The two curves in [fig. 104], giving the relation between response and stimulus, show that in the case of inorganic substances, as in the retina (Waller), magnitude of response does not increase so rapidly as stimulus.
After-oscillation.—When the sensitive surface is subjected to the continued action of light, the E.M. effect attains a maximum at which it remains constant for some time. If the exposure be maintained after this for a longer period, there will be a decline, as we found to be the case in other instances of continued stimulation. The appearance of this decline, and its rapidity, depends on the particular condition of the substance.
When the sensitive element is considerably strained by the action of light, and if that light be now cut off, there is a rebound towards recovery and a subsequent after-oscillation. That is to say, the curve of recovery falls below the zero point, and then slowly oscillates back to the position of equilibrium. We have already seen an instance of this in [fig. 102]. Above is given a series of records showing the appearance of decline, from too long-continued exposure and recovery, followed by after-oscillation on the cessation of light ([fig. 105]). Certain visual analogues to this phenomenon will be noticed later.
Fig. 105.—After-oscillation
Exposure of one minute followed by obscurity of one minute. Note the decline during illumination, and after-oscillation in darkness.
Abnormal effects.—We have already treated of all the normal effects of the stimulus of light on the retina, and their counterparts in the sensitive cell. But the retina undergoes molecular changes when injured, stale, or in a dying condition, and under these circumstances various complicated modifications are observed in the response.
