1. Rikitake, T., Electromagnetism and the Earth's Interior, Developments in Solid Earth Geophysics, 2 (Elsevier, 1966).
2. Parkinson, W. D., Geophys. J., 2, 1 (1959).
3. Reitzel, J. S., Gough, D. I., Porath, H., and Anderson, C. W., Geophys. J., 19, 213 (1970).
4. Lilley, F. E. M., and Bennett, D. J., Geophys. J. (in the press).
5. Born, J., and Wolf, E., Principles of Optics, 24 (Macmillan, New York, 1964).