Inorganic response under the stimulus of light.—It may now be asked whether such a complex vital phenomenon as retinal response could have its counterpart in non-living response. Taking a rod of silver, we may beat out one end into the form of a hollow cup, sensitising the inside by exposing it for a short time to vapour of bromine. The cup may now be filled with water, and connection made with a galvanometer by non-polarisable electrodes. There will now be a current due to difference between the inner surface and the rod. This may be balanced, however, by a compensating E.M.F.
Fig. 96.—Record of Responses To Light given by the Sensitive Cell
Thick lines represent the effect during illumination, dotted lines the recovery in darkness. Note the preliminary negative twitch, which is sometimes also observed in responses of frog’s retina.
We have thus an arrangement somewhat resembling the eye, with a sensitive layer corresponding to the retina, and the less sensitive rod corresponding to the conducting nerve-stump ([fig. 96], a).
The apparatus is next placed inside a black box, with an aperture at the top. By means of an inclined mirror, light may be thrown down upon the sensitive surface through the opening.
On exposing the sensitive surface to light, the balance is at once disturbed, and a responsive current of positive character produced. The current, that is to say, is from the less to the more stimulated sensitive layer. On the cessation of light, there is fairly quick recovery ([fig. 96], b).
The character and the intensity of E.M. variation of the sensitive cell depend to some extent on the process of preparation. The particular cell with which most of the following experiments were carried out usually gave rise to a positive variation of about ·008 volt when acted on for one minute by the light of an incandescent gas-burner which was placed at a distance of 50 cm.