1. Titius, J. D., footnote to Bonnet's Contemplations de la Nature, second edition, 7 (1772).
2. Ovenden, M. W., Vistas in Astronomy (in the press).
3. Ovenden, M. W., Proceedings of NATO Advanced Institute in Dynamical Astronomy, Cortina d' Ampezzo (in the press).
4. Hills, J. G., Nature, 225, 840 (1970).
5. Roy, A. E., and Ovenden, M. W., Mon. Not. Roy. Astron. Soc., 114, 232 (1954).