OPTICAL GLASS.
The manufacture of optical glass--Raw materials--
Mixing--Furnaces and crucibles--Kilns for heating
pots--Transfer of pots from kiln to melting furnace--
Introduction of cullet and raw materials--The fining
process, difficulties and limitations--The stirring
process--The final cooling of the glass--Rough sorting
of the glass fragments--Moulding and final annealing
of the moulded glass--Grinding and polishing of plates
and discs for examination; smallness of yield obtained--
Difficulty of obtaining large blocks of perfect glass _p._ 223