The following four Ellen McArthur Lectures were delivered at Cambridge University in May 1968. In revising the manuscript for publication I made a number of alterations, both stylistic and substantive, and a good many additions, but I did not try to change the tone of an oral presentation. To do so would have seduced me into an attempt to produce a much more comprehensive treatise of the subject, which is something I hope to reserve for another time. It remains for me to express my gratitude to the seminar which met after the last lecture and whose members offered stimulating strictures, and beyond that to the hospitality of the academic community of Cambridge and, above all, to Pembroke College which was my host during that wonderful fortnight of the English spring, which convinced me that it could have been nothing but the requirements of the meter that made Browning change May to April.

I should also like to state my appreciation to the Institute for Advanced Study in Princeton and its director, Carl Kaysen, for providing a year of freedom from academic duties and with it, among other things, the leisure to transform the original draft of these lectures—a task that was greatly facilitated by the competent assistance supplied by my secretary at the Institute, Mrs Anna Maria Holt.