DEVICES FOR SEEING 



297 



diverge. The convex lens may have both faces convex, one 

 plane and one convex, or one less concave than the other is 

 convex. Similarly, concave lenses 

 may be double concave, plano- 

 concave, and the concave meniscus 

 (see Fig. 144). 



If the principles of operation 

 of a convex lens, given above, FlG - ^.-Lenses of severalshapes 



have been grasped, it will be easy to understand the operation 

 of many optical instruments. Let us see why it is that a magni- 

 fying glass magnifies. The object to be examined must be 



FIG. 145. Diagram showing how a magnifying glass magnifies 



placed nearer to the lens than is its focal point. Rays emanating 

 from a point in such an object, as from a in the diagram (Fig. 145), 

 will be less divergent after passing through the lens than they 

 were on entering the lens. When such rays enter the eyes 

 they will be referred back to a point at their intersection, and 

 this point a' is much farther from the lens than is the point from 

 which they really came. Similarly, point b of the little arrow 

 will be referred back to b', and intermediate points of the objects 

 to a position between a' and b'. One therefore sees the object 

 enlarged. Under these conditions no actual image is formed, 

 but the image seen is spoken of as a virtual image. 



