Acknowledgment

I HAVE greatly profited from hints, generously lavished in the course of correspondence, from Professors D.B. MACDONALD, R. NICHOLSON, and LOUIS MASSIGNON, in addition to recent works by the last two. My cordial thanks to these; and also to Professor D.S. MARGOLIOUTH for discussing with me some of the difficult points in the translation. {p. vi} {p. vii}