HOW THE GLASS IN LENSES IS MADE 



on the subject of glass manufacture and especially 

 on the kind used for optical instruments — as a 

 result there are many kinds of glass differing from 

 one another in physical properties and in chemical 

 composition. Although the various chemicals used 

 and their proportions are fairly well standardised, 

 as the result of long experience, it is probable that 

 glass is not a definite chemical compound but a 

 mixture, in which certain of the components act as 

 solvents for the rest. 



The ordinary glass in use in this country, apart 

 from specially prepared optical glass, may be either 

 English flint glass, plate glass or Bohemian glass. 

 The first named is composed of sand, potassium 

 carbonate and red lead; plate glass is made of sand 

 with the carbonates of sodium and calcium, whilst 

 similar ingredients are used for Bohemian glass 

 except that carbonate of potassium is substituted 

 for carbonate of sodium. It is chiefly owing to the 

 requirements of optical instrument makers that, 

 new kinds of glass containing very many previously 

 untried chemicals, have been produced. As a 

 result glasses are now made with specific gravities 

 varying from 2.5 to 5.0, that is to say the weight 

 of a. square inch, or a square foot or a square yard 

 of glass may weigh anything from two and a half 

 to five times more than a square inch, foot or yard 

 of water, according to its composition. 



Before we describe the details of its manufacture 

 let us consider its properties as briefly as possible. 

 At high temperatures it is perfectly fluid and may 



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