HOW THE GLASS IN LENSES IS MADE 



the whole process. After numerous small samples 

 of the molten glass have been taken, on the end 

 of iron rods, to see if the air bubbles have been 

 driven off, the mixture is stirred to render it homo- 

 geneous and to eradicate striae. The stirring is 

 carried out by means of a cylinder of fireclay which 

 is first of all heated to the temperature of the 

 molten glass before it is introduced. To the end 

 of the fireclay cylinder a long, detachable iron 

 handle is fixed so that the man who undertakes 

 the stirring may stand at a distance from the hot 

 furnace. The heat is great and the work of stirring 

 is laborious and for this reason the stirrers are 

 constantly changed. The iron handles must be 

 watched carefully for, owing to the heat they rust 

 rapidly and should any of the rust fall into the 

 molten glass it would impart to it a colour and 

 render it useless for optical purposes. When stir- 

 ring begins the glass is liquid as water, but the 

 stirring is continued during cooling and all the 

 while the glass is gradually becoming more and more 

 solid. During the final stages the operation is hard 

 labour indeed and, finally, it is not possible to stir 

 any more, then the fireclay cylinder is either 

 removed or left in the glass. 



When the glass has solidified and, whilst it is 

 still hot the furnace is sealed and allowed to cool 

 very gradually till it has reached the ordinary 

 temperature of its surroundings ; this may take 

 several weeks. Wlien quite cool, the crucible is 

 removed from the furnace and carefully broken; 



288 



