BINOCULAR VISION 265 



Some rays are brought to a focus before others and a 

 partial blurring results. Astigmatism may be remedied 

 by glasses of a slightly cylindrical form, which make 

 good the general defect of curvature in the lens. 



Perception of distance. For the perception of color 

 and form, one eye is enough, but to accurately deter- 

 mine the distance of objects, two eyes are necessary. 

 With two eyes, we look a little farther around an ob- 

 ject on each side and thereby make it stand out in relief. 

 Besides this, since near objects require a greater con- 

 vergence of the eyes than distant objects, the amount of 

 movement necessary to make the convergence helps us 

 to estimate the distance. 



Binocular vision. That we may see but one object 

 with the two eyes instead of two, it is necessary, first, 

 that the eyes should be controlled so as to converge ac- 



PUHey of upper rotating muscle 

 Part of skull (eye socket) 

 Muscle of upper lid 



Upper rotating muscle 



Muscle turning eye upwar~ 

 Muscle turning eye outward 

 Muscle turning eye downward 



Bone of socket 

 Optic nerve- 

 Lover rotating muscle 



FIG. 146. The eye and its muscles. 



curately upon an object. To accomplish this, there are 

 muscles which pass from the sides of the eyeballs at the 

 front to the back of the eye-sockets. By their contrac- 

 tion, they are able to move the eyes so that they can con- 

 verge upon an object in any direction. It is further 



