WAR-TIME PHOTOGRAPHY 97 



Professor Pope of Cambridge. These plates possess the 

 unusual characteristic of being more sensitive to red than to 

 blue light, and possess at the same time unusually high speed. 

 Produced first in the spring of 1918 these plates proved a God- 

 send to the Allied aerial photographers in the dull days of the 

 last great offensive, when the plates formerly employed by 

 them were only usable a few hours near noon. 



Closely connected with the matter of color sensitiveness in 

 the plate is the question of color filters, to pierce the veil of 

 haze characteristic of the view from high altitudes. How 

 important is the use of a filter is shown in Figure 3, where 

 the picture taken at 10,000 feet without a filter is quite use- 

 less, while its companion, taken at the same time but with a 

 filter, shows the roads, trees, and other details clearly. Here 

 again the Germans were at an advantage both because of their 

 mastery of the manufacture of colored glass and also because 

 of their well-developed dye industry. In this connection it is 

 to be noted that the ordinary yellow filter intended to produce 

 orthochromatic effects is not what is required to pierce haze. 

 The requirement here is for a comparatively abrupt absorption 

 of the blue of the spectrum, which will cut down the green 

 but little and so leave the filter as efficient as possible. 



The problem of producing filters of the required efficiency, 

 through which the exposure would not have to be increased 

 more than two or three times, was completely solved for the 

 American Air Service by two developments. The first was 

 a yellow glass produced by a leading glass manufacturer, and 

 the second a new dye, the " EK " (from the name of the com- 

 pany in whose laboratory it was developed) with which gelatin 

 discs were dyed and afterward mounted between glass plates 

 to form a highly satisfactory filter. 



The simple camera above described, which sufficed for spot- 

 ting, was subject to many improvements aimed at simplifying 

 its manipulation, increasing the speed of operation, enlarging 

 its plate capacity, and a vital point in the military plane 

 making its operation as independent as possible of the attention 



