Heber D. Curtis. Modern Theories of the Spiral Nebulae. Offprint: Smithsonian Report for 1919, pp 123-132, publication 2591, Washington, DC, Government Printing Office, 1921. Original wrappers. Fine copy. $65
- On Curtis, see Robert G. Aitken, “Heber Doust Curtis 1872–1942,” in Biographical Memoirs. National Academy of Sciences, 22 (1943), 275–294.