46 



E. F. MACNICHOL, JR. 



4 3 2 



- LOG. INTENSITY 



Fig. 12. Family of curves representing the effects of excess potassium and calcium 

 in the external medium on the number of impulses discharged by a single unit in the 

 Limnlus optic nerve in response to flashes of light of variable intensity and 0.02 sec. 

 duration. (#) Normal salt concentration. (O) 10 mM KC1 added; other salts normal. 

 (A) Normal salt concentration. (A) 30 mM CaCl 2 added; other salts normal. 



on the rate of dark adaptation. Fig. 13 shows the rate of recovery after ten 

 minutes exposure to a very bright light. Brief test flashes were repeated regu- 

 larly at progressively decreasing intensities and the time of appearance of the 

 first impulses at each light level noted. 



The logarithm of the threshold intensity was plotted against time in the 

 dark. The first run shown by the open circles was a control, then excess potas- 

 sium was used as shown by the right halfmoons. Then potassium-free solution 

 was used (open triangles). Then excess potassium was given again (left half- 

 moons). Then calcium was withdrawn from the solution (open squares). The 

 potassium was withdrawn again (closed triangles) and finally, a control was 

 made (closed circles). 



Clearly excess potassium speeded dark adaptation and its withdrawal slowed 

 it down. 



Indeed, in some experiments when potassium was withdrawn there was a 

 prolonged discharge after the adapting light was turned off. The afterdischarge 

 finally stopped and subsequently very little dark adaptation took place. It 

 may be that during light adaptation potassium leaks out of the receptors which 

 cannot dark adapt until it is restored from the external medium. That the 

 effect is not due just to a change in threshold of the receptors is shown by the 



