But scholars and historians often need, in respect of documents, information not usually supplied by descriptive catalogues; they wish, for example, to know whether such and such a document is known or not, whether it has already been critically dealt with, annotated, or utilised.[41] This information can only be found in the works of former scholars and historians. In order to become acquainted with these works, recourse must be had to those "bibliographical repertories," properly so called, of all kinds, compiled from very different points of view, which have already been published. Among the indispensable instruments of Heuristic must thus be reckoned bibliographical repertories of historical literature, as well as repertories of catalogues of original documents. To supply the classified list of all those repertories (repertories of catalogues, bibliographical repertories, properly so called), together with other appropriate information, in order to save students from mistakes and waste of time, is the object of what we are at liberty to call the "science of repertories," or "historical bibliography." Professor Bernheim has published a preliminary sketch[42] of it, which we have endeavoured to expand.[43] The expanded sketch bears date April 1896: numerous additions, not to speak of revision, would already be necessary, for the bibliographical apparatus of the historical sciences is being renewed, at the present time, with astonishing rapidity. A book on the repertories for the use of scholars and historians is, as a general rule, out of date the day after it has been completed. III. The knowledge of repertories is useful to all; the preliminary search for documents is laborious to all; but not in the same degree. Certain parts of history, which have been long cultivated, now enjoy the advantage of having all their documents described, collected, and classified in large publications devoted to the purpose, so that, in dealing with these subjects, the historian can do all that need be done at his desk. The study of local history does not generally require more than local search. Some important monographs are based on a small number of documents, all belonging to the same collection, and of such a nature that it would be superfluous to look for others elsewhere. On the other hand, a humble piece of work, such as a modest edition of a text of which the ancient copies are not rare, and are to be found scattered in several libraries of Europe, may have involved inquiries, negotiations, and journeys without end. Since the majority of the documents of mediæval and modern history are still unedited, or badly edited, it may be laid down as a general principle that, in order to write a really new chapter of mediæval or modern history, it is necessary to have long haunted the great depositories of original documents, and to have, if we may use the expression, worried their catalogues.