1. Bastin E. W. and Kilmister C. W. Bonnor W. B. 1956 Rend. del Circ. Mat. di Palermo (2)5 187.
2. Bonnor W. B. 1954 Zeit. für Astr.35, 10.
3. Eddington Sir A. S. 1924Mathematical Theory of Relativity: Cambridge.
4. Eddington Sir A. S. 1936Relativity Theory of Protons and Electrons: Cambridge.
5. Eddington Sir A. S. 1940 Proc. Roy. Soc. (A)174, 16.