6 ELEMENTS OF APPLIED MICROSCOPY. 



A biconvex lens works roughly, as if it were made 

 up of an infinite number of prisms arranged about a 

 central axis. Rays parallel to the principal axis of such 

 a lens (the line joining the centers of curvature of its 

 two surfaces, — will be bent from all directions toward 

 that axis, and the rays nearest the outer edge or 

 periphery will be most bent, so that all will meet at one 

 point, known as the Principal Focus or burning-point of 

 the lens (o. Fig. 5). The greater the curvature of the 



Fig. s- — ^Plano-convex Lens and its Principal Focus. 

 (After Hager-Mez.) 



lens the nearer to its optical center this principal focus 

 will lie. 



5. Formation of Images by the Convex Lens. — ^As 

 the principal focus of a biconvex lens is defined as the 

 point at which rays parallel to the principal axis meet, 

 it is obvious that rays proceeding from a radiant point 

 placed at the principal focus will be sent off on the other 

 side of the lens as parallel rays, and will meet to form an 

 image only at an infinite distance {B, Fig. 6). Rays 

 from a point between the lens and its principal focus, 

 being still more divergent, will not even be made parallel, 

 but will still continue to diverge after passing through. 



