Section of the machinery of the Great Rose telescope, 1845.

Engraving from the Illustrated London News, showing the machinery used to move the telescope known as 'The Leviathan of Parsonstown'. The Rose telescope, an instrument with a six foot speculum (mirror), was the largest telescope in the world for 75 years, only being superseded by the 100-inch reflector at Mount Wilson, California in 1917. It was built by William Parsons, the 3rd Earl of Rose, at Birr Castle in Ireland between 1842 and 1844. In the tradition of William Herschel, Rose used reflecting telescopes because their light-gathering power was superior to that of lens telescopes. Rose discovered the spiral structure of the Andromeda nebula with the telescope in 1850.