1. A review has been given by D. F. Brewer,Proceedings of the Tenth International Conference on Low Temperature Physics (Moscow, 1966), Vol. 1, p. 145. See, also, D. F. Brewer, inProceedings of the Symposium on Thin Helium Films, held at Stevens Institute of Technology, Hoboken, New Jersey, June 4 and 5, 1970,J. Low Temp. Phys. 3 (3), 205 (1970).
2. D. F. Brewer, A. J. Symonds, and A. L. Thomson,Phys. Rev. Letters 15, 182 (1965).
3. D. F. Brewer, A. J. Symonds, and A. L. Thomson,Phys. Letters 13, 298 (1964) and to be published.
4. H. P. R. Frederikse,Physica 15, 860 (1949).
5. W. D. McCormick, D. L. Goodstein, and J. G. Dash,Phys. Rev. 168, 249 (1968).