BENT LEI GLASS 899 



aclai)tation. The characteristic effect of light is to lower the membrane 

 resistance. Alter a bright (lash of light, a weak flash provokes a 

 greatly tliminishcil response in comparison ^vitll that which it w^oiild 

 evoke before the i)right flash. On the contrary, a strong depolarizing 

 ciurent passed through the microelectrode has no effect whatsoever 

 on the response to the subsecpient Avcak flash, lluis the adaptation 

 does not reside in the nerve cell itself but occurs somewhere earlier 

 in the train of events. Any action of currents generated elsewhere 

 and passing through the nerve cell, and assumed to lower the re- 

 sistance of the cell membrane, may also be excluded, since Fuortes has 

 found that the membrane resistance remains unchanged as a func- 

 tion of applied current. It woidd seem to follow that light must 

 stimulate by increasing cell permeability, perhaps through the re- 

 lease of some chemical agent, a "transmitter hormone." If so, there 

 is a question whether the effect of light adaptation is to reduce the 

 amoimt of hormone released or to modify the sensitivity of the cell 

 to a fixed amount of hormone. Since recovery of sensitivity in the 

 dark, after the rod and cone pigments in the living retinas of various 

 animal and human eyes have been bleached, corresponds closely in 

 each case to the time required for full regeneration of the pigment, 

 it seems more probable that the first of the suggested alternatives is 

 the correct one. 



The relation between the membrane resistance of the visual cell in 

 Limulus and the light intensity may be expressed by the equation 



(k 



V - Vd = log (7^ + 1) 



where V is the membrane potential, V^ is the value of V in the 

 dark, / is the light intensity, and Ijj is the "dark light," or intrinsic 

 noise of the system. By differentiation, one obtains: aF = A// (/ + 7^) . 

 This righthand expression has been found experimentally (by Riggs 

 and Graham) to be almost exactly constant over the entire range of 

 light intensities; in other words, \V is essentially a constant for in- 

 crement thresholds, and vision in Limulus, at least in its electrical 

 aspects, obeys the classic Weber-Fechner Law — as Stiles has shown to 

 be true of rod vision and several color mechanisms in man. 



The threshold in the dark, obtained by setting I = 0, is A/ = AV'Ip, 

 an expression that at once makes one wonder whether the great rise 

 in A/ upon adaptation of the eye to light is attributable to an increase 

 in the nerve threshold (sV) or to an increase in //,, the noise of the 

 system. Experiments by Rushton on his own cones, so adapted to light 



