The first 48-inch speculum mirror cast by Sir William Herschel in 1785 for his 'Forty-Foot' telescope at Slough, England.
The astronomer William Herschel made this 48-inch concave metal mirror for his '40-Foot Telescope' at Slough, England. Cast in 1785, the mirror is made of speculum, a bronze alloy with arsenic for a more lustrous finish. Weighing only half a ton, the mirror proved to be too thin to hold its shape properly once it had been ground and polished to its final form. A few years later, a heavier second mirror was cast which was used in preference to the original one. Completed in 1787, Herschel's great telescope soon became a wonder of the age. It quickly became a landmark of Slough visited by the rich and famous visiting the king at nearby Windsor.