JEWEL LENSES. 



power, one of glass and the other of diamond, the latter will have 

 less convexity than the former. 



From what has been explained on the subject of spherical 

 aberration, in our Tract upon Optical Images, it will be under- 

 stood, that the more convex a lens is, the less its diameter must 

 be, for if its diameter exceeds a certain limit relatively to its con- 

 vexity, the spherical aberration will become so great as to render 

 all vision with it confused and indistinct. This is the reason 

 why all lenses of high magnifying power and short focal length 

 are necessarily small. 



16. But since the spherical aberration depends on, and increases 

 with the convexity of the lens, other things being the same, it 

 follows that if two lenses, composed of different materials, have 

 equal focal lengths, that which has the less convexity will also 

 have less spherical aberration. 



17. NoWj as according to what has been explained above, a 

 diamond lens has less convexity than a glass lens of the same focal 

 length, it will, if it had the same diameter, have less spherical 

 aberration, or, what is the same, it will admit of being formed 

 with a greater diameter, subject to the same aberration. 



18. In lenses of high magnifying powers, and which are con- 

 sequently of small dimensions, any increase of the diameter which 

 can be made ' without being accompanied with an injurious 

 increase of aberration, is attended with the advantage of trans- 

 mitting more light from each point of the object to the eye, and 

 therefore of rendering the object more distinctly visible. It was 

 on this account that, when single lenses of high magnifying 

 power were thought desirable, ' great efforts were made to form 

 them of diamond, and other transparent gems having a refracting 

 power greater than that of glass. 



19. Sir David Brewster, who first suggested the advantage of 

 this, succeeded in getting lenses of great magnifying power, made 

 of ruby and garnet ; he considered those made from the latter 

 stone to surpass every other solid lens : the focal length of some of 

 those made for him was less than the l-30th of an inch, the mag- 

 nifying power being more than 300. 



20. All these and similar efforts made by Messrs. Pritchard and 

 Varley, aided by the genius and science of the late Dr. Goring, 

 have, however, happily for the progress of science, been subse- 

 quently rendered unnecessary by the invention of methods of pro- 

 ducing good achromatic object-glasses of high power for compound 

 microscopes, so that the range of usefulness of simple microscopes, 

 or magnifying glasses, is now limited to uses and researches in 

 which comparatively low magnifying powers are sufficient. 



21. The most feeble class of magnifying glasses are those occa- 



105 



