1. Richard Berendzen, Richard Hart, and Daniel Seeley, Man Discovers the Galaxies (New York: Science History Publications, 1976), 145-146.
2. Quoted in Raymond S. Dugan, “Max Wolf,” Popular Astronomy, 41, 5 (1933):239.
3. Max Wolf, "On the dark nebula NGC 6960," Astronomische Nachrichten, 219 (1923), 109-116
4. translated by Brian Doyle and Owen Gingerich in Kenneth R. Lang and Owen Gingerich, eds., A Source Book in Astronomy and Astrophysics, 1900-1975 (Cambridge, Mass. and London, England: Harvard University Press, 1979), 568-570. The region chosen by Wolf for analysis is the so-called Cygnus Loop, now recognized as a supernova remnant. Wolf correctly shows by star counts that the obscuration here is caused by dust rather than by gas.
5. M. Wolf, “Die Klassifizierung der Kleinen Nebelflecken,” Publ. Astrophys. Inst. Konigstühl-Heidelberg, 3 (1909), 109-112.