1. 1) ZS. f. Phys., 42 (1927), 779. Also see K. Becker, ibid., 42 (1927), 222.
2. 2) Proc. Imp. Acad., 3 (1927), 132.
3. 3) Proc. Nat. Acad. Sci., 13 (1927), 46. The present authors were not aware of this paper when their work was made.
4. 1) R. Fürth, Schwankungserscheinungen in der Physik, 1920, S. 21.