156 RESPONSE IN THE LIVING AND NON-LIVING 
illumination of A and B alternately, we shall now obtain 
currents flowing alternately in opposite directions. 
Just as in the strain cells the galvanometer contact 
was transferred from the electrolytic part to the metal- 
lic part of the circuit, so we may next, in an exactly 
similar manner, cut this plate into two, and connect 
these directly to the galvanometer, electrolytic connec- 
tion being made by partially plunging them into a cell 
-0005 

Fic. 99.—ReEsponsEs To Licut In Froc’s Retina 
Illumination L for one minute, recovery in dark for two minutes during obscurity D. 
(Waller.) 
containing water. The posterior surfaces of the two 
half-plates may be covered with a non-conducting 
coating. And we arrive at a typical photo-electric cell 
(fig. 98, >). These considerations will show that the 
eye is practically a photo-electric cell. 
We’ shall now give detailed experimental results 
obtained with the sensitive silver-bromide cell, and 
compare its response-curve with those of the retina. A 
series of uniform light stimuli gives rise to uniform 
