1. Bartels, J., Abh. Akad. Wissenschaften (Göttingen, 1951). Chapman, S., and Bartels, J., “Geomagnetism”, 2 (1940), supplemented by the yearly surveys of character figures.
2. Kiepenheuer, K. O., “The Sun”, chapter 6 (1953).
3. Öpik, E. J., Z. Astrophys., 35, 43 (1954).