160 HUMAN PHYSIOLOGY 



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 

 the camera. In the same way, the picture on the retina 

 of the right eye is unlike that in the left eye and most 

 markedly as regards things which are close at hand. We 

 have learned by experience without thinking about it 

 in any analytic way that dissimilarity of retinal 

 pictures is associated with nearness of objects. So 

 binocular vision becomes a great aid in forming judg- 

 ments of distance. 



As this is true for distinct objects so it is for various 

 parts of the same object. Looking at a building with 

 both eyes we have a vivid impression of the projection 

 or recession of its angles. We say that we gain in this 

 way the sense of solidity. Stereo-binocular glasses are 

 made with the object lenses farther apart than the lenses 

 which are held to the eyes. As a result the user has not 

 only a magnified picture to look at but enjoys the 

 advantage that would be his if his eyes could be moved 

 apart and made to converge upon the scene. The 

 principle is that of the surveyor's triangulation and also 

 of the range finder. 



Judgments. In speaking of binocular vision we have 

 given an idea of one way in which we estimate the 



