1. E.R. Lipincott, A.V. Valkenberg, C.E. Weir and E.N. Bunting, J. Res. Nat’l. Bur. Std. 61, 61 (1958);
2. V.P. Cheremisinov, Tr. Fiz. Inst. Akad. Nak (SSSR) 25, 153 (1964).
3. The band-to-band transition will be discussed in later sections.
4. H.A. Robinson, J. Rhys. and Chem. of Sol. 26, 109 (1965) and references contained therein; V. V. Tarasov, New Problems in the Physics of Glass, Chap. I.
5. U.C. Mittels and P.A. Sherrill, Phys. Rev. 93, 1117 (1954).