40ft
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.
The apparatus used for grinding mirrors consists of two iron discs, 4ft in diameter, and 2 inches thick.
The apparatus used for polishing mirrors consists of a circular iron frame, 4ft 6 inches in diameter, with ratcheted arcs all painted black, and a very rusty disc, coated with graphite enriched wax, which, after being cleaned by sand blasting, was then polished.
One disc of the grinding apparatus was found exposed in the centre of the lawn at Observatory House in Slough (the Herschel family home) in or near the middle of the area originally occupied by the mounting of the 40ft telescope. The other was buried a few feet to the South West of it, at a depth of about 15 inches.
The polishing apparatus hung on the north wall of the attic at Observatory House.
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.
The apparatus used for grinding mirrors consists of two iron discs, 4ft in diameter, and 2 inches thick.
The apparatus used for polishing mirrors consists of a circular iron frame, 4ft 6 inches in diameter, with ratcheted arcs all painted black, and a very rusty disc, coated with graphite enriched wax, which, after being cleaned by sand blasting, was then polished.
One disc of the grinding apparatus was found exposed in the centre of the lawn at Observatory House in Slough (the Herschel family home) in or near the middle of the area originally occupied by the mounting of the 40ft telescope. The other was buried a few feet to the South West of it, at a depth of about 15 inches.
The polishing apparatus hung on the north wall of the attic at Observatory House.
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