1. The three books [1], [2], [3] listed below appeared in 1953 and 1955. They deal extensively with the subject of this article. For that reason it has not been tried to reach completness as to the literature before 1953; only some of the most important papers are mentioned. It was attempted to make the later references more complete.
2. Various authors: The Sun. Vol. I of “The Solar System”; ed. G. P. Kuiper. Chicago, Ill.: Chicago University Press 1953.
3. Unsöld, A.: Physik der Sternatmosphären, 2. Aufl. Berlin: Springer 1955.
4. Waldmeier, M.: Ergebnisse und Probleme der Sonnenforschung, 2. Aufl. Leipzig: Akademische Verlagsgesellschaft 1955.