HOW THE GLASS IN LENSES IS MADE 



Good optical glass should be transparent and 

 colourless and, as we have stated, it should be 

 homogeneous — the refraction and dispersion of light 

 rays should be identical over all parts of the glass. 

 It should possess no striae, as they are called. Striae 

 may be seen at the edge of a piece of plate glass 

 as little lines just as though the glass had been 

 formed in layers. Striae detract from the efficiency 

 of optical glass, nevertheless, some very cheap 

 lenses are made of plate glass. Bubbles are almost 

 always present in Jena glass but, unless they are 

 very numerous they do not apear to render the glass 

 less efficient. Hardness and chemical stability are 

 other desirable qualities. Most of these high-grade 

 glasses are soft, as shown by the ease with which 

 they may be scratched ; many of them are not very 

 stable chemically and are easily affected by chemical 

 fumes with the result that their surfaces become 

 covered with a coloured film. With all their draw- 

 backs, for optical work the newer glasses far excel 

 the older. 



The manufacture of optical glass is a costly and 

 lengthy process. The chemicals used in its manu- 

 facture are selected with the greatest care; 

 impurities must be guarded against for they would 

 change the composition of the glass and in doing 

 so alter its physical properties on which everything 

 depends. The chemical substances are used either 

 in the form of oxides, nitrates or carbonates, for 

 the reason that they are easily decomposed by heat. 

 To assist in the melting of the substances a few 



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