1. W. Heisenberg, Zeits. f. Phys. 120, 513, 673 (1943).
2. See also J. A. Wheeler, Phys. Rev. 52, 1107 (1937).
3. W. Heisenberg, Zeits. f. Phys. 123, 93 (1944).
4. C. Moller, Kg1. Danske Videnskab. Selskab, Mat.-fys. Medd. 23, No. 1 (1945); 24, No. 19 (1946).
5. For references see the collection of papers on Quantum Electrodynamics, edited by Julian Schwinger (Dover Publications, Inc., New York, 1958 ).