744 LIGHT AND LIFE 



retinas the cone pigment iodopsin was synthesized much more rapidly 

 than the rod pigment rhodopsin (53) . Frog rhodopsin is synthesized 

 in solution relatively slowly, alligator rhodopsin with great speed; 

 dark adaptation measured in the living animals at the same tempera- 

 ture was found to follow the same time relations (51). In both 

 cases the course of these changes resembled each other only when 

 the logarithm of the sensitivity was compared with pigment concen- 

 tration. 



This argument derived strong support from the much closer com- 

 parisons made possible by Rushton's superb measurements of the 

 rise and fall of visual pigment concentration in the living human 

 eye, (7, 36-38) as related to the course of human light and dark 

 adaptation (46, 47) . Rushton presents a particularly handsome com- 

 parison of this type in the present volume. 



I should like here to review briefly some recent measurements by 

 Bowling in our laboratory, that I think approach this problem 

 unequivocally (14, 15, 12) . 



Normal white rats were light adapted by exposure to brilliant 

 white light, and the recovery of both the visual threshold and the 

 retinal content of rhodopsin were measured during subsequent dark 

 adaptation, the threshold by means of the electroretinogram (ERG) , 

 the pigment concentration by direct extraction. In each such ex- 

 periment, 5 animals, weighing about 150 g, were light adapted for 

 30 minutes. One rat was anesthetized lightly with nembutal 5 minutes 

 before the adapting light was shut off. Other animals were anesthe- 

 tized periodically thereafter in the dark, each one 5 minutes before be- 

 ginning the recording of electroretinograms. Thus the first animal 

 was ready for measurement immediately on shutting off the adapting 

 light, the others at various intervals thereafter. In all, 8 such series 

 of measurments, complete or partial, were performed. 



Fig. 7 shows the results of two such series. Light adaptation caused 

 a rise of visual threshold of about 3.5 log units, or about 3000 times. 

 Afterward in darkness the threshold fell regularly, reaching the mini- 

 mum level in about 2 hours. This is considerably slower than dark 

 adaptation in man, which is completed to this degree in about 45 

 minutes. 



On the following day, once with the same group of animals, once 

 with a fresh batch, the same procedure was repeated, but this time 

 instead of measuring the ERG of the anesthetized animal, it was 

 beheaded to yield a pair of retinas for the extraction and measurement 

 of rhodopsin. 



