speculum metal


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speculum metal

(spek -yŭ-lŭm) An alloy of copper and tin used by Newton and his successors to make telescope mirrors because it could easily be cast, ground to shape, and took a good polish. It was made obsolete by the use of glass provided with a thin highly reflective silver coating.
References in periodicals archive ?
This fully functioning recreation of Sir Isaac Newton's reflecting telescope features a 70-mm f/4 primary mirror (made of glass rather than the speculum metal that Newton used), and a tube wrapped in waxed parchment paper.
Mirrors were cast from molten speculum metal that was poured into molds of loam and charcoal or, in the case of the larger reflectors, compressed horse dung.
Despite the great promise of silvered mirrors made by Leon Foucault in France and others, the overly cautious members of the Southern Telescope Committee doubted the ability of glassmakers to cast a large blank and regarded speculum metal as the tried and tested material.