1. A comprehensive review of the evidence for anisotropy of the superconducting energy gap is given by A.G. Sheplev, Soviet Phys. Usp. 11, 690 (1969). Also see Ref. /2,3a,3f/
2. J.L. Bostock, M.L.A. MacVicar, paper R-6, this volume, p. 213
3. Over the last decade a number of experimental papers on anisotropy have been published by the Low Temperature Group at Georgetown University. Theses, when unpublished, are available from Dissertation Copies, P.O.B. 1764, Ann Arbor, Michigan 48106 a W.D. Gregory, Ph.D.Thesis, MIT 1966 (unpublished); W.D.Gregory, T.P.Sheahen, J.F.Cochran, Phys.Rev. 150, 315 (1966) b W.D. Gregory, L.S. Straus, R.F. Averill, J.C.Keister, C.Chapman Low Temp. Phys. LT 13, vol.3, p. 316 (Ed. Timmerhaus and Hammel, Plenum Publ.Corp.) 1972 c W.D. Gregory, Phys.Rev.Lett. 20, 53 (1968) d W.D. Gregory, M.A. Superata, P.J. Carroll, Phys.Rev. B3, 85 (1971) e W.D. Gregory, M.A. Superata, J.Cryst.Growth 7, 5 (1970) f S.B. Horn, Ph.D. Thesis, Georgetown University 1974 (unpublished) g J.C. Keister, L.S. Straus, W.D. Gregory, J.Appl.Phys. 42, 642 (1971) h R.F. Averill, L.S. Straus, W.D. Gregory, Appl.Phys.Lett. 20, 55 (1972) i W.D. Gregory, R.F. Averill, L.S. Straus, Phys.Rev.Lett. 27, 1503 (1971) j A. Grekas, Ph.D. Thesis, Georgetown University 1976 (unpublished)
4. D. Markowitz, L.P. Kadanoff, Phys.Rev. 131, 563 (1963)
5. J. Clem, Annals of Phys. 40, 268 (1966)