1. Aller, L. H., Ufford, C. W., and Van Vleck, J. H.: 1949, Astrophys. J. 109, 42.
2. Basov, N. G., Boiko, V. A., Voinov, Yu. P., Kononov, E. Ya., Mandel'shtam, S. L., and Sklizkov G. V.: 1967, J. E. T. P. Letters 5, 141.
3. Boiko, V. A., Voinov, Yu. P., Gribkov, V. A., and Sklizkov, G. V.: 1970, Opt. Spectrosc. 29, 545.
4. Bowen, I. S.: 1960, Astrophys. J. 132, 1.
5. Deutschman, W. A.: 1967, Ph. D. Thesis, Department of Astro-Geophysics, University of Colorado Boulder, Colo.