1. NATURE, 132, 929, Dec. 16, 1933.
2. NATURE, 132, 101, July 15, and 710, Nov. 4. See also a letter by Norton, ibid., p. 676, which seems open to the same criticisms, though his method is not very fully described.
3. Proc. Amst. Akad., 30, 974; 1932.