Melinda Stiles, age 13, of Biloxi, Miss., for her question:

HOW DOES MICROFILM WORK?

Microfilm is a small photographic film on which tiny images of printed and other material are photographed. The film stores a great deal of printed material in a small space.

Microfilm became a large industry in about 1928 after the United States Library of Congress began to microfilm books.

An entire average book's contents can be reduced photographically, page by page, on a short strip of microfilm that is 35 millimeters wide. This is film that is about 1 3/8 inches wide. One strip, containing an entire book, can then be wound into a small roll and stored in a fraction of the space occupied by the book.

It is easy to read a microfilm copy of a book by putting the film into a special projection machine. The machine enlarges the image and displays it on a TV like screen.

Some microfilm projection machines will make an enlarged paper copy of the image the same size as the book's original page. Most microfilm is black and white because color is more expensive and, in most cases, is not necessary.

In addition to books, business records and financial reports find their way onto microfilm. The greatest advantage is microfilm reduces the image and makes it possible to store a tremendously large amount of material in a very small space.

When cut into short pieces of film and placed in a plastic card, a strip of microfilm becomes what is called microfiche. A microfiche measures about 4 by 6 inches. Two or three of these plastic cards can hold an entire book's contents.

Individual frames cut from a strip of microfilm can be inserted in punched cards used by high speed business machines. Information on microfilm can then be found quickly by running the cards through a sorting machine.

Microfilm is now used extensively by newspapers, libraries and government offices.

A few small boxes of microfiche cards can store enough material to make an entire small library.

Copies of rare books and manuscripts are now made for schools and libraries for much less than it would cost to print them.

Architects and engineers are also using microfilm these days. They can now easily store large and detailed drawings on microfilm.

The process of making microfilm copies is called microphotography.