Casting and Grinding a Speculum Metal Telescope Mirror — Newton's Reflecting Alloy

When Isaac Newton built his reflecting telescope in 1668, he needed a concave mirror to focus light — but silvered glass mirrors did not yet exist. Instead, he cast his mirror from speculum metal, a bronze alloy of approximately 68% copper and 32% tin that can be polished to a brilliant, highly reflective surface. Speculum metal had been known since antiquity — the Romans used it for hand mirrors (speculum is Latin for mirror) — but Newton was the first to grind it into a precise concave paraboloid for astronomical use. The alloy is hard, brittle, and takes a superb polish, but it tarnishes in air and must be periodically repolished. Every reflecting telescope before the 1850s used speculum metal mirrors, including William Herschel's great telescopes that discovered Uranus and mapped the Milky Way. This blueprint casts a speculum metal disc from copper and tin ingots, then grinds and polishes it into a concave mirror suitable for a Newtonian reflecting telescope.