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menters in the new varieties of optical glass, just at the time 

 when dry-plate photography was coming extensively into 

 vogue. 



Much work was done with a view to the production of 

 such glasses, and this resulted in the manufacture of a 

 number of barium glasses, which have an index of refraction 

 comparable to that of a flint glass, but a dispersion similar 

 to that of a crown glass. 



In this country the investigation of barium glasses was 

 commenced in 1897, and a long series of experiments were 

 put in hand with a view to producing these glasses on a 

 commercial scale. Great difficulties were experienced in 

 obtaining suitable materials, and the many new problems 

 which presented themselves to the glass manufacturer proved 

 very costly and, for a time, could not be successfully solved, 

 though the investigations were carried out under the super- 

 intendence of a highly-qualified chemist. There was also 

 the difficulty of having to meet the requirements of opticians 

 who had, for some years, been using glass of particular 

 optical constants and required these matched before they 

 could be expected to change their source of supply. 



Commercially, there was no inducement to carry on the 

 manufacture; but it was felt that, in the national interest, 

 the manufacture of optical glass was one which should not 

 be dropped, and it was, therefore, persevered in, though with 

 the most discouraging results and without any Government 

 assistance. Eventually, however, some success was obtained, 

 and various types of barium glasses were gradually introduced, 

 so that at the outbreak of war there were a number of the 

 newer types of glasses which could be manufactured in this 

 country suitable for the use of opticians. 



The outbreak of war brought about an entire change in 

 the position of affairs, and an appreciation of the fact that 

 the manufacture of optical glass is a " key " industry. 



Great efforts were made to increase the output of the 

 types of glasses required for war instruments, and such 

 requirements were successfully met. It was found, as a 

 matter of fact, that the types of glasses manufactured by 

 Messrs. Chance Brothers at the outbreak of war were for 

 most purposes sufficient to cover the varieties demanded for 

 optical munitions. Towards the end of 1917, however, it 

 became necessary to extend the types of barium and other 

 special glasses, to meet the requirements of photographic 

 lenses for the Air Board. It was also found that it was not 

 only desirable, but essential, that research work should be 



