speculum

A brittle white alloy of two parts copper to one part tin that takes a very high polish and was used for telescope mirrors until the late 19th century. The manufacture of speculum mirrors reached its apex with William Parsons, third Earl of Rosse, who managed to produce several 36-inch (91-cm) Newtonians and later, in 1845, a colossus 72 in. (1.8 m) in diameter. However, speculum, which is difficult to work with and quick to tarnish (forcing a repolishing of the optical surface itself) was made obsolete in astronomy with the development of silvering and aluminizing.

Related category

• HISTORY OF ASTRONOMY

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