THE EYE 159 



himself with a suitable green necktie and he hastened 

 to choose one. When he displayed it there was a 

 strenuous protest; it was the Harvard crimson. 



The process in the rods and cones must be a photo- 

 graphic one. Some chemicals are present there which 

 are changed in a definite way by the action of light. 

 The changes which occur must furnish the immediate 

 source of the stimuli which start the nerve-impulses 

 on their way to the brain. One difference between the 

 retina and a common photographic plate lies in the 

 fact that in the eye the taking of each picture is fol- 

 lowed by a marvellously rapid recovery of a condition 

 which permits a new set of images to be registered. In 

 the plate the outlines photographed are permanent 

 even though they may be overlaid by those of a second 

 exposure. We may say that in the one case there is 

 great resistance to fatigue while in the other it is almost 

 immediate. The retina, however, does show a measure 

 of fatigue under strong stimulation. 



A retinal picture, when the eyeball is stationary, 

 must consist of a vast number of associated points. 

 In this respect it resembles a half-tone reproduction. 

 In either instance the points are so numerous and so 

 close together that no discontinuous effect is noticed. 

 When we try, in imagination, to correlate these points 

 of excitation in the retina with simultaneous streams of 

 nerve-impulses in thousands of fibers of the optic nerve 

 and with the multitudinous brain-processes which re- 

 sult from their arrival at the centers we realize the 

 hopeless difficulty of the analysis The situation be- 

 comes still more amazing when we consider that we 

 can move the eye, shifting each detail of the picture 

 from certain retinal cells to others, and yet have the same 

 general impression as before. By our movements we 

 cause chosen features of the scene to pass in succession 

 over the fovea while the sense of the larger relationships 

 of all the things we see remains steady and reliable. 



