HOW THE GLASS IN LENSES IS MADE 



then the glass, which may be in one mass weighing 

 as much as 1000 lbs., is freed from particles of 

 fireclay and examined for defects. 



The next stage consists of moulding and anneal- 

 ing. Large pieces of glass are heated till they are 

 just soft, then they are passed into iron or fireclay 

 moulds designed so that the glass is formed into 

 discs or slabs suitable for grinding by opticians. 

 They are allowed to cool very slowly in the moulds. 



When the glass is taken from the moulds it is 

 not yet ready to be made into lenses. It is sub- 

 jected to another and very careful examination, 

 when all defective parts are cut out. Should there 

 be many defects the glass is again heated, moulded 

 and annealed. From a crucible containing 1000 lbs. 

 of molten glass it is unusual to obtain much more 

 than 200 lbs. of optically perfect glass, it is obvious 

 therefore that lenses canot be cheap. 



One would naturally imagine that the minute 

 lenses used in microscope objectives should be 

 cheaper in comparison than the larger lenses used 

 for photography or in telescopes, for it is always 

 more difficult to make minute articles than large 

 ones. As a matter of fact, even allowing for the 

 greater amount of material used in the larger lenses, 

 quality being the same in both cases, they are far 

 more expensive than the smaller microscope lenses; 

 the reason being that it is exceedingly difficult to 

 obtain a perfect specimen of large size. The 

 difficulty arises not only in the actual manufacture 

 of the glass but in the subsequent operations of 



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