Note 10 — Introduction
Reading edition
The giant mirrors of Herschel (1789) and Rosse (1842) were made of an alloy of 71% copper and 29% tin, and 68½% copper and 31½% tin, respectively. This alloy was known as “speculum metal.” The silvered glass mirror was pioneered by Steinhill and Foucault in 1856. In England Dr. A. A. Common made considerable use in the 1870’s of silvered glass mirrors made by George Calver. About 1892-97 Common himself made, but never finished, a 60-inch mirror. It was later refigured and is still in use.
On these matters see King, op. cit. (footnote 6).