SPECULUM

(Lat. speculum, a mirror.) In Optics, the term speculum is usually appropriated to reflectors formed of polished metal; while the term mirror is used to signify a reflector of gla . For the focal distances and laws of reflexion of spherical specula, see Reflexion; and for a description of the method of casting and polishing large specula for reflecting telescopes, see Sir W. Herschel's Description of his Telescope in the Phil. Trans, for 1795; Brewster's edition of Ferguson's Lectures; and Lord Oxmantown's Account of Experiments on the Reflecting Tele sco|>e, in the Phil. Trans, for 1840. SPECULUM METAL, with which mirrors for reflecting telescopes are made, is an alloy of two parts of copper and one of tin; its whitene is improved by the addition of a little arsenic. [s. 1159]