DIAMOND MICROSCOPES

were first suggested by Dr. Goring, and have been well executed by Mr. Pritchard. Previous to grinding a diamond into a spherical figure, it should be ground flat and parallel upon both sides, that by looking through it, as opticians try flint gla , we may see whether it has a double or triple refractive power, as many have, which would render it usele as a lens. Among the 14 different crystalline forms of the diamond, probably the octahedron and the cube are the only ones that will give single vision. It will, in many cases, be advisable to grind diamond lenses, plano-convex, both because this figure gives a low spherical aberration, and because it saves the trouble of grinding one side of the gem. A concave tool of cast iron, paved with diamond powder, hammered into it by a hardened steel punch, was employed by Mr. Pritchard. This ingenious artist succeeded in completing a double convex of equal radii, of about 1 ⁄ 25 of an inch focus, bearing an aperture of 1 ⁄ 30 of an inch with distinctne upon opaque objects, and its entire diameter upon transparent ones. This lens gives vision with a trifling chromatic aberration; in other respects, like Dr. Goring’s Amician reflector, but without its darkne , its light is said to be superior to that of any compound microscope whatever, acting with the same power, and the same angle of aperture. The advantage of seeing an object without aberration by the interposition of only a single magnifier, instead of looking at a picture of it with an eye-gla , is evident. We thus have a simple direct view, whereby we shall see more accurately and minutely the real texture of objects.