The most elaborate camouflage works can be rendered utterly useless, however, by the carelessness of a single soldier, for there is little that escapes the eye of the airman’s camera, particularly when it was fitted, as during the latter days of the war, with a stereoscopic attachment. I remember that in one of the Champagne sectors the Germans had installed a battery of heavy guns which were so ingeniously concealed that the French were unable to locate them. It was believed that they were hidden somewhere in a fringe of woods along a stream, but though there was a considerable area of cultivated land beyond the woods, the aerophotographs of it showed nothing which would suggest a path such as would be made by artillerymen going to and from their guns. One day, however, a new batch of plates, upon being developed, showed a dim gray line, faint as the shadow of a hair, leading across this cultivated area to a small wood on the bank of the stream, where a battery might easily be concealed. Upon studying an enlargement of the picture the intelligence officers became convinced that the shadowy line on the negative really represented the trail left by a soldier crossing the field. Proceeding on the surmise that the soldier was an artilleryman going up to his gun-position, the French gunners registered on that particular patch of woods the following morning, whereupon the fire from the concealed battery abruptly ceased. German prisoners captured a few days later explained how the secret of the battery’s position had been kept so long. The German security officer had issued orders that the artillerymen must under no considerations walk across the fields in order to reach their guns, but that they must instead follow a much-used highroad until they reached a bridge over the stream, drop from the bridge into the water, and wade up the stream until opposite their position. But one night an artilleryman, in a hurry to reach his battery and confident that the tracks left by a single man could do no harm, took a chance and a short cut across the forbidden field. I have told you what happened to his battery as a result of his carelessness. Knowing something of German discipline, I can imagine what happened to him. But it was not often that the Germans were caught napping, and so ingenious were some of their ruses and stratagems, that it required an intelligence officer with the imagination of a Sherlock Holmes to keep up with them. During the operations on the Flanders front a British aviator brought in some photographs of a certain area behind the German lines. The intelligence officer whose duty it was to scrutinize them detected a suspicious something which he was convinced was a cleverly camouflaged German battery, but though it was in the midst of open country there was no suggestion of a path leading to it. After studying the photographs for several hours he suddenly exclaimed: “I have it! They get up to the guns on the covers of biscuit-boxes.”