1. Cooper, L. N., Proc. Seventh Intern. Conf. Low Temp. Phys., 416 (University Press, Toronto, 1961).
2. Marcus, P. M., Proc. Seventh Intern. Conf. Low Temp. Phys., 418 (University Press, Toronto, 1961).
3. Glover, R. E., Proc. Seventh Intern. Conf. Low Temp. Phys., 421 (University Press, Toronto, 1961) (see discussion of paper by Marcus).
4. Broom, R. F., and Rhoderick, E. H., Proc. Phys. Soc., 79, 586 (1962).