1. Ginsburg, V. L., Paris Symposium on Radio Astronomy, 589 (Stanford University Press, 1959). Shklovsky, I. S., Cosmic Radio Emission (Moscow, 1956).
2. Baade, W., and Minkowski, R. L., Astrophys. J., 119, 206 (1954).
3. Minkowski, R. L., Paris Symposium on Radio Astronomy, 315 (Stanford University Press, 1959).
4. Ambartsumian, V. A., Izv. Acad. Nauk Armyanskoi SSR, 11, 9 (1958).
5. Burbidge, G. R., and Burbidge, E. M., Astrophys. J., 125, 1 (1957); 129, 271 (1959).