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 



oo/o 



0005 



'OOOI 



FIG. 99. KESPONSES TO LIGHT IN FROG'S KETINA 



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, b). 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 



