VISION. 



79? 



rays; the other is a change of curvature of the lens. The ciliary 

 muscle is the motive power of accommodation. Its paralysis renders 

 accommodation impossible. The oculomotor innervates the ciliary 

 muscle. Its paralysis by atropine produces both dilatation of the 

 pupil and inability to accommodate. 



To correct anomalies of refraction it is necessary to use lenses. 

 These are transparent media which seem to refract rays of light 



Fig. 343. Refraction of Parallel Rays of Light in Emmetropia (E), 

 Hypermetropia ( H ) , and Myopia ( M ) . ( BALL. ) 



passing through them. They have curved surfaces. The direction 

 which the rays take on emerging from the medium depends upon 

 the nature of the curvature. The chief forms of lenses are convex 

 and concave; convex lenses may be doubly convex, plano-convex, or 

 concavo-convex. A concave lens may have equivalent features. A 

 convex lens converges the rays of light; a concave lens diverges the 

 rays of light. In myopia a concave lens is used; in hypermetropia 

 and presbyopia, a convex lens. 



