Cog. Part of the grinding and polishing apparatus for the mirror for Herschel's 40ft reflector telescope (40ft refers to the length of the tube).

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 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.