SELTG HECHT 515 



It is not my purpose to show how many of the properties of visual 

 reception may be explained in terms of the reversible photochemical 

 reaction postulated for the initial effect in photosensory reception. 

 Outside of dark adaptation, this is possible only in a qualitative way, 

 because much of the needed data does not exist in quantitative form. 

 It is clear, however, that the consequences of the reversible reaction 

 may be calculated and predicted, and experiments devised to test the 

 possibilities. Such quantitative results will be forthcoming in the 

 future, and their analysis will be reserved for that time. 



One caution must be mentioned. The provisions of the Duplicity 

 Theory make a clear distinction between vision in dim light and 

 vision in bright light (Nagel, 1911). Dark adaptation is essentially a 

 phenomenon of dim vision. Therefore all our conclusions must be 

 limited to the mechanism of vision at low intensities only. The 

 properties of photoreception at high intensities, involving as it does 

 color vision, cannot be considered at the present time. It will be 

 remembered that defects in color vision cannot be correlated with any 

 changes in the course or the quantity of dark adaptation (Piper, 1903, 

 p. 191). Indeed the results given graphically in the present paper 

 represent not only normal Trichromats, but anomalous Trichromats 

 as well. 



The fact that our analysis applies only to vision in dim light is of 

 considerable advantage in one respect. This concerns the final 

 meaning of the terms in the equation 



light 



S :^ P + A 



"dark" 



for the photochemical reaction of photoreception. With vision in 

 dim light there has been associated the existence of visual purple. 

 The evidence for the connection of visual purple with vision is quite 

 striking (Trendelenburg, 1911; Henri and des Bancels, 1911). The 

 most significant facts in this respect are those concerned with the 

 threshold of sensitivity, the photochemical action of light on visual 

 purple, and the absorption of energy by visual purple. It is known 

 that spectral light falling on the retina, when so diminished in inten- 

 sity as to be barely visible to the dark-adapted eye, produces a 



