Casting a telescope mirror. This illustration, Casting the speculum metal mirror of the Great Melbourne Telescope (1866), depicts workers at Grubb's works, Dublin. They are casting the mirror by heating the metal in the furnace at right, then lifting it in a pot to the tilting apparatus at lower centre. It will then poured into a circular mould and placed into the annealing furnace at the back of the room. Annealing is a heat treatment that alters the microstructure of a material. The Great Melbourne Telescope remained the largest equatorially mounted instrument for several decades after its completion in 1868.