1. Link, F., and Neuzil, L., Bull. Astro. Insts. Csl., 5, 112 (1954).
2. Elsmore, B., and Whitfield, C. R., Nature, 176, 447 (1955).
3. Boischot, A., Blum, E. J., Ginat, M., and le Roux, E., C.R. Acad. Sci., Paris., 242, 1849 (1956).
4. Rishbeth, H., Austral. J. Phys., 9, 494 (1956).
5. Rishbeth, H., and Little, A. G., The Observatory, 77, 897 (1957).