Described here is a technique with a direct current method by means of which the experimenter is able to measure the magnitude of excitation of rods in the human eye adapted to suprathreshold light stimuli. The method is in principle based on the finding that the electrostimulation, with a single constant current pulse of about 25 milliseconds in duration, exercises a selective effect on the rod mechanism. By means of this technique, rod responses to spectral lights are measured at a temporal retinal region of 15 degrees from the fovea as a function of wavelength. The spectral distribution of rod responses determined in this manner is found to agree satisfactorily with the scotopic visibility curve. Rod responses to a weak white light stimulus are measured at various parts of the retina. The spatial distribution of rod responses so obtained is found to show close resemblance to the rod density distribution of Østerberg.

The magnitude of rod responses increases with the increasing intensity of stimulus light, but decreases at certain intensity and above it. This fact suggests the inactivity of rods in photopic vision, thus verifying the original idea of von Kries that the rods act only in twilight vision and not at all in day vision.

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