It is undoubtedly most fitting to begin by expressing my gratitude and indebtedness to Professor Eggleston, who suggested the
main problem of the book to me and whose critical remarks and
suggestions have been invaluable. As the main body of the book
was approved by the University of London for the award of their
Ph.D. degree, I would also like to thank the D.S.I.R. for a
Research Studentship whilst carrying out this work.