WAR-TIME PHOTOGRAPHY 93 



short exposures but adequate anti-vibration mounting of the 

 camera. 



Before taking up these problems one by one, let us look at 

 the various types of photograph required. The simplest picture 

 of all is produced by the process called " spotting," which con- 

 sists in taking a single photograph of some important detail, 

 such as a trench or a battery. (Figure i.) Pictures of this sort 

 were usually made with as long-focus lenses as possible, in 

 order to secure large magnification. For this purpose cameras 

 of as great focal length as 120 centimeters figured as a regular 

 part of aerial photographic equipment. 



Next come strip or mosaic maps made by a series of succes- 

 sive exposures, at such intervals as to overlap by a quarter 

 or a third of their length. These photographs showed trenches, 

 railways, large manufacturing plants, or other extended areas 

 of military importance. (Figure 2.) 1 



Both these types of picture were *' verticals," that is, made 

 with the camera pointing directly downward through the floor 

 of the plane. " Obliques " were pictures taken with the camera 

 at an angle. At first made with hand-held cameras, these were 

 later taken by cameras slung in the plane at the desired inclina- 

 tion. Obliques closely resemble views made from high build- 

 ings. They show what the verticals do not, the elevations and 

 depressions of the terrain, and because of the natural appear- 

 ance presented by objects so photographed and because of their 

 ease of interpretation, they were of great value during the 

 preparation for local attacks. 



Last of all come a class of pictures which may probably be 

 claimed as an entirely new development due to the war. These 

 are aerial stereoscopic views, produced not with two lenses 

 separated by the distance of the eyes apart, as are ordinary 

 stereograms, a method which, at flying altitudes, would give 

 practically no relief, but with a single camera taking successive 

 exposures separated sometimes by a few seconds, often by a 



1 Acknowledgement is made to the Air Service of the United States 

 Army for figures 2 and 3. 



