1. Allen, C. W.: 1963, Astrophysical Quantities, 2nd ed., Athlone Press, London.
2. Altrock, R. C.: 1967, National Center for Atmospheric Research Cooperative Thesis No. 9.
3. Altrock, R. C.: 1968, Solar Phys. 5, 260.
4. Beckers, J. M. and Schröter, E. H.: 1969, Solar Phys. 7, 22.
5. Bray, R. J. and Loughhead, R. E.: 1965, Sunspots, John Wiley, New York.