Model of Northumberland refractor, 1839. The label displayed with this model at the Royal Greenwich Observatory says: 'Northumberland Refractor 1839. At the beginning of the nineteenth century glassmakers were learning how to make discs of good quality glass suitable for large lenses, and by 1823 Cauchoix in Paris was able to polish lenses of nearly 12 inches diameter. One of these was bought by the Duke of Northumberland in 1839 and used to make a telescope for the Cambridge Observatory.'