594 MANUAL OF PHYSIOLOGY. 



the sensations caused by the two images so that they produce but 

 one idea. 



These points of the retina which are thus habitually stimulated 

 by the same objects are called " corresponding points." 



Besides being of great use in making up for such deficien- 

 cies as the blind spots (which are not corresponding points), 

 binocular vision is useful for the following purposes : 



To judge of distance. When using one eye only, some knowl- 

 edge of distance may be gathered by the force employed to 

 accommodate, but a much more accurate judgment can be made 

 when both eyes are used and the muscular sense of the ocular 

 muscles, employed in converging the eyeballs for near objects, 

 can be used as evidence of their distance. 



In judging of size, in the same way, with one eye, we can only 

 have an idea of the apparent size of an object, which will vary 

 with its distance. With a knowledge of the apparent size and 

 of the distance such as is gained by binocular vision, we can 

 come to a fairly accurate conclusion as to the size of an object. 



To judge of the relative distances of objects so as to see depth 

 in the picture before our eyes, binocular vision is necessary. If 

 one eye alone is used we see a flat picture, without having an 

 accurate idea of the relative distances of the different things we 

 see. With each eye, however, we get a slightly different view of 

 each object, and thus we are helped to conclude as to their exact 

 distances and shape, so as to be able to arrive at fairly correct 

 judgments as to their exact form, etc. 



