Fig. 106.—Transient Positive Augmentation given by the Frog’s Retina on the Cessation of Light L (Waller)
Fig. 107.—Responses in Silver Cell
The thick line represents response during light (half a minute’s exposure), and dotted line the recovery during darkness. Note the terminal positive twitch.
1. Preliminary negative twitch.—When the light is incident on the frog’s retina, there is sometimes a transitory negative variation, followed by the normal positive response. This is frequently observed in the sensitive cell (see [fig. 96], b).
2. Reversal of response.—Again, in a stale retina, owing to molecular modification the response is apt to undergo reversal (Waller). That is to say, it now becomes negative. In working with the same sensitive cell on different days I have found it occasionally exhibiting this reversed response.
3. Transient rise of current on cessation of light.—Another very curious fact observed in the retina by Kuhne and Steiner is that immediately on the stoppage of light there is sometimes a sudden increase in the retinal current, before the usual recovery takes place. This is very well shown in the series of records taken by Waller ([fig. 106]). It will be noticed that on illumination the response-curve rises, that continued illumination produces a decline, and that on the cessation of light there is a transient rise of current. I give here a series of records which will show the remarkable similarity between the responses of the cell and retina, in respect even of abnormalities so marked as those described ([fig. 107]). I may mention here that some of these curious effects, that is to say, the preliminary negative twitch and sudden augmentation of the current on the cessation of light, have also been noticed by Minchin in photo-electric cells.
4. Decline and reversal.—We have seen that under the continuous action of light, response begins to decline. Sometimes this process is very rapid, and in any case, under continued light, the deflection falls.
(1) The decline may nearly reach zero. If now the light be cut off there is a rebound towards recovery downwards, which carries it below zero, followed by an after-oscillation ([fig. 108], a).