94 THE NEW WORLD OF SCIENCE 



goodly fraction of a minute. The result is a pair of pictures 

 with points of view so separated that the stereogram when 

 placed in the stereoscope presents the earth as it would be seen 

 by a giant with a head a hundred feet or more in width, that is, 

 with all the elevations and depressions showing in magnificent 

 relief. Stereo aerial views, both verticals and obliques, were 

 of the greatest importance in the detection of irregularities of 

 level, in differentiating shell holes from " pill boxes," and in 

 piercing the devices of the camoufleur. 



The airplane camera required merely for spotting is a com- 

 paratively simple affair. No provision is needed for focussing 

 since the objects to be photographed are always at photographic- 

 ally infinite distance. All that is necessary is a lens, a box as 

 little subject to expansion and contraction as possible in the 

 extremes of temperature met from ground to upper air, a 

 shutter, and a plate holder. These essentials of an aerial 

 camera, lens, shutter and plate, may profitably be considered in 

 detail before touching on the more complicated types of camera 

 demanded by mapping or by stereoscopic photography. 



The lens should be of the anastigmatic type, covering a large 

 flat field with microscopic definition, and should have the largest 

 possible aperture, preferably not less than F/4.5. Lenses meet- 

 ing these requirements had already been developed before the 

 war and were in fairly common use for the smaller sizes and 

 foci (up to 8 inch), but almost exclusively as a German product, 

 due to the Jena optical industries. As a temporary measure, 

 all available lenses of this kind were commandeered by the 

 Allies for aerial use. Soon, however, flying was forced to 

 10,000 feet and over, and the pictures obtained with such lenses 

 were too small. This fault was partially met in the British 

 service by the regular practice of making enlarged prints from 

 their standard 4x5 inch negatives. But this was not nearly so 

 satisfactory as the process of contact printing from negatives 

 secured by long-focus lenses. The efforts of the Allied lens 

 manufacturers were, therefore, directed toward the production 

 of lenses of a standard focus of 50 centimeters, capable of 



