now THE GLASS IN LENSES IS MADE 



fragments of glass of similar composition are added. 

 The crucibles, in which the chemical components 

 are melted, are covered so that no fumes from the 

 furnace may gain access to them, even the chemical 

 composition of the crucibles is carefully tested that 

 no impurities may contaminate the glass. No 

 crucible is ever used more than once and only 

 a single crucible is heated in each furnace, in 

 order that the temperature may be regulated to a 

 nicety. 



The actual manufacture is then begun after all 

 these preliminaries have been attended to. A clean 

 dry crucible is heated in a furnace — not the one in 

 which the glass making is to take place — to a dull 

 red heat. Then, with iron tongs, it is removed to 

 the previously heated glass making furnace and the 

 temperature is raised very gradually. The next 

 stage sees the addition of the well mixed chemicals 

 to the heated crucible, in small quantities at a time. 

 When the full quantity has been added the crucible 

 contains melted glass full of bubbles, some of them 

 air bubbles released from the raw materials as they 

 were added and some bubbles of gas given off from 

 the chemicals as they act upon one another. The 

 molten glass is then heated strongly so that it will 

 become perfectly liquid and many of the bubbles 

 will be driven off. To reach this stage may occupy 

 anything from thirty-six to sixty hours and 

 constant attention is necessary during the whole 

 time. 



The next stage is perhaps the most important in 

 287 



