Telecope mirror polishing machine. Diagrams of the 19th century telescope mirror polishing machine invented by William Lassell of Liverpool, England, and constructed by Nasmyth Gaskell of Manchester, England. The mirrors were called 'speculum', which was the name of the metal alloy used at the time in telescope mirrors. The mirrors needed to be carefully polished to ensure they were of the correct shape and reflectivity. This diagram was published in 1850 in volume 18 of the Memoirs of the Royal Astronomical Society. In 1856-7, a new method of silvering glass mirrors was devised, and glass mirrors eventually replaced speculum mirrors.