Original mirror for William Herschel's 40 foot telescope, 1785.

The British astronomer Sir William Herschel (1738-1822) made this 48-inch concave metal mirror for his 40 foot telescope at Slough, Berkshire. 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 and 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, visited by the rich and famous visiting the king at nearby Windsor Castle.