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 ?
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.
On the other hand, modern mirrors are better than those made of speculum metal, and eyepieces have advanced enormously from the 19th century's simple, uncoated designs with narrow fields of view.