48 E. F. MAC'NICHOL, JR. 



stant number of quanta would be required. More direct evidence has been 

 provided by some experiments of Rush ton (1952, 1954) and of Weale (1953). 

 They measured the bleaching of the photopigments in intact mammalian eyes 

 spectrophotometrically and at the same time measured the changes in the 

 visual threshold. These experiments showed that enormous changes in thresh- 

 olds were produced when only a small fraction of the pigments was bleached. 



Finally, Wald (1954) has made artificial eyes which duplicated as nearly as 

 possible the conditions in the human eye. They had a layer of photopigment 

 at the back which was estimated to have the same concentration as in the 

 human eye. He exposed these eyes to light and measured the rate of bleaching 

 spectrophotometrically. He then placed the eyes of human observers in front 

 of his light source and measured their rate of adaptation. He found that light 

 sufficient to change the observers' thresholds more than a thousand fold 

 bleached less than one percent of the photopigment in the artificial eyes. 



Wald has postulated a structural arrangement in the outer segments of the 

 receptors that could produce large changes in threshold by bleaching only a 

 small fraction of the molecules of photopigment. There is little evidence for 

 this theory at the present time but it presents an attractive possibility. 



At the present time we do not know where the change in concentration of 

 the potassium ion exerts its effect. We must investigate it further. It appears 

 that very little of light and dark adaptation can take place in the trigger mech- 

 anism which generates the impulses since some rather crude preliminary experi- 

 ments indicate that the slow potentials recorded from the microelectrodes ap- 

 pear to follow a course of light and dark adaptation parallel to changes in the 

 frequency of the spike discharge. It must be concluded that dark adaptation 

 takes place in the electrochemical amplifying mechanism activated by the 

 splitting of the photopigment and resulting in the production of the graded 

 slow potential. 



This mechanism must be complex since it requires some time to act, as shown 

 by Hartline's (1934) experiments on the critical duration. He found that the 

 response to a short flash of light takes place wholly in the dark as shown in 

 Fig. 14. If a short flash of light is given there is a latent period before any 

 impulses are generated or any slow potential appears. In the dark adapted 

 animal this time can be as long as a second and usually is at least some tens of 

 milliseconds. Furthermore the duration of the latent period is determined only 

 by events taking place near its beginning as shown in the following experiment. 

 Two flashes of light separated by a variable time are presented to the eye. If 

 the second flash appears within an interval known as the critical duration it 

 will shorten the latent period of the first response. If it appears after the critical 

 interval it will have no effect on the first response but can affect the time of 

 occurrence of subsequent impulses. The critical duration is from } 3 to }^ the 

 duration of the latent period. Thus there is this considerable period of time in 



