1. On the Distribution of the Solar Spots in Latitude since the Beginning of the Year 1854; with a Map
2. Hale, G. E. & Nicholson, S. B. 1938, Magnetic Observations of Sunspots 1917-1924 (Publ. Carnegie Inst. 498; Washington, DC: Carnegie Inst.)
3. Harvey, K. J. 1992, in ASP Conf. Ser. 27, The Solar Cycle, ed. K. J. Harvey (San Francisco: ASP), 335
4. Kiepenheuer, K. O. 1953, The Solar System, Vol. 1, The Sun, ed. G. P. Kuiper (Chicago: Univ. Chicago Press), 322