Wonderful progress has been made in mathematics during the last thirty years, which have been so productive of memoirs, treatises, journals, encyclopedias, societies, and international congresses, that even an expert is in considerable danger of being overwhelmed by the never-ending torrent. The flow is indeed so great that every one must feel how little he knows, and regret that neither the time nor the energy at his disposal will allow him to explore or even to obtain second-hand information of many of the interesting subjects that are brought to his notice.