1. R. Wizinger and H. Wenning, Helv. Chim. Acta, 23, 247 (1940).
2. L. K. Mushkalo, M. Khabubi, N. N. Mushkalo, and L. V. Fedorova, Ukr. Khim. Zh., 40, No. 9, 957 (1974).
3. A. I. Kiprianov, Usp. Khim., 29, 1336 (1960).
4. P. Grammaticakis and H. Texier, Bull. Soc. Chim. France, No. 4, 1323 (1971).
5. R. Bertelson, Photochromism Tech. Chem. (edited by G. H. Brown), Vol. 3, Wiley-Interscience (1971), p. 56.