The aim and scope of this work are set out in the first chapter. Here I explain the conventions that I have used and thank those who have been kind enough to criticize my efforts.

The work is based on primary printed sources. A few letters and other informal documents have been used but only if they have already been printed. Secondary sources are given when they refer directly to the matter in hand or when they seem to be particularly useful. No attempt has been made, however, to cite everything that is relevant to the background of the subject since this would have led to the inflation of an already long bibliography. This policy has led to a fuller coverage of the 18th century than of the 19th where the secondary literature is potentially vast. In contrast, there are almost no directly useful secondary sources for the 20th century, but here the number of primary sources is impossibly large. It would have been easy to have given ten or more times the number listed. The choice is inevitably biased by the recent aspects of the subject upon which I have chosen to concentrate; others might have made other choices, but no one could give a comprehensive coverage of the last century.