In the spring of 1845, William Parsons, the third Earl of Rosse, began observing with his great six-foot telescope... The Earl was excited by what he was the first human to see: spiral patterns of stars, seemingly swirling in great 'spiral convolutions' about the centre of the galaxy.... No one could ever have seen the spiral pattern of stars in a galaxy unless they had looked through Rosse's telescope or seen his drawings.... I believe that Van Gogh would have seen those drawings in the press following the publicity attracted by them, or in Flammarion's book... and gained his astronomical inspiration from them.
Cosmic Imagery: Key Images in the History of Science (2008)