1. Hale, G. E. and Nicholson, S. B.: 1938, Magnetic Observations of Sunspots (Carnegie Institution of Washington, Washington, D. C.) 1, No. 498, 37.
2. Howard, R. and Harvey, J.: 1966, private Communication.
3. Leighton, R. B.: 1959, Astrophys. J.
130, 366.
4. Livingston, W. C.: 1965, private Communication.
5. Sheeley Jr., N. R.: 1966, Astrophys. J.
144, 723.