Submitted by Christopher Paul Ewels to the University of Exeter as a thesis for the degree of Doctor of Philosophy in Theoretical Physics, in the Faculty of Science, July 1997.
This thesis is available for Library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement.
I certify that all material in this thesis which is not my own work
has been identified and that no material is included for which a
degree has previously been conferred upon me.
C. P. Ewels, July, 1997
Also available in postscript format.