1. Richardson, NATURE, 73, 607; 74, 55; 1906.
2. Kurz, Phys. Zeit., 10, 836; 1909: see also Millikan, NATURE, 126, 14; 1930, for historical studies.
3. Gockel, Phys. Zeit., 11, 280; 1910: also 12, 597; 1911.
4. Millikan and Cameron, Phys. Rev., 31, 930; 1928.
5. Millikan and Cameron, Phys. Rev., 28, 856; 1926.