Vol. 6, 1920 
PHYSIOLOGY: S. HECHT 
response is elicited is always a constant fraction of the amount of pre- 
cursors already present in the sense organ. Conversely, this amount of 
fresh precursors is a direct measure of the concentration of residual pre- 
cursors. If this concentration is high, much fresh precursors are required, 
and much light energy must be used to produce the minimum stimulating 
effect. If the concentration of residual precursors is low, the reverse will 
be true. In short, the sensitivity is thus defined in terms depending on 
the physical constitution of the sense organ. 
Similarly with the second condition which inhibits the interpretation 
of retinal adaptation. Experiments to be published in the Journal of 
General Physiology show a simple relation between the intensity of the 
stimulating light and its photochemical effect in the photosensory process. 
If E is the photolytic effect as measured in units of P and A formed, 
then the results are that 
E = Inl 
where I is the intensity of the stimulating light. The photochemical 
effect of the light is, therefore, a logarithmic function of its intensity. 
Here again the meaning of the minimum intensity is given in terms of the 
ultimate composition of the sensory mechanism. 
If the initial phase of the photic reception of the human retina in dim 
light is similar to the mechanism in these lower forms, the application of 
the two principles just described should yield an analysis of the results 
obtained in dark adaptation. The sensitivity of the eye given in terms of 
the minimum intensity of the stimulating light can now be replaced by a 
direct statement of the photochemical effect of this light. Instead of the 
intensity we may use the logarithm of the intensity to indicate the ex- 
tent of this photolytic activity. This now shows that during dark adapta- 
tion of the eye less and less photochemical decomposition is required to 
produce a visual effect. Assuming a photosensitive substance in the retina, 
this means that as the time goes on less and less of this photosensitive 
material must be decomposed in order to produce a stimulus. The exact 
amount of this decomposition can now be calculated. 
If in addition it is true for the eye that the amount of freshly formed 
decomposition product is always a constant fraction of the residual de- 
composition product present in the retina, the significance of these facts 
becomes clear. During dark adaptation less and less fresh decomposition 
product must be formed. Therefore, the quantity of residual decomposi- 
tion product present in the retina must also become less and less, and at 
the same rate as that of the fresh decomposition product. The ratio be- 
tween the two being constant, the curve of dark adaptation should repre- 
sent the rate of disappearance of the residual decomposition product. 
The many experiments reported on the retinal adaptation of the eye, 
when analyzed in this way, show a uniformly consistent course for the dis- 
