114 
PHYSIOLOGY: S. HECHT 
Proc. N. a. S. 
appearance of the decomposition product. The curve which represents 
all of the published data is the isotherm of a bimolecular reaction 
1 X 
k = 
at a 
in which k has an average value of approximately 0.016. Because of the 
refined technic used in the experiments the agreement between the indi- 
vidual observations and the theoretical curve is frequently diagrammatic 
in its precision. An example is given in figure 1. The observations are 
6.0 
so 
i 
\o 
O /O 20 JO SO 
T/me /n Dark - — ^/nute^ 
Fig. 1. Dark adaptation of the human eye. The points are single observations, 
whereas the curve is the isotherm of a bimolecular reaction. Actually the ordinates are 
the logarithms of the minimum intensities at different periods in the dark ; theoretically 
they represent the number of units of decomposition products still present in the retina. 
taken from an experiment of Nagel's made more than ten years ago. 
The meaning of the representation of the course of retinal dark adapta- 
tion by the curve of a bimolecular reaction is obvious. Two substances 
are decreasing in concentration according to the ordinary velocities of 
chemical reactions. These two substances are, therefore, combining to 
form a third substance. 
The simplest interpretation of these facts is that the substance synthe- 
sized during dark adaptation is the photosensitive substance, and that the 
two materials which form this photosensitive substance are its precursors 
as well as its decomposition products. The initial process in visual re- 
ception in dim light must, therefore, depend on a reversible photochemical 
