1. Harris Clyde, W.Jour. Opt. Soc. Am., 1950,40, 819.
2. Herzberger, M.Quart. App. Math., 1943,69, 1.
3. Ramachandran, G. N. and Thathachari, Y. T.Curr. Sci., 1951,20, 314.
4. Smith, T.“Optics” Encyl. Brit., 1948,16, 815.
5. —Proc. Phys. Soc. Lond., 1945,57, 558.