1. The photograph was taken by H.Kreye and K.Müller, Hochschule der Bundeswehr, Hamburg
2. R.K.Iler: The Colloid Chemistry of Silica and Silicates (Cornell Univ. Press, Ithaca, N.Y. 1955 )
3. S.Henning and L.Svensson: Phys. Scripta 23, 697, (1981)
4. G.Poelz and R.Riethmüller: Nucl. Instr. Methods 195, 491 (1982)
5. G.Poelz: DESY 84–110 (1984), to be published in Proc. of the Seminar on Cherenkov Detectors and their Applications in Science and Technics, Moscow