SPECULUM

, or Mirror, in Optics, any polished body, impervious to the rays of light: such as polished metals, and glasses lined with quicksilver, or any other opake matter, popularly called Looking-glasses; or even the surface of mercury or of water, &c.

For the several kinds and forms of Specula, plane, concave, and convex, with their theory and phenomena, see Mirror. And for their laws and effects, see Reflection and Burning Glass.

As for the Specula of reflecting telescopes, it may here be observed, that the perfection of the metal of which they should be made, consists in its hardness, whiteness, and compactness; for upon these properties the reflective powers and durability of the Specula depend. There are various compositions recommended for these Specula, in Smith's Optics, book 3, ch. 2, sect. 787; also by Mr. Mudge in the Philos. Trans. vol. 67; and in various other places, as by Mr. Edwards, in the Naut. Alm. for 1787, whose metal is the whitest and best of any that I have seen.—For the method of grinding, see Grinding.

Mr. Hearne's method of cleaning a tarnished Speculum was this: Get a little of the strongest soap ley from the soap-makers, and having laid the Speculum on a table with its face upwards, put on as much of the ley as it will hold, and let it remain about an hour: then rub it softly with a silk or muslin, till the ley is all gone; then put on some spirit of wine, and rub it dry with another part of the silk or muslin. If the Speculum will not perform well after this, it must be new polished. A few faint spots of tarnish may be rubbed off with spirit of wine only, without the ley. Smith's Optics, Rem. p. 107.