Published in Physical Review Letters, Volume 18, Issue 24, June 12, 1967, pages 1051-1053.
NOTE: At the time of publication, the author Richard B. Frankel was not yet affiliated with Cal Poly.
The definitive version is available at https://doi.org/10.1103/PhysRevLett.18.1051.
This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Physical Society. The following article appeared in Physical Review Letters.