The Air Service model of film developing and drying machine (Fig. [117]) introduces an essential modification in the drying scheme of the Eastman apparatus. The upper reel is quite independent of the lower reel and is revolved at a high rate of speed, so that a whirling action is introduced into the drying. Large rotating fans at the same time drive a considerable volume of air across the film surface, and the combination of the two agencies makes it possible to dry 35 feet of 18 × 24 centimeter film in 20 to 30 minutes. This for large numbers of pictures makes the use of film even quicker than that of plates. The only practical drawback to the apparatus is its bulk, which calls for a separate room or trailer. This, however, seems to be inevitable in the use of large roll film.
Cut film can be dried with speed only if placed in a draft of warm air. Drying boxes, with a chute or chimney and with fans to drive the air through from an alcohol stove, will dry several dozen films in an hour. The films must not be closer together than about one inch, which makes the drying boxes rather cumbersome.
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.
Fig. 118.—Printing machine.
The printing of aerial negatives may be done either on roll or cut paper, and if films are used, a further alternative is offered of handling it either in the roll or in cut form. Where many prints are to be made from one negative roll paper has some advantages, particularly if a developing and drying machine is available. But for moderate numbers the advantage is small, since cut prints can be developed quite conveniently in goodly numbers in the ordinary trays. But the advantages of keeping film in the roll form are very great, both in respect to storage and in respect to handling during printing, as the rollers provide the necessary tension and prevent the film “getting away.”