1. Avrett, E. H.: 1981, in L. E. Cram and J. H. Thomas (eds.), The Physics of Sunspots, Sacramento Peak Obs., Sunspot, New Mexico.
2. Chistyakov, V. F.: 1962a, Soviet Astron. 5, 471.
3. Chistyakov, V. F.: 1962b, Soviet Astron. 6, 363.
4. Collados, M. and Bonet, J. A.: 1984, Appl. Optics 23, 2827.
5. Collados, M., del Toro, J. C., and Vázquez, M.: 1987, in E. H. Schröter, M. Vázquez, and A. Wyller (eds.), D-E-S Inaugural Workshop, Cambridge University Press, Cambridge.