1. Varian model No. 981-0137.
2. D. M. Zehner, J. R. Noonan, and L. H. Jenkins, Solid State Commun. (to be published).
3. Custom built by Galileo Electro-Optics Corp., now model No. 6-5672.
4. S. P. Weeks and A. Liebsch, Surf. Sci. (to be published).