THE MICROSCOPE. 13 



termed the axial ray. Being perpendicular to both sur- 

 faces, it undergoes no refraction. The rays R S, on 

 emerging from the lens, are bent from the perpendicular 

 C S, and meet in the focus F. In a plano-convex lens the 



FIG. 12. Effect of plano-convex lens. (Hannover.) 



optical centre is placed at the point where the axial ray 

 meets the convex surface ; in a bi-convex lens it is placed 

 within the lens, at its very centre, if the convexity of the 

 surfaces be equal. The distance of the focal point F from 

 the optical centre is the focal distance of the lens. In a 

 plano-convex lens of crown glass the focal distance is almost 

 exactly twice the radius of curvature (C S), while in a bi- 

 convex lens of the same substance having equally curved 

 surfaces, the focal length is very nearly the same as the 

 radius of curvature. But the focal length depends not 

 only upon the degree of curvature of the lens, it also de- 

 pends on the refractive power of its material ; thus, if the 

 focal lengths of two lenses the one of flint, the other of 

 crown glass of equal curvatures be compared, the flint 

 lens is found to have a shorter focus than the other, be- 

 cause of its higher refractive power ( 21). 



24. Principal Focus and Conjugate Foci The 

 principal focus of a convex lens (A, Fig. 13) is the point 

 (F) to which rays that were parallel before entering the 

 lens are brought. (The same is shown in Fig. 14, where 

 P P are the parallel rays, and F the principal focus.) If 

 the rays be divergent (D D, Fig. 14) before entering the 

 lens, they are focalised outside the principal focus (D') ; 

 while, if they be convergent (C C), they are focalised within 



