200 



THE POPULAR SCIENCE MONTHLY. 



position. The image is formed in the eye in the way in which an 

 image is produced and thrown on a screen by a magic lantern. 



When a ray of light passes obliquely from the air through 

 glass, water or other transparent media, it is bent, or refracted, 

 and the angle at which it is bent is called the index of refraction. 

 In passing to the retina, the rays of light pass through the 

 cornea, a watery liquid (the aqueous humor) surrounding the 

 lens, the crystalline lens, and a gelatinous liquid (the vitreous 

 humor) filling the posterior two thirds of the globe, all of which 

 have the same index of refraction. This provides that a ray of 

 light, having once passed through the cornea, is not refracted in 

 passing through the other transparent media, except by the curv- 

 atures of the crystalline, which is a double-convex lens situated 

 just behind the pupil. The rays of light are not reflected within 

 the eye itself, for the opaque parts of the globe are lined with a 

 black membrane (the choroid), as the tube of a microscope is 

 blackened for a similar purpose. Practically, the bending of the 

 rays of light is produced by the curved surface of the cornea and 



FIG. 1. This figure gives a general view of the eyeball, the outer wall of the orbit being re- 

 moved: 1, tendon of origin of three of the muscles of the eyeball ; 2, the external straight 

 muscle divided and turned down so as to expose the lower straight muscle ; 3, 4, 5, 6,7, 8, 

 muscles moving the eyeball; 9, 10, 10, muscle which raises the upper eyelid; 11, optic 

 nerve. (After Sappey.) 



the two curved surfaces of the double-convex crystalline lens. 

 These three curved surfaces bring the rays from an object to a 

 focus exactly at the retina in a normal eye. When, however, the 

 eye is too long, the focus is in front of the retina unless, in near 

 vision, the object be brought very near the eye, and the person is 

 near-sighted. For ordinary vision, such persons must wear prop- 



