1. Baranovsky, E. A. and Stepanov, V. E.: 1959; Izv. Krymsk. Astrophys. Obs. 21, 180.
2. Blackwell, D. E., Mallia, E. A., and Petford, A. D.: 1969, Monthly Notices Roy. Astron. Soc. 146, 93.
3. Brandt, P. N.: 1969, Solar Phys. 7, 181.
4. Bray, R. J. and Loughhead, R. E.: 1964, Sunspots, Chapman and Hall, London.
5. David, K. H.: 1961, Z. Astrophys. 53, 37.