1. A. Peres:Nuovo Cimento,11, 617 (1959). Hereafter referred to as I. All notations throughout the present paper are those of I, unless otherwise stated.
2. Pham Tan Hoang:Compt. Rend.,246, 1497 (1958);Nuovo Cimento,9, 647 (1958).
3. L. Infeld andJ. Plebański:Bull. Acad. Pol. Sc., Cl. III,4, 689 (1956);5, 51 (1957).
4. L. Infeld:Acta Phys. Pol.,16, 177 (1957);Rev. Mod. Phys.,29, 398 (1957).
5. E. Schrödinger:Space-Time Structure (Cambridge, 1954), p. 55.