12 PRACTICAL HISTOLOGY. 



a focus (C). On the other hand, if the angles of the 



FIG. 8. Refraction by prism. FIG. 9. Convergence of rays by prisms. 



prisms be turned towards each other (Fig. 10), the rays A 

 B, are rendered diver- 

 gent. The converging 

 lenses A B and C in 

 Fig. 1 1 act after the 

 manner of the prisms in 

 Fig. 9, while the diver- 

 ging lenses D E and F 

 (Fig. n) act like the 



prisms arranged as in FlG . Io _ Divergence of raysby prisms< 

 Fig. 10. 



FIG. ii. Lenses. 



23. Focal Length. Take the simple case of a plano- 

 convex lens (Fig. 12). It may be regarded as composed of 

 two rectangular prisms joined by their bases, with the oblique 

 surfaces curved so as to form the segment of a sphere whose 

 radius is C S. The rays A R R all strike the plane surface 

 of the lens at right angles, and therefore undergo no refrac- 

 tion until the spherical surface is reached. The ray A S 

 passing through the centres of both surfaces of the lens is 



