speculum metal


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speculum metal

n
(Metallurgy) a white hard brittle corrosion-resistant alloy of copper (55–70 per cent) and tin with smaller amounts of other metals. It takes a high polish and is used for mirrors, lamp reflectors, ornamental ware, etc
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Despite the adaptation of a process for depositing a thin coat of silver on glass to telescope mirrors by physicists Leon Foucault and Carl August von Steinheil in 1857, casting and silvering such a large glass mirror was as yet untried; the long-term stability of glass as a material and silver as a coating were unknown; and silver, although more reflective than speculum metal, tarnished more quickly.
Speculum metal is much heavier than glass and tarnishes quickly, and mirrors required frequent repolishing.
William Herschel discovered NGC 2420 in 1783 with an 18.7-inch reflector that used a speculum metal mirror.