VII 



OPTICAL GLASS FOR WAR NEEDS 

 HARRISON E. HOWE 



THE optical-glass problem, so far as the United States was 

 concerned, can be simply stated. Large quantities of 

 dependable quality were required immediately, the varieties 

 being limited to a half dozen or so necessary for military 

 optical instruments. It should be understood that by optical 

 glass is meant that type of glass which is so made that its 

 physical characteristics may be controlled within rather narrow 

 limits, so that it is suitable for the exacting requirements of 

 photographic lenses, range finders, spotting telescopes, binocu- 

 lars, periscopes, gun sights, and similar modern warfare 

 requisites. 



In order t 1 at the complexity and magnitude of this problem 

 may be more clearly understood, it will be well to examine 

 briefly the history of its development elsewhere and understand 

 the condition which prevailed in our country prior to 

 August, 1914. 



Prior to 1886 the glass makers were offering a very limited 

 variety of optical glass to the makers of refracting instruments, 

 and the perfection of the various microscopes, telescopes, etc., 

 was necessarily limited to the possibilities presented a few 

 crown and flint glasses. The possibility had been established 

 of combining two lenses made from the available glasses into a 

 doublet so as to bring pairs of colors to a common focus on the 

 optical axis of the lens, thereby diminishing chromatic aberra- 

 tion. Means to render the image almost entirely free of 

 spherical aberration had also been devised, but no attempts were 



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