OCULAR REFRACTION 313 



ated just behind the iris. Its function is to refract the rays of 

 light, and its action in this respect is similar to such lenses in 

 optical instruments. It is held in place by the suspensory liga- 

 ment. Its anterior convexity is more marked than its posterior. 

 It is enveloped by a thin transparent capsule. 



The Suspensory Ligament is a continuation of the anterior 

 layer of the hyaloid membrane of the vitreous humor. When 

 this layer reaches the edge of the lens (coming forward) it divides 

 into two parts, one passing in front of and the other behind that 

 body; the divisions are continuous respectively with the anterior 

 and posterior portions of the capsule of the lens. The ligament 

 supports the lens. 



The Aqueous Humor is behind the cornea and in front of 

 the lens and suspensory ligament. The iris has been said to 

 separate this cavity into anterior and posterior chambers com- 

 municating through the pupillary opening. The aqueous hu- 

 mor is colorless and perfectly transparent. It serves to refract 

 the rays of light, having for that purpose the same index as the 

 cornea. 



The Vitreous Humor occupies about the posterior two-thirds 

 of the globe, and is back of the lens and suspensory ligament 

 surrounded by the delicate hyaloid membrane. It is of a gelat- 

 inous consistence, and is divided into numerous compartments 

 by very delicate membranes radiating from the point of entrance 

 of the optic nerve. It is a transparent refracting medium. 



Ocular Refraction. In order for the image of an object to be 

 distinct the rays passing from it must fall on a single portion of 

 the retina, viz., the fovea centralis. The sensibility of the 

 retina to light decreases in passing away from the fovea. All 

 rays would not meet on the retina unless they were refracted; 

 and for this purpose there are the cornea, the aqueous humor, 

 the lens and the vitreous humor. The surfaces of the cornea 

 and lens are the most important of these. Since the two surfaces 

 of the cornea are parallel, the external surface alone is concerned 



