#After-oscillation.#--In the case of the sensitive silver cell, we have seen (fig. 105), when it has been subjected for some time to strong light, that the current of response attains a maximum, and that on the stoppage of the stimulus there is an immediate rebound towards recovery. In this rebound there may be an over-shooting of the equilibrium position, and an after-oscillation is thus produced. If there has been a feeble initial current, this oscillatory after-current, by algebraical summation, will cause the current in the circuit to be alternately weaker and stronger than the initial current. #Visual recurrence.#--Translated into the visual circuit, this would mean an alternating series of after-images. On the cessation of light of strong intensity and long duration, the immediate effect would be a negative rebound, unlike the positive after-effect which followed on a short exposure. The next rebound is positive, giving rise to a sensation of brightness. This will go on in a recurrent series. If we look for some time at a very bright object, preferably with one eye, on closing the eye there is an immediate dark sensation followed by a sensation of light. These go on alternating and give rise to the phenomena of recurrent vision. With the eyes closed, the positive or luminous phases are the more prominent. This phenomenon may be observed in a somewhat different manner. After staring at a bright light we may look towards a well-lighted wall. The dark phases will now become the more noticeable. If, however, we look towards a dimly lighted wall, both the dark and bright phases will be noticed alternately. The negative effect is usually explained as due to fatigue. That position of the retina affected by light is supposed to be 'tired,' and a negative image to be formed in consequence of exhaustion. By this exhaustion is meant either the presence of fatigue-stuffs, or the breaking-down of the sensitive element of the tissue, or both of these. In such a case we should expect that this fatigue, with its consequent negative image, would gradually and finally disappear on the restoration of the retina to its normal condition. We find, however, that this is not the case, for the negative image recurs with alternate positive. The accepted theory of fatigue is incapable of explaining this phenomenon. In the sensitive silver cell, we found that the molecular strain produced by light gave rise to a current of response, and that on the cessation of light an oscillatory after-effect was produced. The alternating after-effect in the retina points to an exactly similar process.