During a recent trip to London, I paid a visit to the Science Museum. In the space gallery were the old favourites of the mock-up Lunar Module and various rockets and space vehicles. The one item
that really caught my eye however, was the first mirror that Willaim Herschel made for his great 40-foot telescope in 1785. This is a 36-inch mirror and is cast from Speculum metal. For nearly 60 years this was the largest telescope mirror in the World!
Despite weighing half a ton the mirror was actually too thin and so Herschel decided to make a second, thicker mirror in 1789. In spite of its problems, this original mirror was often used whilst the new main mirror underwent repolishing. Using the 40-foot telescope, Herschel discovered two new moons of Saturn, Enceladus and Mimas.