1. Bøggild, J. K., Kgl. Danske Vidensk. Selsk. Mat.-fys. Medd., 23, No. 4 (1945).
2. Bohr, N., Kgl. Danske Vidensk. Selsk. Mat.-fys. Medd., 18, No. 8 (1948); § 5.2 (in the press).
3. ibid., §§ 2.4 and 5.2.
4. Cf. ref. 1, and Bower, I. C., Bretscher, E., and Gilbert, C. W., Proc. Camb. Phil. Soc., 34, 290 (1938).