0 a type of mirror made from speculum metal (= a mixture of copper and tin) that was used in old telescopes (= devices for looking at stars and other things that are far away):
A speculum mirror will bend three times as much as a glass one because it will be three times as heavy.
The largest ever speculum-mirror-based telescope was built by William Parsons.
The radius of curvature of the speculum mirror was 6.55 metres.
Lassell's 47-inch reflector, made in 1860, was the last large reflector with a speculum mirror.
