It was the morning after Crewe's visit to Riversbrook, and the detective sat in his private office glancing through a note-book which contained a summary of the Hampstead mystery. Crewe was a painstaking detective as well as a brilliant one, and it was his custom to prepare several critical summaries of any important case on which he was engaged, writing and rewriting the facts and his comments, until he was satisfied that he had a perfect outline to work upon, with the details and clues of the crime in consecutive order and relation to one another. Experience had taught him that the time and labour this task involved were well-spent. If an unexpected development of the case altered the facts of the original summary Crewe prepared another one in the same painstaking way. The summaries, when done with, were methodically filed and indexed and stored in a strong room at the office for future reference, where he also kept full records of all the cases upon which he had been engaged, together with the weapons and articles that had figured in them: huge volumes of newspaper reports and clippings; photographs of criminals with their careers appended; and a host of other odds and ends of his detective investigations--the whole forming an interesting museum of crime and mystery which would have furnished a store of rich material for a fresh Newgate Calendar. It was an axiom of Crewe's that a detective never knew when some old scrap of information or some trifling article of some dead and forgotten crime might not afford a valuable clue. Expert criminals frequently repeated themselves, like people in lesser walks of life, and Crewe's "library and museum," as he called it, had sometimes furnished him with a simple hint for the solution of a mystery which had defied more subtle methods of analysis.