1. J. Fritzsche. C. R. Seances Acad. Sci., Vie Acad., 1867, 64, 1035. https://fr.wikisource.org/w/index.php?title=Page:Comptes_rendus_hebdomadaires_des_s%C3%A9ances_de_l%E2%80%99Acad%C3%A9mie_des_sciences,_tome_064,_1867.djvu/1044&action=edit&redlink=1
2. E. ter Meer. Ann. Chem., 1876, 181, 1. https://doi.org/10.1002/jlac.18761810102
3. W. Marcwald. Z. Phys. Chem., 1899, 30, 140. https://doi.org/10.1515/zpch-1899-3007
4. Y. Hirshberg. C. R. Seances Acad. Sci., Vie Acad., 1950, 231, 903. https://gallica.bnf.fr/ark:/12148/bpt6k3183z/f903.image.r=Hirshberg?rk=21459;2
5. Main Photochromic Families: Organic Photochromic and Thermochromic Compounds, Vol. 1 / Eds. J. C. Crano and R. J. Guglielmetti. New York, London: Plenum, 1999.