Not long ago, most historians had to be travelers.【C1】______ written accounts of historical events, they had to travel to the places where the accounts 【C2】______ . To read rare old books and other documents, they often had to visit museums and libraries 【C3】______ home. Today, historians can 【C4】______ many old books and documents without travelling very far. Rare books, old letters, and other historical documents are 【C5】______ . Many different 【C6】______ can now have copies. Microfilming is a photographic process which copies and reduces pages of printed or wirtten matter【C7】______ small tells of film. To read microfilm, the user 【C8】______ the reel in a machine. The machine then【C9】______ a lull-sized image on a glass screen. 【C10】______ 5,000 book pages can be copied on one reel of microfilm. A reel takes up less storage space than a book and can be mailed 【C11】______ . 【C12】______ . a rare book or other document has been microfilmed, the 【C13】______ can be stored and protected from damage or theft. Some microfilms are stored in fireproof 【C14】______ . They will be available even if the originals are 【C15】______ or lost. American students of European history can now see, on microfilm, documents 【C16】______ back to the time of King Henry Ⅶ England. They can examine 【C17】______ manuscripts from the Vatican Library in Rome. A European student of American history can consult the records of the first United States Congress, or the 【C18】______ of the first United States Census of 1790. Each year, more old and rare material is 【C19】______ to microfilm. Today's newspapers, printed on a kind of paper that deteriorates rapidly, are being copied on microfilm. Historians are already consulting 【C20】______ . 【C1】