The simplest _means of communication_ is of course direct conversation. But this is only possible in those planes, such as the DH-9, in which pilot's and observer's cockpits are immediately together, so that, by shouting, any desired information can be conveyed with fair ease. When the distance is increased to four or five feet, as in the DH 4, the loudest shouts are totally lost in the roar of the engine and the blast of the wind. Speaking tubes and telephones are now fairly good, but are none too comfortable or convenient to have strapped on one's head and face. A primitive device used to some extent in the war was merely a pair of reins attached to the pilot's arms, by which he could be directed which way to steer. There is much to be said for a simple semaphore system, where an indicator in the observer's cockpit actuates a similar dial in front of the pilot, indicating “right” or “left,” “picture obtained,” “try again,” etc. If the observer has a sight by which he can see far enough ahead to correct the pilot's error of pointing, the need for an accurate sight for the pilot is diminished. [Illustration: FIG. 128.—Diagram showing relationship between focal length and area covered by plate.] [Illustration: FIG. 129.—Diagram giving data on area covered at various altitudes by representative lens.] [Illustration: FIG. 130.—Burchall Slide Rule, for calculating intervals between exposures, and for other aerial photographic data.] In considering the question of sights, attention may again be called to the poor “visibility” from the pilot's seat in the present prevailing type of two-seater tractor plane. Blind directly in front, beneath, and to either side (Figs. 7, 8 and 9), it is no unusual thing for a pilot to entirely miss an objective, such as a railway line, which he can only estimate to be beneath him by judging its distance from those objects to either side which he can actually see. The English practice of leaving a clear space of six inches to a foot between the fuselage and the beginning of the wing fabric, allows the pilot to look down over the side, a decided advantage. But for photographic purposes nothing can compare with a good negative lens carrying fore and aft lines or wires, so that the pilot can see his objective in ample time to head directly for it. The lens should either be large enough so that its rear edge gives the view directly downward, or supplemented by an additional lens pointing directly down, so that the covering of the target is assured. To locate such a lens in the front cockpit, free of all controls, is a very hard task; even so its view is likely to be badly interrupted by the landing gear. Nevertheless, so important is it, both in photography and in bombing, to have a sight by which the plane can be accurately directed that designers of planes should recognize this need and make every effort to provide a suitable location. [Illustration: