1. Bray, R. J. and Loughead, R. E.: 1964, Sunspots, Chapman and Hall, London.
2. Danielson, R. E.: 1961, Astrophys. J. 134, 289.
3. Hale, G. E. and Nicholson, S. B.: 1938, Magnetic Observations of Sunspots, Carnegie Inst., Washington, p. 44.
4. Livi, S. H. B., Martin, S. F., Wang, H., and Ai, G.: 1989, Solar Phys. 121, 197.
5. Mosher, J. M.: 1976, BBSO Preprint, No. 159.