REFRACTION. 



563 



way as parallel rays of light from the sun may be focussed on a 

 near object by means of an ordinary convex lens. 



Only so much light reaches the fundus of the eye as can pass 

 through the opening in the iris, so that a comparatively narrow 



FIG. 221. 



Diagram showing the course of parallel rays of light from A, in their pas- 

 sage through a biconvex lens L, in which they are so refracted as to bend 

 towards and come to a focus at a point F. 



and varying beam is admitted to the chamber in which the nerve- 

 endings are spread out for its reception. 



If we hold a biconvex lens at a certain distance from the eye 

 and look out of the window through it, we see an inverted image 

 of the landscape. If we place a piece of transparent paper behind 



FIG. 222. 



Diagram showing the course of diverging rays, which are bent to a point 

 further from the lens than the parallel rays in last fig._ 



the lens, we can throw a representation of the picture on it, which, 

 however, will be seen to be inverted. This power of convex lenses 

 is employed in the instrument used for taking photographic pict- 

 ures, called a camera, which consists of a box or chamber into 

 which the light is allowed to pass through a convex lens, so that 

 an inverted image of the objects before it is thrown upon a screen 

 of ground glass within the box. When the sensitive plate re- 



