1. Högbom, J. A., and Shakeshaft, J. R., Nature, 189, 561 (1961).
2. Westerhout, G., B.A.N., 14, 215 (1958).
3. Mezger, P. G., Z. Astrophys., 46, 234 (1958).
4. Heeschen, D. S., Astrophys. J. (in the press).
5. Hagen, J. P., McClain, E. F., and Hepburn, N., Proc. Inst. Rad. Eng., 42, 1811 (1954).