A Newtonian primary mirror of speculum with a black metal cover. The mirror and cover are in a padded wooden box. The box is labelled:
'9 inch Plane Speculum Mirror'.

This mirror belonged to the Royal Greenwich Observatory (RGO) and was presented to the Museum in 1981 when the astronomers were based at Herstmonceux Castle (after leaving the Observatory's original site in Greenwich in the 1950s and before moving to their final location in Cambridge in the 1990s). The mirror is thought (presumably because of its dimensions and the type of metal) to have been made by William Herschel though the astronomers at the RGO had no accompanying documentation about its origin to corroborate this.

If one of those made by Sir William Herschel in the late 18th, early 19th century this would have been used with one of Herschel's 10 foot telescopes which he made both for himself and for sale.