1. Dreiser, Theodore: 1914, The Titan, World Publishers, Cleveland, Ohio.
2. Hale, G. E.: 1892, Astron. Astrophys. 11, 790.
3. Hale, G. E.: 1895, Astrophys. J. 1, 80.
4. Hale, G. E.: 1905, Contributions from the Mount Wilson Observatory 1, 29.
5. Wright, Helen: 1966, Explorer of the Universe, Dutton, New York.