1. W.H. Bragg, Phil Mag.
30, 305 (1915).
2. S. Nishikawa, Proc. Tokyo Math.-Phys. Soc.
8, 199 (1915).
3. T.F.W. Barth and E. Posnjak, Z. Krist.
82, 326 (1932).
4. E.W. Gorter, Philips Res. Rep.
9, 321 (1954).
5. E.W. Gorter, Philips Res. Rep.
9, 403 (1954).