1. Beckers, J. M.: 1964, Dissertation, Utrecht (= AFCRL Environmental Research Paper No. 49).
2. Beckers, J. M.: 1966, The Fine Structure of the Solar Atmosphere (ed by K. O. Kiepenheuer), Franz Steiner Verlag, Wiesbaden, p. 56.
3. Beckers, J. M.: 1968, Solar Phys. 3, 258.
4. Blamont, J. E. and Carpentier, G.: 1967, Solar Phys. 1, 180.
5. Edmonds, F. N., Jr.: 1961, Astrophys. J. Suppl. Ser. 6, 357.