6 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



point of meeting ; and conversely, the greater the refractive index, 

 the more will the oblique rays he deflected towards the axial ray, 

 and the nearer will be their point of convergence. A lens made 

 of any substance whose index of refraction is 1"5, will bring 

 parallel rays to a focus at the distance of its diameter of curvature, 

 after they have passed through one convex surface (Fig. 2), and at 

 the distance of its radius of curvature, after they have passed 

 through two convex surfaces (Fig. 3) ; and as this ratio almost 

 exactly expresses the refractive power of ordinary Crown or plate 



Fig 



Eays diverging froni the farther extremity of one diameter of 

 curvature of a doable-convex Lens, brougnt to a focus at tne same 

 distance on the other side. 



Glass, we may for all practical purposes consider the ' principal 

 focus ' (as the focus for parallel rays is termed) of a double-convex 

 lens to be at the distance of its Radius, that is, in the Centre of 

 curvature, and that of a plano-convex lens to be at the distance of 

 twice its radius, that is, at the other end of the Diameter of its 

 sphere of curvature. 



5. It is evident from what has preceded, that as a Double-convex 

 Lens brings parallel rays to a focus in its Centre of curvature, it 

 will on the other hand cause those rays to assume a parallel direc- 

 tion, which are diverging from that centre before they impinge 

 upon it . (Fig. 3) ; so that, if a luminous body be placed in the 

 principal focus of a double-convex lens, its divergent rays, falling 

 on one surface of the lens as a cone, will pass forth from its other 

 side as a cylinder. If, however, the rays which fall upon a double- 

 convex lens be diverging from the farther extremity of the 

 Diameter of its sphere of curvature, they will be brought to a 

 focus at an equal distance on the other side of the lens (Fig. 4) ; 

 but the more the point of divergence is approximated to the centre 

 or principal focus, the farther removed from the other side will be 

 the point of convergence (Fig. 5), until, the point of divergence 



