112 PHYSIOLOGY: S. HECHT Proc. N. A. S. 
HUMAN RETINAL ADAPTATION 
By SkIvIG Hecht 
Physioi^ogical Laboratory, CohhHcn of Medicine, Creighton University, Omaha 
Communicated by J. Loeb, January 2, 1920 
Although the magnitude and the course of the dark adaptation of the 
human eye are well known, no fundamental interpretation has been made 
of the results. The data are simple. In the dark the eye becomes in- 
creasingly sensitive to light as the time goes on, until after about an hour 
it reaches a maximum of irritability. The measure of this changing sen- 
sitivity is given in terms of the least intensity which is just perceptible 
to the eye. The data which have been accumulated in this way are plenti- 
ful and accurate.^ 
The lack of theoretical significance of these data is caused by two con- 
ditions. The first is that the term sensitivity, even as measured in units 
of intensity, has no ultimate material basis. The fundamental question 
is: what objective changes in the makeup of the retina determine the 
variations in its sensitivity? This cannot at present be answered from 
experiments on the eye. However, even if this situation were satisfactorily 
elucidated, a second limitation bars the way to a final interpretation. 
This second condition hinges on the photochemical effect of the light. 
Since the data of retinal dark adaptation are given in terms of intensity, it 
is necessary to have some quantitative estimate of the photol3rtic action 
of light at the various intensities used for stimulation. This condition 
again cannot be fulfilled from the reported experiments with the eye. 
Indeed, in the present state of our knowledge it is difficult to devise 
experiments for the purpose of investigating these two conditions. 
Fortunately the physicochemical analysis of the photoreception of lower 
forms has reached the point where these questions may be answered 
with considerable accuracy and a fair degree of confidence.^ With regard 
to the first limitation of retinal interpretation, the experiments with Mya 
and Ciona are unequivocal. A photosensitive substance 5 is decomposed 
by the light into its two precursors P and A. This reaction is reversible, 
the regeneration of photosensitive material 5 from its two precursors P 
and A following the course of a bimolecular reaction. Thus the equation 
of the reaction is 
light 
5:::=^:p + A. 
"dark" 
The important point in this mechanism is that the sensitivity of the 
sense organ at any moment is dependent on the concentration of pre- 
cursor substances P and A . Quantitatively expressed this means that the 
amount of fresh precursors that must be formed by the light before a 
