19 



convex, plant" convex, convex meniscus and double 

 concave, plane concave, concave meniscus. 



In the convex lenses, parallel rays are converged 

 to one point as shown in Fig. 6, in which c is 



d 



c 



Fig. 6. 



tt\& principal focus, or focal point and the distance 

 from b to c the focal distance. With the same 

 lens, if a flame be placed at c, all the rays which 

 strike the lens will be emitted in a parallel direc- 

 tion on the opposite side. The straight line d b 

 c which passes through the middle of the lens is 

 called the principal axis and the line a c' the 

 radius. 



The radius determines the power or converging- 

 quality and as it becomes longer, reduces the 

 power by making the lens longer focus. If in a 

 double convex lens the radius of each surface is 

 1 inch, the lens has a focus of 1 inch and if 

 in a plane convex lens, the convex surface has the 

 same radius, the focus is twice as long, or -2 inches. 



In a concave lens the action of the lens is oppo- 



