I've recently come across references to Lord Rosse's original sketch of the M51 galaxy, made in 1845, and found both versions on the internet — one where the spiral structure is depicted in white on a black background, and another where it appears black on a white background. This has left me wondering which representation is actually the original drawing?
His original drawings were, as far as I can tell, on paper, and would have been black-on-white. The drawing of M51 is reproduced from a modern photograph in Hoskin, Michael. “The First Drawing of a Spiral Nebula.” Journal for the History of Astronomy 13 (1982): 97-101. There it's black-on-white. According to that article, it was first published in J. P. Nichol, “Thoughts on some important points relating to the system of the world”, plate after p.22, where it's white-on-black. But other drawings of his were first published as black-on-white, so I doubt that he had a strong preference.
Neither of the depicted images is the original. The white on black appears to be a lithographic print; these were produced from around 1860 onward by others discussing Rosse's work. The black on white appears to be an inversion of the white on black image.
Examples of the drawings as published, which were engravings, can be seen in https://archive.org/details/s3id13528860/page/912/mode/2up e.g. on page 912. These are black on white.
The original drawings in the observatory ledger, were hand sketches using pencil on white paper, and comprised notes, multiple rough sketches, and dimensioned sketches of features. It is believed that the rough sketches were combined into a single ink on paper drawing that was sent to an engraver. There is a note somewhere in one of Rosse’s papers that he accidentally used standard ink rather than india ink, which ran when the mail on the way to the engraver got wet, and the plates from the engraver required rework before they were suitable for publishing.
