1. Javan, A., Phys. Rev. Letters, 3, 87 (1959).
2. Bennet, jun., W. R., Applied Optics Maser, Supp. 1, 24 (1962).
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4. White, A. D., and Rigden, J. D., Proc. Inst. Rad. Eng., 50, 1697 (1962).
5. See, for example, McDonald, A. D., and Brown, S. C., Phys. Rev., 76, 1634 (1949), and Francis, G., Handbuch der Physik, 22 (Springer Verlag, Berlin, 1956).