1. Euler and Heisenberg, Ergeb. exak. Naturwiss., 17, 1 (1938).
2. See also Blackett, NATURE, 142, 169 (1938).
3. Auger, Ehrenfest, Freon and Fournier, C.R., 204, 257 (1937). See also Rossi, NATURE, in the Press.
4. Johnson, Phys. Rev., 43, 307 (1933).
5. Blackett, Phys. Rev., in the Press.