1. Lacroix, R...Helv. Phys. Acta, 1957,30, 374.
2. Van Vleck, J. H. and Penney, W. G.Phil. Mag., 1934,17, 961.
3. Feher, G. H.E.D. Scovil,Phys. Rev., 1957,105, 760.
4. Weger, M. and Low, W...Phys. Rev., 1958,111, 1526.
5. Upreti, G. C. ..Ph.D. Thesis, submitted to the Indian Institute of Technology, Kanpur, India, 1966.