In the composition of this book I owe much to the kindness and encouragement of Professor W. R. Sorley, Professor H. H. Joachim and Mr J. S. Boys-Smith, and their copious criticisms have enabled me to avoid many pitfalls and to correct many blunders. They are not, of course, responsible for the mistakes and imperfections which remain, but without their generous help the defects would have been even greater.