1. Mathews, W. G., Astrophys. J., 157, 583 (1969).
2. Wehner, G. K., Phys. act., 112, 1120 (1958).
3. Henschke, E. B., Phys. Rev., 121, 1286 (1961).
4. Greenberg, J. M., Nebulae and Interstellar Matter (edit. by Middle- hurst, B. M., and Aller, L. H.), 243 (Univ. Chicago Press, Chicago, 1968).
5. Mathews, W. G., Astrophys. J., 147, 965 (1967).