=Impression of Relief Produced by Motion.=—An appearance of solidity can be obtained in _moving pictures_ by the simple expedient of slowly moving the camera laterally as the pictures are taken. As an illustration, if the moving picture camera is carried on a boat while structures on the shore are photographed, when these are projected on the screen they appear in relief, due to the relative motion of foreground and background. As relief of this sort is not dependent on the use of the two eyes, it demands no special viewing apparatus. This idea has been utilized to a limited extent in ordinary moving picture photography by introducing a slow to-and-fro motion of the camera, but this can hardly be considered satisfactory, since this motion is so obviously unnatural. In moving pictures made from the airplane the normal rapid motion of the point of view is ideal for the production of the impression of relief in the manner just described. For instance, in moving pictures of a city made from a low flying plane, the skyscrapers and spires as they sweep past stand forth from their more slowly moving background in bold and satisfying solidity. In fact, such pictures probably constitute the most satisfactory solution yet found of the vexing problem of “stereoscopic” projection. No better medium can be imagined for the travel lecturer to introduce his audience to a foreign city than to throw upon his screen a film made in a plane approaching from afar and then circling the architectural landmarks at low altitudes. CHAPTER XXIX THE INTERPRETATION OF AERIAL PHOTOGRAPHS Oblique aerial photographs if on a large enough scale are even easier to interpret than are ordinary photographs taken from the ground, since they practically preserve the usual view, and add to it the essentials of a plan. With verticals, however, this is far from the case. In them all natural objects present an appearance quite foreign to the ordinary mortal's previous experience of them. This may be easily demonstrated by taking any aerial view containing a fair amount of detail and trying systematically to identify each object. A necessary preliminary to doing this accurately is acquaintance with and study of the ground photographed, or of similar regions, and of objects of the same character as those likely to be included. The interpretation of military aerial photographs is of such importance, and has become such an art, that it is the function of special departments of the intelligence service. Extended courses in the subject are now given in military schools. This instruction must cover more than the interpretation of aerial photographs as such. General military knowledge is essential, so that not only may photographed objects be recognized, but the significance of their appearance be realized. Whether attack or retreat is indicated; whether a long range bombardment is in preparation, or a mere strengthening of local defences.