1. See, for example, R. S. Halford, J. Chem. Phys. 14, 8 (1946).JCPSA60021-9606
2. H. C. Kremers, J. Opt. Soc. Am. 37, 337 (1947).JOSAAH0030-3941
3. A. H. Pfund, J. Opt. Soc. Am. 37, 558 (1947).JOSAAH0030-3941
4. A. Elliot and E. J. Ambrose, Nature 159, 641 (1947). NATUAS0028-0836
5. Also A. Elliot, E. J. Ambrose, and R. Temple, in an article to appear in a forthcoming issue of J. Opt. Soc. Am. We are indebted to Dr. Elliot for sending us a copy of this manuscript in advance of its publication.