convex ', plane 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 



Fig. 6. 



the 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- 



