Speculum is an alloy of copper and tin with a dash of arsenic---which gives it that extra kick---invented for reflector telescope mirrors by Isaac Newton (1643--1727).
It is brittle, tarnishes easily, and reflects only about 16 % of incident visible light.
By the later 19th century, reflector primary mirror were made of glass (eventually borosilicate glass: see Wikipedia: Borosilicate glass: optics) with a thin reflective coating first of silver and later of aluminum (see Wikipedia: History of the telescope. No more speculum.
The telescope mount was an altazimuth mount with such limited range in azimuth that was close to a transit mount (see Wikipedia: Leviathan of Parsonstown: Side note).
Astronomical objects could be observed for about an hour centered on their transits of the meridian.
The discovery 'twas by Himself, Lord Rosse, in 1845.