26 CONSTRUCTION OF THE MICROSCOPE. 



be refracted into r l,r' I', which diverge accurately from 

 the virtual focus. 1 



Spherical aberration is not so much connected with the 

 focal length of the lens as depending on the relative con- 

 vexity of its surfaces, and is much reduced by observing 

 a certain ratio between the radii of its anterior and pos- 

 terior surfaces; thus the spherical aberration of a lens, 

 the radius of one surface of which is six or seven times 

 greater than that of the other, as in fig. 14, is very much 



Fig. M. 



less when its more convex surface is turned forward to 

 receive parallel rays, than when its less convex surface is 

 turned forwards. 



This is still better effected, or even got rid of altogether, 

 by using combinations of lenses, so disposed that their 

 opposite aberrations shall correct each other, whilst mag- 

 nifying power is gained. For it is seen that, as the aber- 

 ration of a concave lens is just the opposite of that of a 

 convex lens, the aberration of a convex lens 

 placed in its most favourable position may be 

 corrected by a concave lens of much less 

 power in its most favourable position. This 

 is the principle of a combination proposed by 

 Sir John F. W. Herschel, fig. 15, an " aplanatic 

 doublet," consisting of a double-convex lens 

 and a meniscus; a doublet of this kind is 

 found extremely useful and available for micro- 

 scopic purposes : it affords a large field, like 

 the Coddington lens. 



Chromatic aberration. Another and serious difficulty 

 arises, in the unequal refrangibility of the different 



(1) Brewster's "Treatise on Optics." 



Fig. 15. 



