The Form and Magnitude of the Electrical Response of the Eye to Stimulation by Light at Various Intensities

The photo‐electric reaction of an isolated eye is specially adapted to the study of the effect of stimuli of very different intensities. In our experiments light stimuli have been used whose energy varies from 3·05 × 10−12 to 3·95 × 10−2 g. cal. per sec. Short illumination with the strongest light has not been found to damage the eye, while the weakest light was capable of producing a fairly considerable photo‐electric reaction. 2. The form under which the photo‐electric reaction manifests itself under different conditions gives ground for the supposition that there occur in the eye three separate processes, and each of these may be dependent upon a separate substance. We speak of three substances for the sake of convenience. 3. The first substance reacts more rapidly than the other two. On lighting it develops a negative, on darkening a positive potential difference. Its action comes strongly into prominence in a light eye and appears almost unmixed on sudden darkening of short duration (a flash of darkness). The second substance reacts less rapidly than the first, and in an opposite sense. On lighting it develops a positive, on darkening a negative potential difference. Its action appears almost unmixed in a dark eye which is illuminated for a short time with weak light. The third substance reacts in the same sense as the second, but much more slowly. Its action fails in a completely light eye, and also in a dark eye which is illuminated very weakly for a short time. 4. The latent period of the photo‐electric reaction is in a high degree dependent upon the intensity of the stimulus. With strong stimuli it is of the order of 0·01 sec., while with very weak stimuli it may be lengthened to more than 2 sec. These values are in agreement with the latent periods of light perception in the human eye. 5. For each of the three substances the rule holds good that with moderate and strong light the energy of the stimulus increases much more quickly than the energy of the reaction. 6. Although the eye is far more sensitive than the most sensitive artificial bolometer, the energy of the reaction remains below that of the stimulus even in the most favourable circumstances. The photo‐electric reaction of the eye is in this respect comparable with the action current of a muscle or nerve.