PROTECTION AGAINST AIR OBSERVATION The purpose of camouflage is to render objects indistinguish able or unrecognizable by means of imitation or disguise. Con cealment in the limited sense of "hiding from view" is not the primary aim. The ideal is non-interference with the natural or normal aspect of the locality as viewed from the air, with which the enemy has become familiar through observation and pho tography, the latter being the more accurate. A photograph will always record something. The art of camouflage lies in conveying a misleading impression as to what that something signifies. The photograph records colours and accidents of ground (such as bare earth, vegetation, woods, etc.) in terms of light and shade, form ing a patchwork or pattern of black and white meeting in varying intensities of grey. A cultivated district presents a regular chess board pattern, with large rectangular expanses of monotone, the only accidents to break the monotony being large hedges, banks or houses, with their attendant shadows. Broken ground, such as demolished villages, shelled areas or patchy vegetation, pre sents a highly complex pattern, full of merging lights and shades.
Photographically, the effect of colour is not so marked or important as the effect of light and shade. Earth is toward the white end of the scale and grass or vegetation toward the black— not because of their respective colours but on account of the amount of contained shadow or "texture." Contrasts in tone are much accentuated on snow and the effects of shadows are more marked. Consequently it is essential, when judging the colours of a locality, to view it vertically and not obliquely as one is accustomed to see a flower bed.
Therefore the most practical method of planning the conceal ment of any work is to plan it with reference to a recent photo graph which records the ground pattern, and the natural facilities for concealment which exist in the locality. Such facilities abound in a neighbourhood the photographic pattern of which is complex. Any slight error in exact reproduction may escape notice in the prevailing complexity, because detection depends on comparison and comparison is made difficult by the intricacy of the pattern.
There are certain characteristic clues which betray new work to the reader of aerial photographs. They are disturbance of soil, tracks, shadows, regularity and blast marks of guns. To achieve success these clues must be suppressed from the very beginning; conversely, if deception is to be achieved by the use of dummies, these clues must be supplied.
Shadows.—The form of any erection, or excavation, is re vealed in a photograph by the shape of the shadow cast. Two intersecting planes, e.g., the two sides of the roof of a building, will show differently on the photograph (except for a very brief period every day) because they receive light at different angles, and therefore reflect it differently. It follows that an artificial reproduction of locality must be erected parallel to the contours of that locality, or in other words the planes of the imitation and the real must not intersect. A mound must be imitated by a mound, and a flat surface by a flat surface. Any departure from this principle is most easily detected in a photograph taken when the sun is low, the shadows being long in consequence.
Natural materials such as branches, grass, etc., are of little use for camouflage purposes if protection from view is required for more than two or three days, for, when withered, they be come very conspicuous. The most satisfactory cover is afforded by some form of net having an opaque centre of a boldly irregular shape, with a border becoming decreasingly opaque towards the edges. The centre should be only so opaque as to prevent the object underneath (gun or excavation, as the case may be) being visible from the air and the border opaque enough to mask the shadow of the object without casting a shadow itself. Such a cover is erected at a suitable height above the object to be concealed, and parallel with the contour of the ground. It follows that the higher the erection the greater must be the area required to cover object and shadows. Evidence afforded by tracks is perhaps the most difficult of all to eliminate. Positions which are admirably concealed in every other way are betrayed by the tracks leading up to them, so much so that it is often possible to count the number of guns in a battery by the paths leading to each gun-pit and to distinguish between gun positions and other works.
The following afford good illustrations of methods of con cealing approaches to positions in the open : (a) Leading the track close past the gun position and on to join an existing track.
the connection to each pit being treated with camouflage material or cut grass, etc. (b) Similarly, but close in front of the gun pits in order to use the track to hide blast marks. This method has the disadvantage of restricting traffic while the guns are in action. (c) Siting a battery in the midst of an existing network of tracks, taking precautions to reproduce on the camouflage any path interrupted by a gun-pit. It is not practicable to conceal long trenches. If a covering sags or differs materially in tone from its surroundings the mere length and regularity will betray its presence.
Material and Production.—The principal desiderata in ca mouflage material are durability and portability. It is almost inevitable that production must be standardised in the case of operations on a large scale. It is possible to deal individually only with a few special positions and the exterior of observation posts and the like. Square, woven, weather-proofed fish netting provides the most portable foundation. For a field gun 3o ft. x 3o ft. is sufficiently large, as larger sizes than this become un wieldy and difficult to erect. Wire netting is more durable and therefore best suited for large works. Small rolls-3o f t. x 6 f t.— are a convenient size. Open meshed canvas (scrim) is very suitable material for use on the ground and in conjunction with wire or fish netting. Fireproofing should be adopted wherever this is possible.
For coloration, sunproof dyes are the ideal medium, failing this, water paints. Oil paints are a source of danger owing to their tendency to cause spontaneous combustion in stacked ma terial. In order to achieve standardisation it will probably be necessary to adopt four appropriate colorations, viz., all vegeta tion, all earth and two mixtures of vegetation and earth.