 

 extent. When corrected for constant pupil diameter, the necessary 

 minimum for a response after 2 seconds of foveal adaptation is about 

 70 X 10"'* ml. The extrapolated value at zero becomes, after pupil 

 correction, 103 X lO""* ml. After 20 minutes the intensity as given in 

 Fig. 6 is about 6X10"* ml. This range of change represents noth- 



