152 RESPONSE IN THE LIVING AND NON-LIVI9G 
phenomenon as retinal response could have its counter- 
part 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 cur- 
rent due to difference between the 
and the rod. This may be balanced, 
compensating E.M.F. 
inner surface 
however, by a 
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(a) (Db) 
Fic. 96.—Recorp 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 
