Threshold voltages of alternating currents passing through the head as determined with flickering phosphenes as an index were plotted as a function of frequencies, and 7 conspicuous minima were found in the curve. It was shown that these minima are the result of differential sensitivity of 7 types of receptor having their own time-constants. Based on the fact that the electrical sensitivity of the eye as measured by an ac of any optimal frequency is selectively enhanced on exposure of the retina to weak spectral lights of wavelengths of a certain limited range, a spectral response curve was determined for each kind of receptor. The response curves determined in this way had so sharp peaks well localized on definite parts of the spectrum that the optimal wavelength for each kind of receptor could be determined with sufficient accuracy. The optimal wavelengths and resonance frequencies of the receptors are as follows: red (650 mµ) 77 cycles per sec; orange (610 mµ) 62.5 cps; yellow (575 mµ) 52.5 cps; green (515 mµ) 42.5 cps; blue (465 mµ) 35 cps; violet (415 mµ) 28.5 cps; rod or scotopic receptor (507 mµ) 20 cps.
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