1. Invited Discourse byM. Ryle andA. Sandage at the XIII. General Assembly of the IAU, 1967.
2. J. N. Bachall et al., Ap. J. Letters,157, L 77, 1969.
3. G. R. Burbidge andE. M. Burbidge, Nature,224, 21, 1969 and lit. quoted there.M. B. Bell, Nature,224, 229, 1969.C. L. Cowan, Nature,224, 665, 1969.
4. S. H. Plagemann et al., Nature,224, 875, 1969.
5. O. Heckmann andE. Schücking, a) in “Handbuch der Physik” LIII, p. 515 (Springer, 1959); b) in “Gravitation, an introduction to current research” (ed. L. Witten), p. 438 (J. Wiley, New York, 1962).