The photographic section was divided into two branches, land and air, the latter being, perhaps, from a military standpoint, the more important of the two for the reason that airplanes were used primarily for reconnaissance work and were, when equipped with cameras, literally the eyes of the army. The airplane being the eye of the army, the camera may be said to have been the pupil of the eye. In order to provide the large and highly trained personnel required for this service, there was established at Rochester, New York, a School of Aerial Photography—the largest in the world—where candidates received, in addition to a thorough military training, a course of instruction in everything relating to modern photography, from the manufacture of plates and films through the selection and use of lenses, shutters, and light-filters, to the printing of the picture itself. In addition to becoming familiar with these details of commercial photography, they were instructed in all the special phases of military photography, such as map-plotting, mosaics, enlargements, and the study of topography from a negative made many thousands of feet in the air. As in that chapter dealing with the Air Service I have described in considerable detail the methods and instruments used in aerial photography, it is enough to say here that the aerial branch of our Photographic Service attained such a degree of efficiency that, in the closing months of the war, it became virtually impossible for the Germans to dig a dozen yards of new trench, to transfer a platoon, to change the position of a machine-gun, without being detected by the all-seeing eyes of our cameras.