SELIG HECHT 505 



attend the sensory reception of lights of low intensity. At present we 

 know very little of the basic reactions, chemical and physical, that 

 underlie retinal stimulation. One thing, however, may be stated 

 with considerable confidence. This is that the initial effect of the 

 light on the retina consists of the photochemical alteration of some 

 photosensitive substance. Each test of the visual threshold involves 

 a primary photochemical effect. The data of dark adaptation, if 

 properly treated, should give us information about this photochemical 

 change. 



Objectively stated, the facts are that as adaptation proceeds, less 

 and less light energy is necessary to produce the initial photochemical 

 effect required for a visual response. In other words, progressively 

 less and less of the photosensitive substance must be decomposed in 

 order to initiate the subsequent processes concerned in photo- 

 reception. 



Granting this qualitative decrease, what are its quantitative im- 

 plications? We have the minimum intensities from the data. What 

 is the relation between the intensity of the stimulating light and its 

 objective effect in the photochemical decomposition of the sensitive 

 material of the retina? An exact relation undoubtedly exists be- 

 tween the two. Its quantitative expression must be known if the 

 adaptive changes in the retinal mechanism are to be stated objec- 

 tively. Unfortunately this condition cannot be fulfilled from any 

 experiments on the retina. We have here, then, the first of the 

 inherent difficulties in our knowledge that prevent the proper funda- 

 mental interpretation of retinal adaptation. 



This, however, is not the only obstacle. Let us tentatively assume 

 that the relation between intensity and photochemical effect is 

 known quantitatively. In this way we will know the exact quanti- 

 ties of photosensitive substance required to be decomposed in order 

 to initiate a minimal retinal effect. This follows from the fact that 

 the photochemical effect E is some function of the intensity I 



even though the exact nature of the function is not stated. 



During adaptation in darkness the amount to be decomposed 

 changes regularly. What determines the magnitude of the quantity 



