Part of the work reported here was done by J. R. Sanford who held a National Science Foundation Cooperative Graduate Fellowship. Phys. Rev. Lett. 6, 62 Show morePart of the work reported here was done by J. R. Sanford who held a National Science Foundation Cooperative Graduate Fellowship. Phys. Rev. Lett. 6, 624–625 (1961) http://link.aps.org/doi/10.1103/PhysRevLett.6.624 Show less