R. F. Barrett

• Department of Physics, The University of Western Australia, Nedlands, W.A., 6009, Australia

B. A. Robson^*

• Research School of Physical Sciences, The Australian National University, Canberra, A.C.T. 2600, Australia and Sektion Physik, Universität München, 8046 Garching, West Germany

W. Tobocman

• Physics Department, Case Western Reserve University, University Circle, Cleveland, Ohio 44106

• ^*Permanent address: Research School of Physical Sciences, The Australian National University, Canberra, A.C.T. 2600, Australia.