The light-induced current as measured using a voltage clamp (holding voltage at resting potential) is attenuated when sodium ions in the bathing solution, Nao, are replaced by Tris, choline, or Li or when NaCI is replaced by sucrose. After replacement of NaCl by sucrose, the reversal voltage, Vrov, for the light response becomes more negative. In this case, the slope of the Vre, vs. log Nao near Nao = 425 mM is approximately 55 mV/decade increase of Nao (mean for 13 cells). The slope decreases at lower values of Nao. Choline is not impermeant and partially substitutes for Na; the slope of Vrov vs. log Nao is 20 mV/decade (mean for three cells). Vrev does not change when Na is replaced by Li. Decreases in the bath concentrations of Ca, Mg, C1, or K do not affect V,,v. When Na = 212 mM, Vrev becomes more positive when Ko is increased. Thus, light induces a change in membrane permeability to Na and probably also to K.

Intracellular recording from single retinular cells of a white eye mutant of the cockroach Periplaneta americana stimulated between 316 and 704 nm reveals the presence of two classes of cells. In the dorsal part of the eye they are present in approximately equal numbers. Both have resting potentials of about 40 mv; neither responds with spikes.Ultraviolet receptors are maximally responsive at 365 nm (A,,,); sensitivity falls steadily at longer wavelengths and is down two log units at 440 nm. The membrane responses of ultraviolet receptors usually consist of a large graded transient which can completely depolarize the cell, followed by a smaller and slower plateau of depolarization which lasts as long as the stimulus.Green receptors have Xma, at about 507 nm. A taiI of sensitivity extends through the violet and near ultraviolet, but sensitivity falls steadily to the long wavelength side of the X , , , , , and is down two log units at 615 nm. The membrane responses typically consist of a graded transient and lack the later plateau.

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