708 



LIGHT AND LIFE 



2 



Fig. 1. Intracellular recording from a visual cell in Limulus. First and last 

 record of each line, response of 0.5 sec flash of light of intensity /. Middle record 

 A, response of cell to bright light 4000 / which leaves the cell insensitive to the 

 next flash. Middle B shows the discharge resulting from 1 sec of depolarization 

 of 10—^ amp, which does not change the sensitivity to the next flash. 



IB shows a similar procedure (upon the same cell) but instead of 

 using bright light, a strong depolarizing current was passed through 

 the microelectrode (middle record) . This lasted a second and caused 

 a more massive discharge of impulses than did the bright light, but it 

 had no effect upon the subsequent weak light flash. Thus light adapta- 

 tion is not caused by the activity of the recorded cell. Something must 

 have happened earlier in the train of events and altered the excitation 

 of the cell caused by the next test flash. But how is the cell excited 

 by the test flash? 



Fuortes (3) has measured the resistance of the cell membrane in 

 conditions of various steady lights and steady currents passed through 

 the microelectrode. The resistance was found to remain unchanged 

 as a function of current but to diminish with increasing light in- 

 tensity. It follows that light cannot act upon the cell by generating 

 somewhere a current which then stinudates the cell directly, for this 

 would not cause the fall in resistance which in fact light produces. 

 Light clearly must stimulate by producing some agent which increases 

 cell permeabilitN — presumably some transmitter hormone. The effect 

 of light adaptation is therefore either to reduce the amount of hor- 

 mone prodiucd by a given flash or to reduce the sensitivity of the 

 cell to a fixed amount of hormone. So long as we can detect the 



