HOW THE GLASS IN LENSES IS MADE 



cutting, grinding and polishing when fractures are 

 very liable to occur. 



From this brief account of the manufacture of 

 optical glass it is clear that a good lens is always 

 worth a high price. Its components must be of 

 the purest quality obtainable, the process of manu- 

 facture requires highly skilled labour and it is 

 laborious and exacting work. Constant attention 

 is needed from start to finish and much of the glass 

 is never sufficiently good to pass the rigorous tests 

 which it must undergo. Lastly, in the final pre- 

 parations of the completed lens, mishaps are fre- 

 quent. Added to all these trials of the lens maker 

 is the one oustanding fact that the process can 

 never be hurried at any stage, the efficient anneal- 

 ing of optical glass is one of the most important 

 stages in its manufacture. A very full account of 

 the manufacture of optical glass is given in the 

 Encyclopsedia Britannica, whence much of our 

 information in this chapter is derived. 



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390 



