Measurement of the electrical excitability of the eye provides a useful means for analyzing visual function. Determination of electric thresholds of the eye necessitates, however, the following special precautions.
(1) A single constant current pulse of 0.1 sec. in duration is adequate as an electric stimulus, because it can stimulate uniformly all kinds of retinal elements. On the contrary, a brief condenser discharge and alternating currents of definite frequency stimulate selectively a certain kind of element.
(2) A weak phosphene caused by an electric stimulus near the threshold can hardly be distinguished from intrinsic light of the retina unless the effect of the stimulus in question is compared with that of a stimulus far below the threshold.
(3) The threshold for disappearance of phosphenes can be determined more accurately than that for appearance.
(4) Retinal elements of different kind show different thresholds, and this is the most important thing to be borne in mind in determination of the electrical excitability-of the eye following illumination.