W. A. H. RUSHTON 



719 



peiinients ol Hecht, Shlaer, and Pirenne (4) . The results showed 

 that when, as a restdt ol light adaptation, the threshold rose 100 

 limes, the number ot quanta used also rose very greatly — perhaps 

 100 times. It is as though every quantum still produced an eftect, 

 but the effect was only 1 per cent ot that in the dark-adapted state, 

 so that 100 ot them were needed to reach threshold. 



Something ot this kind is suggested by Fig. 7, which shows an 

 intracellular record trom a cell in an ommatidium of Limulus 

 (Fuortes and Rushton, unpub.) . The three sections of this slow 

 ink record ran consecutively. The upper line shows that in very 

 weak constant light there is an irregular series of sudden slow potential 

 changes, here enhanced in size by hyperpolarization of the cell. These 

 slow pulses, first described by Yeandle (15), may be the responses 

 to the absorption of single quanta, and may represent a contribution 

 toAvards the potential V upon which nerve excitation depends. 



JO 



30mV 

 20 



10 



-■o 



-L 



i_ 



Jl 



10 



15 sec 



Fig. 7. Intracellular recording from visual cell of Limulus in full dark adapta- 

 tion. Irregularities possibly indicating responses to single quanta. Stronger light 

 exposed for 10 sec in middle trace, leaving diminished responses. The three tracings 

 follow continuously. 



