An 18.7-inch primary Herschelian mirror of speculum metal with a focal length of around 20ft. This is ‘No. 3’ of three mirrors, taken to and brought back from the Cape (South Africa) by Sir John Frederick William Herschel. The other two are at the Radcliffe Observatory in Oxford and the Royal Observatory, South Africa (now the South African Astronomical Observatory). This mirror was made by John and his father William working together.

This mirror has a diameter of 18.7in and rests in an iron cell, covered by a tin lid. The mirror and container are then fitted in a square wooden box with lid and iron handles. The polish on the mirror is in good condition, and there is very little tarnishing, except for two or three small patches. The present figure of the mirror is due to Sir J F W Herschel.