1. Avignon, Y., Martres, M. J., and Pick, M.: 1964, Ann. Astrophys. 27, 3.
2. Beckers, J. M. and Schröter, E. H.: 1969, Solar Phys. 10, 384.
3. Bhatnagar, A.: 1967, Kodaikanal Observ. Bull. No. 180, Series A.
4. Bumba, V.: 1960, Izv. Krymsk. Astron. Observ. 23, 253.
5. Bumba, V., Kleczek, J., Olmr, J., R??i?ková-Topolová, B., and Sýkora, J.: 1968, in K. O. Kiepenheuer (ed.), ?Structure and Development of Solar Active Regions?, IAU Symp. 35.