An unfinished 9-inch Newtonian primary mirror of glass. When finished this mirror would probably be made to have a focal length of around 10ft. It has been attributed to Herschel which is unusual as it is made of glass rather than speculum metal.

In a Newtonian reflector telescope the eyepiece is at the side of the tube. Light comes in at one end, is reflected off the objective (or primary) mirror and onto a secondary mirror which directs the light towards the eyepiece. William Herschel, with help from his brother Alexander and his sister Caroline made a number of these mirrors to go in the telescopes they made to use and to sell. A mirror of this size and focal length would have been used in one of their 10ft telescopes.

The actual diameter of this mirror is 9.29 inches (235.97mm), 0.33 inches thick. It is plane & rough-ground on the back. The back and the edge are stained with rouge. The surface is partly polished, the edges being least finished. Its focal length is of the order of 10ft. Probably used with 20ft before 20ft was converted to the front view form.