1. The main references are listed at the end of the chapter “General physical properties of external galaxies”, p. 311.
2. Earl of Rosse: Phil. Trans. Roy. Soc. Lond. 1850; 1861.
3. S. Alexander: Astronom. J. 2, 95.
4. M. Wolf: Veröff. Königstuhl-Heidelberg 3, Nr. 5 (Plate V).
5. See Ann. Lund Obs. 6 (1937); 9 (1941); 10 (1942).