1. Angel, J. R. P., Novick, R., Vanden Bout, P., and Wolff, R.: 1969, Phys. Rev. Letters 22, 861.
2. Barkla, C. G.: 1906, Proc. Roy. Soc. A 77, 247.
3. Beigman, I. L.: 1974, P. N. Lebedev Physical Institute, Academy of Sciences of the U.S.S.R., Moscow, Preprint No. 68.
4. Berthelsdorf, R., Linke, R. A., Novick, R., Weisskopf, M. C., and Wolff, R. S.: 1973, Astrophys. Letters 14, 171.
5. Bragg, W. H.: 1914, Phil. Mag. 27, 881.