Abstract
1. S-potentials from Luminosity-units in the excised eye of the tench (Tinca) were excited by white lights of various intensities and spatial distributions.2. When a small light spot of fixed size and intensity was presented at various distances from the recording electrode, the S-potential was found to suffer an exponential attenuation with distance (Fig. 3).3. A circular patch of light centred upon the electrode and varied in radius gave concordant results.4. The effects at the electrode site of a distant light flash was not due to light scattered to the electrode which acted upon the retina there; the light acted locally and it was the current that spread.5. With red and blue light stimuli superposed on the retina, it is the potentials not the lights that add.6. These experimental results seem to indicate that S-potentials can spread laterally across the retina within a thin layer at about the horizontal cell level.7. The layer is bounded by leaky membranes which appear to be electrogenic, no doubt as a result of transmitters released by light which change the permeability. Electric currents have no demonstrable effect.8. Two simple results were found. (a) The membrane contains an element whose conductance is proportional to the light intensity. (b) The S-potential is a hyperbolic function of light intensity.9. All these experimental results can be explained quantitatively in terms of the formal model of Fig. 8F. But the required change of membrane permeability with light has not yet been demonstrated.