WAR-TIME PHOTOGRAPHY 95 



covering a plate 18 x 24 centimeters in size. Their problem 

 lay chiefly in securing optical glass, of which the Germans had 

 almost a monopoly. 



The optical glass problem is one which the Allies collectively 

 did solve, but in so far as the dense barium crown glass required 

 for modern photographic lenses is concerned, the greatest suc- 

 cess was attained by the French and English, the latter indeed 

 now bidding fair to oust the Germans from their primacy in 

 the optical industries. While the American optical glass 

 development did not succeed in producing the greatly desired 

 dense barium crown, American manufacturers were able to 

 utilize some substitute glass, and, by using English glass, de- 

 veloped new lens formulae admirably adapted to aerial use, 

 so that lenses in satisfactory quantities were produced of 50 

 centimeters focus, of aperture F/6. 



It is one of the severe limitations of airplane photography 

 (but not of photography from dirigibles) that all exposures must 

 be strictly instantaneous. Calculation shows that the condi- 

 tions are rare when a speed of less than i/ioo of a second 

 may be used without fatal blurring. And the faster the plane, 

 and the lower it flies, the faster does the image move on the 

 plate, and the quicker must the shutter act. 



The common type of shutter used on the smaller commercial 

 cameras, situated between the lens elements, was not suitable 

 for airplane use, because it could not be made in large, sizes; 

 nor was it at all efficient at high speeds. There was, however, 

 already at hand the focal plane shutter, a rapidly moving 

 slotted curtain, traveling close to the plate, originally developed 

 for the photography of rapidly moving objects on the earth's 

 surface, such as race horses and automobiles. With very few 

 exceptions all aerial cameras were equipped with shutters of 

 this type. But the existing designs were found to be defective 

 in many respects. The speeds developed were insufficient ; 4 

 the means for varying speed were inadequate. Most common 

 of all, the speed was greatly different at the beginning and at 

 the end of the travel across the large plates used, so that strip 



