1. Babcock, H. D. and Babcock, H. W.: 1952, in G. Kuiper (ed.),The Sun, University of Chicago Press, Chicago, Ill., p. 704.
2. Cacciani, A., Varsik, J., and Zirin, H.: 1989,Solar Phys. 125, 173.
3. Grossmann-Doerth, U.: 1994,Astron. Astrophys. 285, 1012.
4. Hale, G. E. and Nicholson, S. B.: 1938,Magnetic Observations of Sunspots, 1917?1924, Part I, Publ. Carnegie Inst. No. 498.
5. Jefferies, J. T. and Mickey, D. L.: 1991, in L. J. November (ed.),Solar Polarimetry, National Solar Observatory, Sunspot, N.M., p. 373.