There is another precaution essential to be observed regarding the more costly and rare possessions of the library. Such books should not be placed upon the shelves with the ordinary books of the collection, but provided for in a repository under lock and key. In a large library, where many hundred volumes of books of especial rarity and value are to be found, a separate room should always exist for this class of books. They will properly include (1) Incunabula, or early printed books; (2) Manuscripts, or unique specimens, such as collections of autographs of notable people; (3) Illuminated books, usually written on vellum, or printed in color; (4) Early and rare Americana, or books of American discovery, history, etc., which are scarce and difficult to replace; (5) Any books known to be out of print; and (6) Many costly illustrated works which should be kept apart for only occasional inspection by readers. Where no separate room exists for safe custody of such treasures, they should be provided with a locked book-case or cases, according to their number. When any of these reserved books are called for, they should be supplied to readers under special injunctions of careful handling. Neglect of precaution may at any time be the means of losing to the library a precious volume. It is easy for an unknown reader who calls for such a rare or costly work, to sign his ticket with a false name, and slip the book under his coat when unobserved, and so leave the library unchallenged. But the librarian or assistant who supplies the book, if put on his guard by having to fetch it from a locked repository, should keep the reader under observation, unless well known, until the volume is safely returned. Designing and dishonest persons are ever hovering about public libraries, and some of the most dangerous among them are men who know the value of books. This class of reserved books should not be given out in circulation, under any circumstances. Not only are they subject to injury by being handled in households where there are children or careless persons, who soil or deface them, but they are exposed to the continual peril of fire, and consequent loss to the library. There are often books among these rarities, which money cannot replace, because no copies can be found when wanted. In the Library of Congress, there is a very salutary safe-guard thrown around the most valuable books in the form of a library regulation which provides that no manuscript whatever, and no printed book of special rarity and value shall be taken out of the library by any person. This restriction of course applies to Members of Congress, as well as to those officials who have the legal right to draw books from the library. CHAPTER 12. THE FACULTY OF MEMORY.