ir. A. H. RUSH TON 



709 



hormone only by its effect it is impossible to distinguish with cer- 

 tainty between these alternatives, yet a very different kind of argu- 

 ment has some cogency. 



It is now possible to measure the bleaching and regeneration of 

 rod and cone pigments in the living retina of animals and man, and 

 it is found that human cones take 8 minutes, rods 30 minutes, cat's 

 rods 100 minutes, for the full regeneration of the pigment. But these 

 intervals are just about the times for full return of dark adaptation. 

 Thus the recovery of sensitivity in the dark appears closely connected 

 with the regeneration of the visual pigment. Returning now to the 

 question of the stimulation of the Limulus cell by transmitter hor- 

 mone, it seems much more likely that during dark adaptation the 

 hormone is increased in quantity rather than that the cell is in- 

 creased in sensitivity, since the hormone is liberated in some way by 

 photolysis and hence might well be coupled to pigment regeneration, 

 whereas the cell sensitivity would be expected to have an entirely 

 independent (and much faster) course of recovery. 



(c) The Respojise to Intensity 



Fuortes (3) found that the characteristic effect of light upon the 

 Limulus cell was to lower the membrane resistance, and Fig. 2 shows 

 the relation between the resistance change and log I, where / is the 



Fig. 2. Abscissae give log light intensity, ordinates (black circles) show corre- 

 sponding membrane resistance (or membrane potential), of a single visual cell in 

 Limulus. The continuous curve sliows function log (////> + 1). Upper interrupted 

 curve is Stiles' log, increment threshold (man): lower is Donner and Rushton's 

 light excitability function for the frog. 



