1. Lamla, E.: Naturwissenschaften54, 35 (1967).
2. Arley, N.: Proc. 14th Scand. Math. Congress Copenhagen 1964 (in publication).
3. Schlegel, R.: Time and the Physical World. East Lansing: Michigan State Univ. Press 1961 (cf. also papers in Nature reported here).
4. Einstein, A.: Geometrie und Erfahrung. Berlin: Springer 1921.
5. Pauli, W.: Enc. d. math. Wiss. V19, 570 (1921).