A hand-coloured illustration of William Herschel's massive reflecting telescope with a focal length of forty feet, which was erected at his home in Slough. Completed in 1789, the telescope became a local tourist attraction and was even featured on Ordnance Survey maps. By 1840, however, it was no longer used and was dismantled, although part of it is now on display at the Royal Observatory, Greenwich. This image of the telescope was engraved for the Encyclopedia Londinensis in 1819 as part of its treatment of optics.