A Newtonian primary mirror made of glass in a box. The mirror is in its original iron cell with a narrow brass retaining rim and a close-fitting tin cover. It is transparent and is mounted on velvet. It was not removed from its cell. The general figure of the mirror is very even, except for a small central spot of long focus, surrounded by a narrow zone of short focus of radius 1 inch and a narrow zone of short focus about 1 inch from outer edge.

William Herschel, with help from his brother Alexander and his sister Caroline made a number of Newtonian primary mirrors in speculum metal to go in the telescopes they made to use and to sell. This mirror was found in the Herschel family home in the 1920s, though there is no supporting evidence to suggest the family made mirrors in any material other than speculum metal. It is possible this mirror was bought from a maker outside the family or that it was an experiment by the family.