158 0)1 Light in some of its Relations to Disease, 



the transparent portions, to reach the retina, which is the 

 extremely delicate membrane constituting the inner lining 

 of the posterior part of the eye, and is the sensitive surface 

 on which the image is formed. But, in passing through 

 these transparent portions, to reach the retina, the rays of 

 light are bent, or, in technical terms, are refracted, so that 

 they may be brought to a focus. In this diagram is re- 

 presented the form of a section of an eye, showing the ar- 

 rangement of the parts for refracting the rays of light. 



The tough opaque shell of the eye, s {sclera), maintains 

 its form, and holds in place the transparent humors which 

 fill up its space. In front, this shell is changed to a trans- 

 parent window, c, the cornea. Light from a luminous 

 object passes through this transparent substance, and then 

 through a body called the crystalline lens (c. reaching 

 at length the retina r.^ In order to form an image here 

 these rays are refracted so as to bring them to a focus upon 

 the retina ; but, as objects are seen at different distances, 

 some provision must be made for changing the focus of 

 the eye. This you know is done in the opera glass by 

 turning the screw, so as to throw the lens forw^ard for near 

 objects, and backward for distant ones. In the eye this 

 change of focus is effected by the crystalline lens. 



Fig. 1. 



