A mahogany disc (8.9 inches diameter, 0.8 inches thick) on which the pitch polishers were formed and on top of which the mirrors were moved in polishing.

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.

The face is smooth and slightly convex with a few particles of pitch adhering to it.