What is "speculum metal"

Speculum \Spec"u*lum\, n.; pl. L. Specula, E. Speculum. [L., fr. specere to look, behold. See Spy.]

1. A mirror, or looking-glass; especially, a metal mirror, as in Greek and Roman arch[ae]ology.

2. A reflector of polished metal, especially one used in reflecting telescopes. See Speculum metal, below.

3. (Surg.) An instrument for dilating certain passages of the body, and throwing light within them, thus facilitating examination or surgical operations.

4. (Zo["o]l.) A bright and lustrous patch of color found on the wings of ducks and some other birds. It is usually situated on the distal portions of the secondary quills, and is much more brilliant in the adult male than in the female.

   Speculum metal, a hard, brittle alloy used for making the reflectors of telescopes and other instruments, usually consisting of copper and tin in various proportions, one of the best being that in which there are 126.4 parts of copper to 58.9 parts of tin, with sometimes a small proportion of arsenic, antimony, or zinc added to improve the whiteness.

n. A mixture of around two-thirds copper and one-third tin making a white brittle alloy that can be polished into a highly reflective surface.

Speculum metal is a mixture of around two-thirds copper and one-third tin making a white brittle alloy that can be polished to make a highly reflective surface. It is used primarily to make different kinds of mirrors including early reflecting telescope optical mirrors. Speculum metal can also be used as the metallic coating on glass mirrors (as opposed to silver or aluminium) giving a reflectivity of 68–78% in the visible light range of 450–650 nanometres when evaporated onto the surface.