A primary Gregorian mirror of speculum metal. William Herschel made and used Gregorian telescopes before moving on to Newtonian and later Herschelian (to his own design) telescopes.

This mirror is without a cell and is lying loosely in a tin case with a cover. The general figure of mirror is very even from edge to centre, except for a fairly narrow and shallow zone of short focus just outside the central hole. From a zonal test it was found that the mirror is considerably over-corrected, as tested under a slightly rising temperature. A falling temperature would probably increase the over-correction still further.