1. F. i D. Colegrove and P.A. Franken, Phys. Rev. 119, 680 (1960).
2. F., D. Colegrove, L.D. Schearer and G.K. Walters, Phys. Rev. 132, 2561 (1963).
3. L,,D. Schearer and G.K. Walters, Phys. Rev. [39, AI398 ( I965 ); L. Schearer, Thesis, Rice University (1966).
4. W.A, Fitzsimmons, Master!s Thesis, Rice University (I966).
5. M. Pavlovic and F. Laloe, J. Phys, (Paris), 31, 173 (1970). F. Laloë, Thesis, Paris (1970),- Annales de Physique, 6, 5 (1971).