1. The authors are indebted to Dr.Lanius for his kind lending them several plates from the Berlin part of the I-stack.
2. W. Heitler:The Quantum Theory of Radiation (Oxford, 1954).
3. A. B. Migdal:Phys. Rev.,103, 1811 (1956).
4. L. L. Landau andI. A. Pomerančuk:Dokl. Akad. Nauk SSSR,92, 535, 735 (1953).
5. M. L. Ter-Mikaelyan:Dokl. Akad. Nauk SSSR,94, 1033 (1954).