900 LIGHT AND LIFE 



that the threshold was raised 1000 times, agreed precisely with the 

 shift of the curve expected from an increase in /^ alone, and diverged 

 several hundred-fold from that expected if only AF had changed. 

 Hence the effect of adaptation to a bright light is to increase the 

 intrinsic noise of the retina. 



Rushton points out that the photochemical theory of vision is 

 clearly inadequate to explain the alterations of the increment thresh- 

 olds during light and dark adaptation. He proposes instead an elec- 

 trophysiological interpretation, in which the increment threshold 

 should depend upon (a) a nearly instantaneous logarithmic trans- 

 formation of the light intensity — a transformation the nature of which 

 remains obscure — and upon (b) the response of the nerve to a con- 

 stant increment in the transformed signal. On the other hand, Rush- 

 ton points out that the photochemical theory explains very satis- 

 factorily the rise of the absolute visual threshold upon exposure to 

 light and its subsequent fall in the dark, as a function of the concen- 

 tration of the visual pigments. Experiments using the new technique 

 of retinal densitometry to measure the concentrations of the pigments 

 in the living eye indeed show that dark adaptation is closely related 

 to the pigment concentration, as Wald and others have thought. The 

 change in pigment density can be measured by the change in the 

 logarithm of the reflected light. Again, Rushton made measurements 

 upon his own cones, starting both from the dark-adapted state and 

 from the light-adapted condition. He found "that the absolute thresh- 

 old depends very greatly iqoon the amount of pigment bleached, but 

 the increment threshold very little" — precisely as expected. The for- 

 timate availability of a person with rod monochromat vision per- 

 mitted further conclusions. In the absence of cones, the behavior 

 of the rhodopsin of the rods may be followed over the entire course 

 of the enormous rise (3 million times) in the rod threshold during 

 the course of light adaptation. Rod function is not inhibited by the 

 activity of cones, since it appears at a given level whether there are 

 cones or not, and follows the same dark adaptation curve over the 

 extent of rod function that is observable in normal eyes. There is a 

 linear relation between amount of pigment bleached and log threshold, 

 as Wald has previously proposed. 



Other experiments with Limulus, recorded from a single visual cell 

 by means of an intracellular microelectrode under a weak, steady light, 

 revealed numerous irregular slow pulses of potential, interpreted as 

 responses to the absorption of single quanta. The effect of adaptation 

 to a much brighter light was to produce far more numerous, and also 



