book did not allow satisfactory reproduction till the contrast of the

negatives was greatly increased by the arts of the photographic laboratory. But, even at its best, no photograph taken vertically affords an adequate idea of the height of hills or the depth of hollows. Only shadows that are particularly well defined can be distinguished as shadows, while small elevations and depressions affect the negative no differently than a difference in marking or color. In military defenses, if the mere surface of the camouflage is sufficiently realistic, the ordinary camera is even more easily deceived than the human eye. It is a well-known fact that man and other animals of the higher order see objects in relief, within a certain range of vision, because the eyes convey to their respective retinas slightly different images of the same object which the brain combines into a relief image. The stereoscopic camera has long been used for the same purpose. Its principle, with certain adaptations that need not be discussed here, has been to some extent employed in airplane pictures, with such excellent results that it is claimed by some that by further development actual contouring will be possible by this means. It is reported that in military reconnaissance stereoscopic pictures render ordinary camouflage useless and that bridges, observation towers, gun emplacements, etc., are shown in relief and, therefore, easily detected.[2]