==References==;Notes;Further reading== External links ==*Angular Rotations of Spiral Nebulae, Hubble, E., "ApJ", 81, 334 (1935) – NADS*Internal Motions in Spiral Nebulae, Van Maanen, A., "ApJ", 81, 336 (1935) – NADS*Adriaan van Maanen and internal motions in spiral nebulae: A historical review, Quarterly Journal of the Royal Astronomical Society, Vol.
* Angular Rotations of Spiral Nebulae = Угловое вращение спиральных туманностей, Эдвин Хаббл, "ApJ", 81, 334 (1935) — NADS* Internal Motions in Spiral Nebulae, Van Maanen, A., "ApJ", 81, 336 (1935) — NADS* Adriaan van Maanen on the significance of Internal Motions in Spiral Nebulae, Hetherington, N.S., "J Hist Astron", 5.