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A systematic study on the electric stimulation of the eye by the sinusoidal alternating current was made by means of the electronic rheotome within a wide range of frequencies extending from 2 to 1, 000 c.p.s. under the various levels of illumination.1) The threshold strength-frequency curve for the dark adapted eye has at least two main minima in the vicinity of 7 and 30 c.p.s., while the curve for the light-adapted eye has a distinct minimum at about 20 c.p.s. There exists always an abrupt inflection of the strength-frequency curve at around 100 c.p.s.2) The shape of the threshold strength-illumination curve depends complicatedly upon the frequency of a.c. According to the curves' shape, all curves obtained were classified into four types, each corresponding to four ranges of frequency of 2-9, 9-25, 25-50 and 50-1, 000 c.p.s. respectively in approximateestimation.3) It was especially noted that the electric threshold for the frequency around 3 c.p.s. varies in a qualitatively parallel relation to the threshold for an adaequate stimulus with increase in illumination, and that the subjective sensation evoked by such low frequencies is also most alike to the sensation evoked by an adaequate stimulus, and that if the higher frequency is used for stimulation, only the onset and cessation of stimulation causes the electrical phosphenes to be perceived.4) It was discussed that the electrical phosphenes evoked by very low frequencies of sine waves seemed to have a close relation to the “on” elements in the retina, whereas those evoked by the higher frequencies, to the “on-off” and “off” elements.

An apparatus named “electronic rheotome” was devised by us for electric stimulation of the eye with sinusoidal alternating currents. The electronic rheotome enables us to start the alternating currents at 0°phase and to stop them at 180°or 360°phase. Combining the electronic rheotome with an A.C. oscillator, we can settle precisely such factors of a stimulus as intensity, duration, frequency and phase of the beginning and the termination, that the measurements of the electric threshold can be made accurately. The electronic rheotome of the 2nd type, furthermore, can produce currents of different forms such as square, anodal and cathodal waves.As a first application of the electronic rheotome we examined the dependence of the electric threshold of the eye upon duration and frequency of the sinusoidal alternating currents. It was found that the characteristics of the strength-duration curve at any frequency are quite different between both ranges above and below a definite frequency in the vicinity of 50 to 60 cps. The threshold values for a frequency above 60 cps. differ strikingly according to whether the phase at the termination of the current is 180°or 360°, while the threshold values for a frequency below 50 cps. decrease steadily with the increase in duration of stimulation, till they reach the lowest value in the duration of 2 to 3 cycles, and they thereafter remain almost constant, no matter at which phase of 180°or 360°the stimulating currents end. That is, a few cycles at the onset of the stimulating currents cause the summation of stimuli. This summation of stimuli takes place most remarkably in the neighbourhood of 20 cps.

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.