Conyers Herring^*

• University of Texas, Austin, Texas, and Bell Telephone Laboratories, Murray Hill, New Jersey

M. H. Nichols^†

• Princeton University, Princeton, New Jersey

• ^*Present address, Bell Telephone Laboratories, Murray Hill, New Jersey.
• ^†Most of the work was done while this author was a National Research Fellow at Princeton University. The work was completed at the University of Michigan where he is presently located.