1. Hüniger, cited by Birus, K., Ergeb. Exakt. Naturw., 20, 221 (1942). Hill, C. G. A., Trans. Farad. Soc., 42, 666 (1946).
2. Kröger, F. A., "Some Aspects of the Luminescence of Solids" (Elsevier Publishing Co., Amsterdam and New York, 1947), in the press.