For getting the exposed plates to the photo section, messengers, on motorcycles if possible, are employed. In some cases, where hangars and photographic hut are forced to be widely separated, recourse has been had to parachutes (Fig. 112), a device also employed to distribute prints to infantry during an advance. [Illustration: FIG. 112.—Receiving pictures from plane by parachute.] For warfare of movement, especially in sparsely settled or devastated country, where cellars are unavailable, the dark room must be taken along. _Motor trucks and trailers_ (Figs. 113, 114, 115), the former for hauling supplies and electric light generating plant, the latter fitted as a complete developing and printing laboratory, form the headquarters of each photographic section in the field. Usually altogether too small for the amount of work required, they were extended by tents and lean-to's, or ingeniously used as a nucleus for the organization of the favored stable or cellar. [Illustration: FIG. 113.—Mobile photographic laboratory.] =Methods of Plate Development.=—Where speed is not required the simplest and commonest mode of developing plates is in the tray, one plate at a time. Common practice is to examine the plate at intervals during development, and discontinue the operation on the basis of its appearance. This is only possible if the plates used are insensitive to some light by which the eye can see. Deep red light is suitable for ordinary and most orthochromatic plates. A faint blue-green may be used with some panchromatic plates. The best practice, however, is to develop by time in total darkness, whereby all chance of dark room fog is avoided. Development time for plates of the average exposure of the one to be developed is either known from previous experience, or is found by trial on the first one. Development by time results in negatives of densities varying with the exposures, but, as was brought out in the discussion of sensitometry, this difference can be compensated for by the choice of the paper used for printing, and by its treatment. [Illustration: FIG. 114.—Interior of photographic trailer, developing room.] Where larger quantities of plates are to be handled _tank development_ is adopted. In ordinary tank development the plates are placed in grooved tanks, into which is poured first the developer, next the rinsing water, and then the hypo. It has been customary in tank development as practiced for peace-time work to use dilute developer, requiring from ten to thirty minutes, but speed requirements in war-time aerial photography dictate the use of full-strength quick-acting developer. An improvement on the simple grooved tank is provided by metal cages or racks, each holding a dozen or more plates, which may be introduced or removed from the tank as a unit (Fig. 116). [Illustration: FIG. 115.—Interior of photographic trailer. Enlarging camera and printer.]