Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. The present volume is not a book in the true sense of the word, but a collection of articles and lectures written at different times during the past few years. In pre paring these for publication I have made no attempt to bring them up to date or to remove the inevitable element of repetition and minor inconsistency.