Fig. 99.—20-inch focus automatic film camera mounted vertically in photographic DH-4.
Fig. 100.—DeHaviland 4 re-constructed as a special photographic plane, showing 20-inch camera mounted for oblique photography.
The mounting of the regular camera auxiliaries—releases, sights, propellers, speed controls, motors—is usually a great bother, due to lack of space and to the severe restrictions on methods of fastening. Screws in longerons or uprights are taboo. Metal straps to go around structural parts are the approved device, but with variations in the size of these members, the holes, straps, bolts and nuts provided are very apt not to fit. Changes of construction, such as that from skeletons covered with fabric to veneer bodies, also interfere with any standard means of attachment, and leave this, like many other problems in war-time aerial photography, to the resourcefulness of the man in the field.
Magazine racks must be tucked away in any available space. Under the seat is a position frequently utilized. Especially with plates is it desirable to carry the extra magazines in a position to interfere as little as possible with the balance of the plane. In the DH 4 this means that they should be carried if possible forward of the observer, even though he must turn completely around to get and insert each magazine.
IV
SENSITIZED MATERIALS AND CHEMICALS
CHAPTER XVII
THE DISTRIBUTION OF LIGHT, SHADE AND COLOR IN THE AERIAL VIEW
The general appearance of the earth as viewed from above has already been described and illustrated (Figs. [10] and [11]). It remains to deal with the earth's appearance in a more analytic and quantitative manner, in order to decide upon the characteristics to be sought in our photographic sensitive materials.
Range of Brightness.—The absence of great contrasts so apparent in the view of the earth from a plane is confirmed by photometric observations. These show that the average landscape, as seen from the air, rarely presents a range of brightness of more than seven to one, even when seen without the presence of veiling haze. It is to be remembered that shadows constitute no important part of the aerial landscape. Vertical walls in shadow, which form a substantial part of the surfaces seen by an observer on the ground, are invisible or greatly foreshortened from the air. Moreover, they are never contrasted against the sky, which is photographically often the brightest part of the ordinary picture. To the aviator's eye shadows on the ground are only of any length at early and late daylight hours. Even at these times they cover but a small area, since the number of high vertically projecting objects in a representative landscape is small. Lacking shadows, the brightness range is only that between various kinds of earth, water, and vegetation. Chalk (from freshly dug trenches), reflected sunlight from water, or marble buildings, furnish almost the only extensions to the brightness scale as above given.