20 Mr. Pritchard on Diamond 



be supplied with the finest washed powder till the lens is com- 

 pletely polished. During the process of grinding, the stone 

 should be examined by a magnifying lens, to ascertain whether 

 the figure is truly spherical ; for it sometimes will occur that 

 the edges are ground quicker than the centre, and hence it will 

 assume the form of a conoid, and thus be rendered unfit for 

 microscopic purposes. 



The spherical aberration of a diamond lens is extremely small, 

 and when compared with that of a glass lens the difference is 

 rendered strikingly apparent. This diminution of error in the 

 diamond arises from the enormous refractive power possessed 

 by this brilliant substance, and the consequent increase of am- 

 plification, with very shallow curves. The longitudinal aberra- 

 tion of a plano-convex diamond lens is only 0.955, while that of 

 a glass one of the same figure is 1.166 ; both numbers being 

 enumerated in terms of their thickness, and their convex surfaces 

 exposed to parallel rays. But the indistinctness produced by 

 lenses, arises chiefly from every mathematical point on the sur- 

 face of an object being spread out into a small circle ; these 

 circles, intermixing with each other, occasion a confused view 

 of the object. Now this error must necessarily be in the ratio 

 of the areas of these small circles, which being respectively as 

 the squares of their diameters, the lateral error produced by 

 a diamond lens will be 0.912, while that of a glass lens of like 

 curvature is 2.775 ; but the magnifying power of the diamond 

 lens will be to that of the glass as 8 to 3, their curves being 

 similar ; (or, in other words, the superficial amplification of an 

 object, with the perfect diamond lens before mentioned, is 

 22500 times, while a similar 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 can be well made is 

 about the jr^Qih. of an inch, while that of a diamond, worked in 

 the same tools, would be only the -^Jo^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 aberration increases inversely 

 as the square of the radius, (the aperture and position remain- 



