into Lenses for Microscopes. 153 



their curves being similar ; (or, in other words, the superficial 

 amplification of an object, with the perfect diamond lens before- 

 mentioned, is 225000 times, while a single magnifier made of 

 glass, amplifies only 3136 times, reckoning 6 inches as the 

 standard of distinct vision .) Thus the diamond will enable us 

 to gain more power than it is possible to procure by lenses of 

 glass ; for the focal distance of the smallest glass lens which I 

 have been able to grind and polish is about the ^^^th of an 

 inch focal length, while that of a diamond, worked in the same 

 tools, would be only the ^uo^h of an inch. 



If we wish to compare the aberrations of the two lenses when 

 of equal power, the curvature of the glass must be increased ; 

 and as it is well known the lateral aberrations increases inverse- 

 ly as the square of the radius, (the aperture and position re- 

 maining the same,) the aberration of the diamond lens will on- 

 ly be about ^^^th of that produced by the glass one, even when 

 their thickness is the same ; but as the curvature of the dia- 

 mond is less, the thickness may be greatly diminished. The 

 chromatic dispersion of the adamant being nearly as low as that 

 of water, its effects in small lenses can barely be appreciated by 

 the eye, even in the examination of that valuable class of test 

 objects, which require enormous angles of aperture to be ren- 

 dered visible, which it is evident must be of easier attainment 

 by diamond magnifiers than by any other sort of microscope. 



A mathematical investigation of the spherical aberration of 

 the diamond when formed into lenses, I hope to lay before the 

 public at a future opportunity. The comparative numbers here 

 taken from the longitudinal aberration are, I believe, sufficient- 

 ly accurate for practical purposes. 



Andrew Peitchard. 



312, Strand, opposite Somerset House. 



Postscript. — Since writing the above paper, my attention has 

 been steadfastly devoted in search of a substitute in place of the 

 diamond, which might rank next to that invulnerable substance, 

 and superior to glass, so that I could procure superior amplifi- 

 cation over lenses formed of the latter, combined with the most 

 important property of the diamond, viz. that of obtaining a given 

 power with shallow curves. For the difficulty of working that 



