1. C. V. Raman and N. S. Nagendra Nath,Proc. Ind. Acad. Sci., 1935,2, 406 and 413; 1936,3, 75, 119 and 459.
2. R. Bär,Helv. Phys. Acta., 1936. Bär realised it by diminishing the frequency of the supersonic waves.
3. R. Bär,Helv. Phy. Acta., 1935,8, 591.
4. L. Brillouin,Act. Sci. et Ind., 1933,59.
5. S. Parthasarathy,Proc. Ind. Acad. Sci., 1936,3, 594.