Speculum metal

Speculum metal is a very hard white alloy of four parts copper to one part tin, or according to other sources, 67% copper and 33% tin; some compositions contained 1–2% of arsenic. Composition with 45% tin has more resistance to tarnishing.

It was used by some early telescope makers. For instance, Lord Rosse in 1845 used the alloy for the 72-inch mirror of his "Leviathan of Parsonstown" telescope. The metal has the unfortunate property of tarnishing rapidly, requiring constant re-polishing that can easily disfigure a telescope mirror.

Use of speculum metals for mirrors declined after 1859, when Leon Foucault published his results on silvered glass parabolical mirrors.