THE EYE, AND THE PHENOMENA OF VISION. 351 



The two parts into which the iris divides the eye are 

 called the anterior and posterior chambers. 

 What are the ^hc anterior chamber, or the space before 

 TitieoSa hu"! ^^^ ^"^j ^^ filled with a fluid resembling pure 

 mora? water, and therefore called the aqueous hu- 



tnor ; and the posterior chamber, or the space behind the 

 iris, is filled with a thick hquid, somewhat resembling the 

 "white of an egg, called the vitreous humor. 



In Fig. 283, he represents the aqueous humor, and h the vitreous humor, 

 thie last occupying all the interior of the chamber of the eye. 



The crystalline lens is composed of a more solid sub- 

 stance than either the aqueous or vitreous humor. It is 

 inclosed within a transparent bag, or capsule, having the 

 form of a double-convex lens, and is suspended imme- 

 'iiately behind the iris, and between the aqueous and 

 vitreous humors. 



Its form and position are represented at/ Fig. 283. 

 low do we by ^^'^^ I>'ays of light proceeding from an ob- 

 Ae e?r"pe?- j^^* ^^^ entering the eye, are refracted by the 

 *eive objects? comca and crystalline lens, and made to con- 

 Verge to a focus at the back of the eye, and form an 

 image upon the retina. This image, by producing a sen- 

 sation upon the optic nerve, conveys in some unknown 

 way to the mind a perception and knowledge of the ex- 

 ternal object. 



Fig. 284 represents the manner in FiG. 28-i. 



which the image is formed upon the 

 retina in the perfect eye. The curva- 

 ture of the cornea, s s, and of the 

 erystalline lens, c c, is just sufficient 

 to cause the rays of light proceeding 

 from the image, 1 1', to converge to 

 the right focus, m m, upon the retina. 



When does dig- Distinct vision can only take place in the 

 puc«V"°°'^''* ^ys when the cornea and crystalline lens have 

 such convexities as to bring the rays of light 

 proceeding from an object to an exact focus upon the 

 retina. 



