1. Curran, Dee and Petržílka, Proc. Roy. Soc., A, 169, 269 (1938).
2. NATURE, 143 [759] (1939).
3. Halpern and Crane, Phys. Rev., 55, 260 (1939).
4. Burcham and Smith, Proc. Roy. Soc., A, 168, 176 (1938).
5. Bainbridge and Jordan, Phys. Rev., 51, 384 (1937).