One of the most striking and valuable developments in aerial photography has been the use of stereoscopic views. Pairs of pictures, taken with a considerable separation in their points of view and studied later by the aid of the stereoscope, show an elevation and a solidity which are entirely wanting in the ordinary flat aerial vista. Often, indeed, these attributes are essential for detecting and recognizing the nature of objects seen from above. Stereoscopic aerial photography has been justly termed "the worst foe of camouflage".