1. Lagerqvist, A., Nilsson, N. E., and Barrow, R. F., Ark. Fys., 12, 543 (1957).
2. Shimauchi, M., Science of Light (Japan), 7, 101 (1958).
3. Goodlett, V. W., and Innes, K. K., Nature, 183, 243 (1959).
4. Coheur, F. P., and Rosen, B., Mem. Soc. Roy. Sci. Liège, 10, 405 (1941).