1. Piddington, J. H., and Minnett, H. C., Aust. J. Sci. Res., A, 4, 459 (1951).
2. Bolton, J. G., Westfold, K. C., Stanley, G. J., and Slee, O. B., Aust. J. Phys. (in the press).
3. Mills, B. Y., Aust. J. Sci. Res., A, 5, 266 (1952).
4. Bolton, J. G., Stanley, G. J., and Slee, O. B., Aust. J. Phys. (in the press).