516 DARK ADAPTATION OF EYE 



sensation of light without color. The minimum stimulating energy 

 at different wave lengths has been determined. Similarly the mini- 

 mum energy necessary to produce a bleaching effect on visual 

 purple has been measured. And finally the light absorbed by the 

 pigment at different wave lengths has also been described. When 

 put into graphic form, the curves of these three measurements all 

 follow the same course (Henri and des Bancels, 1911). This is a 

 powerful argument for the participation of visual purple in the 

 process of photosensory reception. 



The reversible character of the chemical behavior of visual purple 

 is well known (Kiihne, 1879). The bleached pigment in the retina, 

 and even in vitro under certain conditions, regenerates its color when 

 placed in the dark. It may therefore be quite possible that the 

 photosensitive substance 5 which our analysis requires is really visual 

 purple. If this is true, it follows that visual purple when bleached by 

 light breaks down into two substances. An investigation of the 

 dynamics of the regeneration of visual purple will furnish deciding 

 evidence for the identification of visual purple with the hypothetical 

 substance S. It is not necessary to suppose that the decomposition 

 and the synthesis represent elaborate changes. A process of reduc- 

 tion, or oxidation, or perhaps of hydrolysis may accomplish all that 

 is required in the way of chemical changes. In fact, considering the 

 extremely small quantities of energy necessary to produce a visual 

 effect, it must follow that the initial photochemical transformation 

 is not only delicate but very simple as well. 



The significant point of all this is that the analysis of dark adapta- 

 tion with which this paper has been concerned is consistent with what 

 we know of the changes in the eye. If further experiments will show 

 the identity of visual purple with the hypothetical photosensitive 

 substance, it will be a distinct advance in our knowledge of the basis 

 of visual reception. 



SUMMARY. 



During the dark adaptation of the human eye, its visual threshold 

 decreases to a small fraction of its original value in the light. An 

 analysis of the quantitative data describing this adaptation shows 

 that it follows the course of a bimolecular chemical reaction. On the 



