1. Black, W. S., Booker, D., Burton, W. M., Jones, B. B., Shenton, D. B., and Wilson, R., Nature, 206, 654 (1965).
2. Blake, R. L., Chubb, T. A., Friedman, H., and Unzicker, A. E., Astrophys. J., 137, 3 (1963).
3. Russell, P. C., Nature, 205, 684 (1965).
4. Tousey, U., Quart. J. Roy. Astro. Soc., 5, 123 (1964).
5. Touaey, R., Austin, W. E., Purcell, J. D., and Widing, K. G., Space Research III, 772 (North-Holland Publ. Co., 1963).