1. See the report by K. Przibram in Z. Phys., 102, 331 (1936); 107, 709 (1937); and also NATURE, 141, 970 (1938).
2. Groot, W. de., Arch. Neerland., iiia, 7, 207 (1924), had already found two regions of excitation in fluorite: below 230 mµ and between 340 and 380 mµ.