162 HUMAN PHYSIOLOGY 



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. 



Binocular Vision. How are we served by having two 

 eyes instead of one? In answering this question it is 

 necessary to point out that we do not profit from our 

 double equipment in the same way that some animals 

 do. The chief value of binocular vision in a fish is that 

 stimuli can be received from nearly opposite directions 

 at the same time. Roughly speaking, if one -eye is 

 looking east the other is looking west. We gain only 

 slightly in the width of our visual field through having 

 two eyes. Our principal gain is in what is called stereo- 

 scopic vision. 



Stereoscopic views are photographs taken in pairs from 

 points somewhat separated. Such views are not du- 

 plicates. Their more distant features are nearly identical 

 but the details of the foreground are differently placed 

 in the two and the more differently as they are nearer 



