1. Allen, C. W.: 1963, Astrophysical Quantities, 2nd ed., The Athlone Press, London, p. 41.
2. Bely, D. and Van Regemorter, H.: 1970, Ann. Rev. Astron. Astrophys. 8, 329.
3. B�hm, K.-H.: 1960, in Stellar Atmospheres (ed. by J. L. Greenstein), University of Chicago Press, Chicago, p. 97.
4. Burgess, A., Hummer, D. G., and Tully, J. A.: 1970, Phil. Trans. Roy. Soc. A266, 225.
5. Burke, P. G., Cooper, J. W., and Ormonde, S.: 1966, Phys. Rev. Letters 17, 345.