SIMPLE MICROSCOPICAL LEN323. 



31 



got, lenses would not only be very nearly perfect, but 

 spherical aberration would be nearly overcome. But 

 even this serious defect can be considerably reduced in 

 practice by observing a certain ratio between the radii 

 of the anterior and posterior surfaces of lenses ; thus 

 the spherical aberration of a lens, the radius of one 



FIG. 16. 



surface of which is six or seven times greater than 

 that of the other, as in fig. 16, will be much reduced 

 when its more convex surface is turned forward to 

 receive parallel rays, than when its less convex surface 

 is turned forwards. 1 



Two forms of lenses may be so combined, that their 

 opposite aberrations shall neutralize each 

 other, and magnifying power be gained. The 

 aberration of a concave lens is exactly the 

 opposite of that of a convex lens, so that 

 the aberration of a convex lens placed in 

 its most favourable position may be cor- 

 rected by a concave lens of much less power 

 in its most favourable position. This prin- 

 ciple of a combination was proposed by Sir 

 John F. W. Herschel; his "aplanatic doublet," fig. 17, 

 consists of a double-convex lens and a meniscus. A 

 doublet of this kind is an extremely useful and avail- 

 able one for microscopic purposes. By a skilful com- 

 bination of crown and flint glass lenses with spherical 

 curves assisted by the Lister adjusting collar, or, 

 what is even more efficient, the homogeneous immersion 



(1) It must be borne in mind that in lenses having curvatures of the kind 

 the object would only be correctly seen in focus at one point the mathe- 

 matical or geometrical axis of the lens. 



