Fig. 119.—Interior of McIntire printer, showing lamps adjustable in position for “dodging.”
Fig. 120.—Film printing machine.
For the American Air Service, cut paper has been used exclusively. For film printing, the Ansco machine has been equipped with roll pivots to take film 24 centimeters wide which may be advanced in either direction by turning large milled heads (Fig. [120]). If we put rollers on the two remaining sides of the box to handle paper we transform the printer into the same form as a French machine, in which paper and film are moved at right angles to each other. A disadvantage of this modification, however, is the difficulty of examining the negative to be printed.
Stereo Printing.—To make separate prints from the two elements of a stereoscopic pair and mount them side by side after proper orientation is too slow a process if quantities of prints are needed. One method of multiple production is to make a master stereogram, and then produce photographic copies of it, but there is inevitable loss of quality in this copying process. An intermediate method is to print from both negatives on the same sheet of paper. In order to do this the negatives must be placed in rather large frames, with mats properly located to guide the placing of the paper. The Richard double printing frame is a practical device which simplifies the necessary manipulations. It consists essentially of a platform pierced with three illuminated openings. The two negatives are compared, superposed, and orientated over the central opening and then shifted laterally, one to each of the two side openings, which serve both as printing frames and masks. The printing back slides on a rod, permitting the paper to be lifted up and moved between exposures. Once the negatives are properly placed, stereo prints can be turned out quickly and easily.
Enlarging.—In the French service contact printing was the rule during the war. The English practice, on the other hand, was to take small negatives—4 × 5 inches, with 8 to 12 inch lenses—and enlarge them, usually to 6½ × 8½ inches. For this purpose a regular part of the English photo section equipment was the enlarging camera (Fig. [115]). This may be briefly described as a short focus camera in which the subject to be photographed is a negative, illuminated by transmitted light, whose image is thrown by the camera lens on the paper or other sensitive surface. By making the distance between negative and lens less than that between lens and paper, the resulting print is an enlargement, and vice versa. The scale of enlargement or of reduction is varied over limits set only by the length of the camera and the amount of light available.
The lens employed must of course possess sufficiently high quality to preserve all the sharpness of the negative, and focussing must be done with accuracy. Next to the lens the most important element is the light source. This may be of the point form, such as a concentrated filament electric lamp, an oxy-acetylene lime light, or an acetylene flame. The latter was extensively used in the English service, while acetylene generators for emergency purposes formed part of each American photo truck equipment. With point light sources we must use condensers to focus the light into the projecting lens. Much less efficient, but the only recourse where large condensers are not available, is a diffusing glass behind the negative, illuminated either by a bank of electric lamps with mirrors or by a U tube mercury vapor lamp, where proper current can be got.
The device for holding the printing paper must permit quick changing, but insure good contact. We may use either a spring plate to hold the paper against plate glass from behind, or else a weight acting on a lever arm of sufficient length.
The need for some automatic means of focussing an enlarging camera has been very generally felt. An illustration of such an enlarging camera is that put out by Williams, Brown & Earle, of Philadelphia, known as the “Semperfocal” (Fig. [121]). In this camera the movements of the lens, paper easel and negative are so inter-related and actuated with respect to each other that the correct focus of the instrument is maintained for any degree of enlargement or reduction. This feature is a great help in making up mosaic maps, where prints of continuously varying scale ordinarily occasion serious delay for individual focussing.