Page:Journal of the Asiatic Society of Bengal Vol 1.djvu/279

1832.] of the metal, whereas the thick parts of the Japan mirror are those which appear illuminated. A short analysis of the facts in either case will readily explain to what these discrepancies are attributable, but it will be necessary to have recourse to a diagram.

Let A B F.VI.Jig. 1, be a plane mirror, upon which the rays of light R impinge : they will be reflected uniformly in R/ forming a clear image.

Now let A B C D E F G be another reflecting surface, having two convexities, B C, E F; and one concavity in the centre, D ; (the condi- tions nearly of the brass button.) In this case the light R R reflected from the outer concave flexures of the protruding portions of the surfaces B C, E F, will converge in the foci b, c, and e, /*, respectively, at distances corresponding to the radius of their curvature : the effect will of course be visible within wide limits of the actual focus. In most of the buttons however the central depression is so great, that it collects the rays in a focus f/, a few inches only in front of the surface ; and when the spectrum is thrown farther off, the rays, crossing, form two less distinct luminous foci at d' d'.

It follows from analogy that the thin parts, or tympanum, of the Japanese mirror, are slightly convex with reference to the rest of the reflecting surface, which may have been caused either by the ornamental work having been stamped, or partially carved with the hammer and chisel on its back ; or, which is more probable, that part of the metal was by this stamping rendered in a degree harder than the rest, so that in polishing it was not worn away to the same extent. That the curvature is extremely small, is proved by the circumstance of the image being nearly equally well defined at the distance of two or twenty feet, or within all the limits at which I had an opportunity of trying it.

Advantage might be taken of this exceedingly simple principle, to produce a multitude of curious effects ; and I have little doubt that, in the skilful hands of an English artist, it will be converted into a philosophical toy, as amusing as the optical paradox, or even the far-famed kaleidoscope itself.

IV. — Description of the Native Manufacture of Steel in Southern India. [Extracted from the manuscript journals of the late Dr. Voysey.]

Kona samundram, (corner of the sea,) situated about 12 miles south of the Godaveri, and 25 from Ntrmul, is celebrated for its manufacture of steel, the chief part of which goes to Persia ; the following are the chief details of this manufacture, collected by repeated visits to the place, aud personal inspection of the process.