When the United States entered the war the British, French, and Italians were using plates exclusively and we followed their lead, it not being until some months later that we turned to films. At this time the British were using 4 × 5 plates, and cameras equipped with lenses of from 8 to 12 inch focus. Instead of making contact prints from these negatives, enlargements 6½ × 8½ were made on glossy paper, it being claimed that this process gave greater control in printing. Whether the British system really had all the advantages claimed for it is open to question, but in any event we adopted it and followed it through the first nine months of the war. The great masters of photography in Rochester were by no means content to let another nation set the pace for the United States, however, and in January, 1918, a concern in that city completed a very remarkable aero camera, radically different from anything which had been seen in Europe up to that time, which was promptly adopted by the War Department. This camera, which took an 18-cm. by 24-cm. picture, had a focal length of 20 inches, held a roll of film on which 100 successive exposures could be made, and weighed only 35 pounds. Its most novel feature was the “vacuum back,” consisting of a perforated sheet which extended across the top of the chamber and over the face of which the film passed. A slight air-suction, produced by a Venturi tube placed where it would catch the rush of air past the plane, served to hold the film absolutely flat—for the slightest curvature of its surface would play havoc with the perspective of a picture taken from a height, say, of 10,000 feet. This ingenious instrument was driven by an electric motor which changed the film and automatically set the shutter, the observer having only to start the machinery going and regulate its speed according to the rate of travel of the airplane in order to obtain a series of pictures forming a continuous photograph of the territory over which the machine was passing.