IO Readings in European History and criticism of the sources have led to the preparation of a number of indispensable works of reference which do not aim to deal directly with history but to serve as a guide to those in search of the material upon which the historian must rely. A very few of the most noteworthy will be mentioned here as illustrations of the apparatus necessary in all professional historical study. To learn what the sources are and where they may be found is the first requisite of historical investigation. A French writer, Langlois, has published a very useful bibliography of historical bibliographies, 1 a catalogue of the best lists of sources and of historical treatises. Such lists are very numerous and often voluminous. The most useful and scholarly is Potthast's Wegweiser^ or "guide," in two stout volumes. 2 The compiler has, with infinite patience, sought to bring together in an alphabetical list the sources for the history of western Europe from the year 400 to 1500, and to state when and where they have been printed. One anxious to learn whether there has been a new critical edition of a partic- ular chronicle, or whether there are any lives of St. Boni- face, or Gregory VII, or Frederick Barbarossa, written by those who lived in their times, can obtain the desired information from Potthast, as well as a list of modern works relating to the topic under consideration. Admirable guides exist for the study of particular coun- tries. German scholars have compiled a list 3 of all the 1 Langlois, Manuel de bibliographic historique, Part I, " Instruments bibliographiques," 2d ed., Paris, 1901, 4 fr. 2 Wegweiser dtirch die Geschichtsivtrke des Europaischen Mittel- alters bis fjoo, 2 vols., 2d ed., Berlin, 1895-1896, M. 26.50. 3 Dahlmann-Waitz, Quellenkunde der deutschen Geschichte, 6th ed., Gottingen, 1894, M. 12.
