1. Bleeker, J. A. M., Burger, J. J., Scheepmaker, A., Swanenburg, B. N., and Tanaka, Y.: 1966, The Royal Dutch Academy of Sciences Preprint.
2. Bowyer, C. S., Field, G. B., and Mack, J. E.: 1968, Nature
217, 32.
3. Clark, G. W., Garmire, G. P., and Kraushaar, W. L.: 1968, Astrophys. J. 153, L203.
4. Felten, J. E. and Morrison, P.: 1966, Astrophys. J. 146, 686.
5. Ginzburg, V. L. and Syrovatskii, S. I.: 1964, Usp. Fiz. Nauk
84, 201. Translation in 1965: Soviet Phys. Uspekhi 7, 696.