1. Beckers, J. M. and Wagner, W. J.: 1970, Appl. Opt. 9, 1933.
2. Billings, D. E.: 1966, A Guide to the Solar Corona, Academic Press, New York.
3. Charvin, P.: 1965, Ann. Astrophys. 28, 877.
4. Charvin, P.: 1971, in R. Howard (ed.), ?Solar Magnetic Fields?, IAU Symp. 43, 580.
5. Dunn, R. B.: 1966, Instr. Soc. Am. 5, 119.