POWERHOUSE COLLECTION

Speculum mirror for reflecting telescope made by Sir William Herschel

Object No. H10259

There are two main types of telescopes. One uses a curved reflecting mirror to capture an image of the astral bodies the other uses a refracting lens to magnify its image. This metal speculum mirror was made for use in an early reflecting telescope and were usually made from composition of two parts copper to one part tin. These early mirrors were made before the invention of technologies which could place a silvered mirror surface on glass and were instead made entirely from metals, mainly copper. The molten metal was cast and then slowly cooled before being carefully ground and polished, a process that required a high level of patience and expertise. However in these early years it was easier to make large speculum mirrors than large lenses made from imperfect glass. While they could reflect around 65 % of the available light they could also get very heavy, for example the speculum mirror and box for the 'Great Melbourne Telescope' weighed 3500 lbs. This speculum mirror is said to have been made by the famous English astronomer Sir William Herschel for use in a reflecting telescope. At the age of 35, Herschel embarked on what would become an astonishingly successful investigation of the stars. Initially he used cheap homemade lens telescopes but he quickly moved to reflecting mirrors and made his first speculum mirror in 1773. In 1782 he moved to Datchet near Windsor Castle where he began to make astronomical equipment commercially and this mirror was probably made sometime after this. These mirrors were extremely labour intensive to produce and in a Memoir published by Herchel's sister Caroline in 1876 she describes her brother polishing a mirror continuously for sixteen hours. The mirror's association with this pioneer astronomer and its early date define the international significance of this object. References Glass, I. S., Victorian Telescope Makers; the Lives and Letters of Thomas and Howard Grubb, Institute of Physics Publishing, Bristol and Philadelphia, 1997 Gascoigne, S.C.B, 'The Great Melbourne Telescope', in Historical Records of Australian Science, Volume 10, Number 3, Panther Publishing, Canberra, Australia, 1995 King, H., C., The History of the Telescope, Dover Publications, New York, 1955 Geoff Barker, March, 2007