1. The phenomenon of dark adaptation of the eye of Limulus is reflected in the behavior of the action potentials obtained upon stimulation by light. The method of obtaining and recording these action potentials has been described in an earlier paper.
2. By determining the intensity of stimulus necessary to produce an electric response of a given magnitude (as to maximum action potential), at various times during dark adaptation, a quantitative analysis of the process may be made. This analysis is identical with that of Hecht for the dark adaptation of the human eye.
3. The results of this analysis indicate that the process of dark adaptation in the Limulus eye may be represented by a chemical reaction of the second order—the recombination of products of photolysis to renew the depleted supply of photosensitive material. This is in complete accord with Hecht's conception of the photosensory process, and is in quantitative agreement with the results obtained by other methods, in several different animal forms.
4. The experimental relation between strength of stimulus and magnitude of electric response reduces the assumption originally made by Hecht to account for the data on the human eye to an equivalent form; that the magnitude of electric response, provided it be sufficiently large, is directly proportional to the concentration of the photosensitive material in the sense organ.