Speculum metal
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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 used it for the mirror of his "Leviathan of Parsonstown" telescope. Has the unfortunate property of tarnishing rapidly, requiring constant re-polishing.
Use of speculum metals for mirrors declined after 1859, when Leon Foucault published his results on silvered glass parabolical mirrors.