Polishing disc or possibly grinders for large mirrors.

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 copper 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 too 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.

Grinders for large mirrors - these are 2 iron discs, one is 34 inch diameter, 1 inch thick, and pierced by a central hole 5 inches in diameter. The other is 21.5 inches in diameter, 1,75 inches thick and solid throughout.

These discs were found at Observatory House leaning up against the wall behind the section of the 40ft telescope. They were much rusted at the time, but in the 1980s were grit blasted and waxed at the NMM.