The transparentizing solution is put on the paper with a brush or sponge and then blue prints are made in the ordinary manner. There are several different makes of the trans-

* A chapter on " Approximation and Accuracy " will be found in " The Elements of Statistical Method," by Willford I. King published by the MacmiUan Company, New York City.

GENERAL METHODS 329

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 necessitj^ for waiting on the part of the person ordering the copy.