THE EYE 163 



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 

 comparative distances of things seen. There are other 

 ways in which this is done. Even with one eye we 

 have a fair basis for drawing conclusions. It may be 

 laid down as a general principle that motor acts are 

 attended with characteristic sensations. This is true 

 of the act of accommodation. When the attention is 

 directed to a surface like the page of a book which is 

 only a few inches away there is a strong tension on the 

 part of the muscle that increases the curvature of the 

 lens. A feeling goes with the maintenance of this 

 tension and this feeling has, from early life, been asso- 

 ciated with looking at that which is near at hand. 

 One source of the bewilderment experienced on first 

 wearing glasses is that the accommodation effort for a 

 given distance is greater or less than before. New 

 interpretations become necessary. 



