This is a polishing frame, a ratchet for polishing mirrors up to 7-inches diameter. It is a component of one of William Herschel's smaller polishing machines.

When the Herschels were polishing telescope mirrors they fixed them in a special frame to ensure an even finish. The mirrors were made of a metal alloy (speculum, an alloy of coper and tin) which tarnished fairly easily, so they had to be re-polished at intervals.

William Herschel developed machines for polishing mirrors while building his 40ft telescope when the mirrors he was making became to large for the work to be done by hand. After 1789 when the 40ft was completed he started to apply the same principles to building polishing machines for smaller mirrors.