1. Harries, J. E., and Burroughs, W. J., Quart. J. Roy. Met. Soc. (in the press).
2. Harries, J. E., and Burroughs, W. J., NPL Divisional Rep. No. 7 (1971).
3. Harries, J. E., thesis, Univ. London (1971).
4. Murcray, D. G., Kyle, T. G., Murcray, F. H., and Williams, W. J., Nature, 218, 78 (1968).
5. Murcray, D. G., Kyle, T. G., Murcray, F. H., and Williams, W. J., J. Opt. Soc. Amer., 59, 1231 (1968).