1. Ranganadham, S. P. (1931). Ind. Jour. Phys. 6, 421; Mathur, R.N. (1931). Ind. Jour. Phys. 6, 207.
2. Rao, S. R. and G. Sivaramakrishnan (1931). Ind. Jour. Phys. 6, 509; Bhatnagar, S. S. and R. N. Mathur (1931). Phil. Mag. 11, 914.
3. Farquharson, J. (1932). Nature 129, 25; Farquharson, J. and M. V. C. Sastri (1937). Trans. Far. Soc. 33, 1472.
4. Cabrera, B. and A. Madinaveitia (1932). An. Soc. Esp. Fis. Quim. 30, 528.
5. Kido, K. (1932). Sci. Rep. Tôhoku Univ. 21, 385; 22, 835.