1 52 RESPONSE IN THE LIVING AND NON-LIVING 



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 witli 

 water, and connection made with a galvanometer by 

 non-polarisable electrodes. There will now be a cur- 

 rent due to difference between the inner surface 

 and the rod. This may be balanced, however, by a 

 compensating E.M.F. 



FIG. 96. KECORD OF KESPONSES 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 



