Page:Graphic methods for presenting facts (1914).djvu/349

 *parentizing solution on the market and a supply can be obtained from most shops selling drafting materials.

During the last few years very convenient photographic machines have been put on the market which photograph directly on sensitized paper without the use of any negative. With this type of machine, copies of drawings can be made very cheaply and clearly. One of the convenient features in doing work by this machine is that drawings can be enlarged or reduced within a wide range of sizes. The machine most commonly used for this work is called the photostat. In most large cities there are companies equipped with the photostat apparatus who will at reasonable cost make copies of drawings sent to them, much as the blue-printing companies make copies from tracings. Some of the best equipped libraries have now installed photostat machines as a convenience to their patrons. In a library so equipped it is possible to have a copy made from any page in any book or periodical in the library. In the New York Public Library, the reader need only fill out an order form giving the exact page and the name of the publication from which the copy is to be made, and state the size desired in the reproduction. Usually the copy is available within a few hours, but, if desired, it may be mailed, thus avoiding any necessity for waiting on the part of the person ordering the copy. In case of rare books or manuscripts, copies may be made page by page so that a complete copy of the book is obtained without prohibitive expenditure.

Charts from which plates must be made for printing are nearly always drawn considerably larger in size than the completed illustration. Most of the charts in this book were drawn two or three times as large as seen here. A photographic reduction in the size of the chart tends to eliminate minor irregularities and gives a much better result than can possibly be obtained from drawings in the exact finished size. In making the original large-size drawings it is almost essential that a reducing glass should be used to make certain that the finished drawing will have the desired appearance. With complex drawings it is often difficult to tell whether the lettering and figures are of large enough size to be read easily after they are reduced to the size to be used for printing. By looking through a reducing glass it can be determined at once whether the drawing is in correct proportions. A reducing glass is similar in appearance to the ordinary magnifying glass, but the lens is ground concave instead of convex so that everything seen through the glass appears of smaller size. The ordinary reducing glass can be