1. Bradley, D. J., Engwell, M., Magyar, G., McCullough, A. W., and Richardson, M. C. (to be published).
2. Bradley, D. J., Magyar, G., and Richardson, M. C., Proc. Eighth Intern. Conf. on Phenomena in Ionized Gases, Belgrade, August 1965 (in the press).
3. Connes, P., Rev. d'Optique, 35, 37 (1956).
4. Bradley, D. J., Bates, B., Juulman, C. O. L., and Majumdar, S., App. Op., 3, 1461 (1964).
5. Hughes, T. P., Nature, 195, 325 (1962).