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recognisable, by means of imitation or disguise. Concealment 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. This is an ideal which can only be reached by close attention to detail, and by the exercise of forethought and imagination. Preliminary study of an aeroplane photograph of the locality will enable the effects of preparatory work, and subsequent active occupation, to be foreseen, and consequently make it easier to plan methods of combating them. These methods must be put into force before commencing work. To do so afterwards is futile, unless it is certain that no observa- tion from the air has been possible during the progress of work. The processes of successful camouflage are closely analogous to those of successful crime namely, preliminary reconnaissance, suppression of clues, provision of false clues, variety of method and concealment of the crime itself.

In " the following study of the principles ' of camouflage the subject is dealt with in relation to the concealment of gun posi- tions. In practice many other works were also concealed, such as machine-gun emplacements, defences, dumps, mine spoil, gas projector installations; but similar problems are encountered in all these cases.

Gun positions can be located by (a) aeroplane photography, (b) air observation, (c) flash spotting, (d) sound ranging. The two last furnish certain limited information. Beyond screening flashes, no method of frustrating them has yet been evolved. The manifest remedy (failing a silent, flashless propellant) is the skilful employment of dummy flashes and synchronized reports. But it is principally by means of photographs taken from the air that positions are definitely located on a map. The chief opponent to be overcome, therefore, is the expert, who, with the advantages of time ami undisturbed concentration, which are lacking to the aeroplane observer, is able to interpret what is recorded on photographs. The aeroplane observer cannot, how- ever, be altogether disregarded, and, although the main efforts must be directed towards defeating the air photograph expert, it must be done in such a way as not to draw the attention of the observer.

The camera is a most accurate witness, and a photograph will always record something. The art of camouflage lies in con- veying a misleading impression as to what that something sig- nifies. The photograph records colours and accidents of ground (such as bare earth, vegetation, woods, etc.) in terms of light and shade, and is a patchwork or pattern of black and white meeting in varying intensities of grey. The pattern may be large and simple like that on a chess-board, or intricate and confused like that on a painter's palette. A cultivated district presents a regular chess-board pattern, with large rectangular expanses of monotone, the only accidents to break the monotony being occasional hedges, banks, or houses, with their attendant shadows. Broken ground, such as demolished villages, shelled areas, or patchy vegetation, presents 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 towards the white end of the scale, and grass or vegetation towards the black not because of their respective colours but on account of the amount of contained shadow or " texture."

A billiard-table or top-hat illustrates this quality. Brush them the wrong way, against the nap, and their tone is low- ered to dark green in the one case, and dead black in the other; brushed the right way they appear very noticeably lighter in tone. The reason is that they gain " texture " when brushed the wrong way, and lose texture when brushed the right way. In other words, they absorb light in the former case, and reflect light in the latter. Nap is constituted of countless slender hairs, each one throwing a shadow when erect, but casting little when flat. Grass, or vegetation, possesses this same property to a marked degree. The longer it is the darker it appears on a photograph; but when it is pressed down, the amount of shadow thrown is lessened, and consequently it appears lighter. Hence the obvious-

ness, on a photograph, of a slightly worn track in grass which is scarcely noticeable when viewed from the ground. Earth, on the contrary, contains little texture, and the longer it has been turned up and exposed to rain and sun, the less it contains. A beaten track is, however, conspicuous as it contains no texture at all, and will therefore reflect more light.

The reason for the mottled effect, in a photograph, of a patchy mixture of grass and earth, which blend imperceptibly into each other, is therefore evident. The appearance of snow can be divined from the foregoing. Contrasts in tone are much ac- centuated, and the effects of shadows are more marked, partly owing to the fact that snow usually falls at a time of year when the sun's path in the sky is low.

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. A field of young corn, surveyed from the ground, appears green, but from above, probably the earth only is seen, darker in tone than the normal, owing to the shadows cast by the young blades of corn. Similarly, with a field of ripe corn the actual light tone of the straw and ear will be somewhat dark- ened by their shadows.

It is of the first importance to grasp this principle of regarding any locality purely from the point of view of the pattern it will present on a photograph. Therefore, the most practical method of planning the concealment of any work is to plan it with refer- ence to a recent photograph which records the ground pattern, and the natural facilities for concealment which exist in the locality. Such facilities abound in a neighbourhood whose photo- graphic pattern is complex, and become less frequent as the pattern becomes less complex. Any slight error in exact repro- duction may escape notice in the prevailing complexity, because detection depends on comparison, and comparison is rendered perplexing by the very intricacy of the pattern; the difficulty is enhanced by the variations present in successive photographs of the same place, due to dissimilar conditions of light. A simple analogy is the comparative visibility of an ink stain on a patch- work hearthrug and on a table-cloth.

There are certain characteristic clues which will always betray new work to the reader of aerial photographs. They are: (a) disturbance of soil; (b) tracks; (c) shadows; (d) regularity; (e) blast marks of guns. To achieve success, these clues must be suppressed from the very beginning. Or if deception is to be achieved by the use of dummies, these clues must be supplied.

The prolonged duration of the period of trench warfare was responsible for the introduction of many new methods of waging war scientifically. Among these was the systematic study of the enemy's normal activities, as gauged by observation over a long period, to determine such things as average intensity of gunfire, movements behind the lines, density of traffic, number of hospi- tals, size of dumps, etc. The chief evidence was obtained from photographs, taken at regular intervals, of the whole enemy front to a depth of several miles. Comparative analysis of this photo- graphic diary revealed departures from the normal from which deductions could be made. It was therefore of the utmost im- portance to preserve an appearance of " normality."

Clues (a), (b) and (e) call for no special comment, but some further explanation may be added in the case of shadows and regularity.

Shadows. The form of any erection, or excavation, is revealed 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 differ- ently 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.

Regularity. No shape in nature is of regular outline; conse- quently anything of a regular shape in a photograph invites scrutiny because it must be the work of human hands. In a battery position, regularity is usually displayed in the geometric shape of the gun-pit, and the regular spacing and alignment of the guns.