12 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS 



It must, however, be understood that there is a very important 

 difference between the action of spherical lenses, which is dm to the 

 different positions of the normals. 



In the prisms (figs. 8, 9) the incident surface A B is a plane ; 

 and as the normals are perpendicular to it, they must be parallel to 

 one another, whether near the base or near the apex. Thus the 

 normal at E is parallel to the normal at K ; therefore, whatever 

 angle D E makes with the normal at E, H K will make a similar 

 angle with the normal at K, because the normals are parallel and the 

 incident rays are parallel. 



But in the case of a spherical lens the normals are radii ; 

 parallelism is therefore impossible, and parallel incident rays will 

 not make equal angles with them, and so the refracted rays will not 

 be parallel. 



This explains how it is that when rays parallel to the axis fall 

 on the prism (see fig. 8) those which pass through the prisms near 

 their bases cut the axis nearer the prisms than those which pass 

 through near the apex. 



But in a convex lens the reverse takes place ; the rays passing 

 through near the middle of the lens cut the axis farther from the 

 lens than those which pass through the edge of the lens. The 

 typical form of a biconvex or magnifying lens is shown in fig. 10, 



FIG. 10. Front and edge views of a biconvex lens. 

 (From the ' Forces of Nature.') 



both in perspective, as seen from the edge, and with a full view of 

 the disc ; while the various forms which for various optical purposes 

 are given to lenses is shown in figs. 11 and 12. 



Now, if we study the four following figures, we shall see the 

 principal action of lenses on light incident on their surfaces. Fig. 

 1 3 shows that if a radiant is placed at the principal focus of a con- 

 verging lens, the rays are rendered parallel ; conversely, if parallel 

 rays fall on a converging lens, they are brought to a principal focus 

 or point upon the axis. 



Fig. 14 shows that if a radiant be placed beyond the principal 



