1. Altrock, R. C.: 1968, Solar Phys. 5, 260.
2. Beckers, J. M.: 1969a, Solar Phys. 9, 372.
3. Beckers, J. M.: 1969b, AFCRL Physical Sciences Research Papers No. 371.
4. Gasson, R. E. M. and Pagel, B. E. J.: 1966, Observatory 86, 196.
5. Grevesse, N. and Swings, J. P.: 1968, Compt. Rend. 266, 110.