=Marking Negatives.=—After development and drying, and before filing or printing, each plate should be marked with data for purposes of future identification. This is most easily done with pen and ink on the film side (in reversed lettering) either along an edge in the unexposed portion covered by the sheath or in a corner, so as to lose as little of the photograph as possible. Just what data shall be inscribed is dictated by the purpose for which the negative was made. The date, altitude, time of day, true north (from known permanent features or from shadow direction and time of day), number of the camera used, the focal length of the lens. Other records, such as the plane and squadron numbers, or even the pilot's and observer's initials, may be called for (Fig. 75). For mapping work the scale of each of a set of negatives, once found, may be marked, either in figures or by means of a line of length corresponding to a fixed distance on the ground. Rectifying data can similarly be inscribed, so that the negative can be printed in the enlarging and rectifying camera with the minimum of delay. CHAPTER XXIV PRINTING AND ENLARGING =Contact Printing.=—Single prints are made most simply in a printing frame held at a short distance from a light source. When any quantity must be made, as in turning out prints at high speed for distribution to an army before an attack, _printing machines_ are employed. These consist essentially of a light box, a printing frame of plate glass, and a pressure pad. In the commercial models, such as the Crown and the Ansco, which are equipped with electric light, merely bringing the pressure pad down and clamping it automatically turns on the light, while release of pressure terminates the exposure. The question of regulating the distribution of light is of considerable importance with negatives taken by focal-plane shutters of non-uniform rate of travel. In the McIntire printer (Fig. 119), the separate electric bulbs are on long necks in ball and socket joints, so that they can be brought individually closer to the printing surface or farther away from it, thus permitting a wide range of “dodging.” This printer also has an automatic time control for the light, a valuable device where many prints from the same negative are desired. These machines are well suited for printing aerial negatives, either plate or cut film, if used where a source of electric current is available. The chief defect, which may be caused by faulty construction, is imperfect contact between paper and negative, a cause of serious unsharpness on prints destined for minute study in interpretation. [Illustration: FIG. 118.—Printing machine.]