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Using light sensation (electrical phosphenes) produced by the sinusoidal alternating current flowing through the eyeball as an indicator for determination of the electric threshold of the eye, we studied dependence of the electric threshold upon intensity of illumination as well as the time course of the electric threshold during dark adaptation. Frequencies of electric current ranging from 2 to 20 cps were used for stimulation. 1. The form of the electric threshold strength-log-illumination curve varies according to the frequency of the alternating current. Among the curves obtained, the curves for 2, 3 and 4 cps are of duplex nature and their form is quite similar to Ferry-Porter's law of critical fusion frequency (CFF), namely, the electric threshold for frequencies around 3 cps has the closest correlation with CFF. But the same correlation is not found in case of the other higher frequencies. 2. The electric threshold for frequencies at 2, 3 and 4 cps decreases progressively in the course of dark adaptation. This relation is, though qualitatively, in accordance with the progressive decrease of the light threshold during dark adaptation. There appears an abrupt inflection on the electric threshold strength-time curve for 2-4 cps, when the preillumination is sufficiently intense and long. This inflection is not coincident in time for appearance with the so-called “Kohlrausch's Knick” lying on the normal dark adaptation curve obtained by the adequate stimulus. 3. It is suggested that the structures stimulated by the sinusoidal alternating current at 3 cps may be the cone and rod cells, while those stimulated by the current at 15 or 20 cps may be supposedly the bipolar cells.