1. N.C. Christofilos, unpublished report (1950).
2. N.C. Christofilos, U.S. Patent no. 2.736,799, filed 1950, issued 1956.
3. E.D. Courant, M.S. Livingston, H.S. Snyder, Phys. Rev. 88, 1190--1196 (1952).
4. O. Gildemeister, diploma thesis, Bonn (1955).
5. G. Knop, doctoral thesis, Bonn (1957).