© Science Museum/Science and Society Picture Library
This is the first of two metal mirrors made by Sir William Herschel for the Great Forty-Foot reflecting telescope that he built in the garden of his home in Slough. The mirror was cast at a London foundry in 1785, using speculum metal, a reflective alloy of mainly copper and tin. It remained the largest mirror in existence until 1845 when Lord Rosse made a 72-inch mirror.
Although weighing over half a ton, Herschel's first mirror was too thin to hold its shape and sagged at the centre when mounted in the telescope. Despite these problems he managed to grind and polish the mirror satisfactorily, coordinating the efforts of ten workmen. In 1787, impatient to try it out, Herschel directed the telescope towards the Orion nebula and crawled into the tube itself with a hand-held eyepiece.
In 1789 the first mirror was superseded by another twice its thickness which, owing to its different composition, tarnished easily. The original mirror continued to be used while the later one was re-polished periodically, using a machine of Herschel's own design.
Although the telescope was a considerable technical achievement, problems with the mirrors and the unwieldiness of the instrument meant that it did not show a substantial improvement over his smaller instruments. With it, however, Herschel discovered Enceladus and Mimas, the sixth and seventh moons of Saturn.