1. Brooks, H., Nuovo Cimento, 7 Supplement 2, 166-244 (1958).
2. Wigner, E.P. and Seitz, F., Solid State Physics, 1 97–126 (1955).
3. Fröhlich, H., Proc. Roy. Soc, A158 97–110 (1937).
4. Raimes, S., Phil. Mag., ser. 7, 43 327–337 (1952).
5. Raimes, S., Proc. Phys. Soc, A66 949–950 (1953).