HOW THE GLASS IN LENSES IS MADE 



be poured from vessel to vessel as easily as water; 

 at lower temperatures it is viscous, i.e., semi-fluid 

 and can be rolled with an iron roller as dough 

 is rolled with a rolling pin; it can be moulded 

 into any desired shape, blown out into flasks 

 and bottles or drawn out into threads so fine 

 that they may be woven into a fabric. It is a 

 bad conductor of heat and for this reason it is safer 

 to pour very hot liquids into a thin glass vessel than 

 into a thick one. With thick glass the inner layers 

 expand with the heat before the outer layers are 

 even warm and the result is a crack or often ab- 

 solute fracture. Sometimes during manufacture 

 glass vessels which are suddenly cooled will appear 

 satisfactory, but the particles of glass remain in so 

 high a state of tension that at the slightest touch 

 the vessel will break up into thousands of pieces 

 On account of this property of glass it must be 

 cooled very slowly indeed; the process is known 

 as annealing. 



Optical glass unlike most other kinds must be 

 manufactured in thick blocks — some of the large 

 lenses on telescopes are of considerable thickness. All 

 glass for scientific instruments must also be homo- 

 geneous, which our dictionary tells us means of the 

 same kind. To be more explicit each particle of 

 optical glass should be precisely the same in com- 

 position and properties as every other particle. 

 In the very early days of manufacture it was diffi- 

 cult to obtain homogeneous pieces of glass and 

 Guinard, in the 18th century, conceived the idea of 



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