1. Ehrenstein, G., Townes, C. H., and Stevenson, M. J., Phys. Rev. Letters, 3, 40 (1959).
2. Shklovsky, I. S., Dok. Akad. Nauk. (SSSR), 92, 25 (1953).
3. Townes, C. H., Intern. Astro. Union Symp., No. 4, edit. by van de Hulst, H. C., 92 (Cambridge Univ. Press, 1957).
4. Barrett, A. H., and Lilley, A. E., Astro. J., 62, 5 (1957).
5. Penzias, A. A., Abst. Paper New England Radio Eng. and Manufacturers (Nov. 4–6, 1963).