quite unfit for use. To avoid this settling and secure a clear, homogeneous glass, that is the problem.
At the present time the best optical glass is probably made in France, and the methods there in use are consequently most worthy of examination. During the melting process the crucible is placed in the center of a domed furnace. The flames play around the crucible on all sides, making an intense heat possible.
Fig. 1.—Flint-glass Furnace. The hood prevents the furnace gases from acting upon the compounds of lead and reducing them to the metallic state. The well-mixed batch is introduced in small quantities into the thoroughly heated crucible, and the charging process continued until the pot is completely filled. This will require from six to ten hours. The heat is then continued for perhaps four hours, at the end of which time the molten glass is vigorously stirred with a wrought-iron rod incased in a fire-clay cylinder. Then comes a second period of quiet heating and a second stirring. After this the stirrings succeed each other at every hour. When these hourly stirrings have been repeated perhaps half a dozen times, the crucible is allowed to cool down for a couple of hours. It is then heated to the utmost that the furnace will permit. As the result of this intermittent treatment, the glass is very liquid and is quite free from bubbles and striations. During the gradual cooling which succeeds this firing, a constant stirring is maintained for at least a couple of hours. When the stirring becomes too difficult, it is discontinued, and crucible and furnace are allowed to cool during a period of ten days or more.
The secret of making fine optical glass lies in this stirring. It was first carried out by Guinand, in Switzerland, in the early part of the century, and was introduced in Paris by Bontemps.
When the cooling is accomplished, the crucible is removed from the furnace and broken, so as to free the mass of flint glass which it incloses. In spite of all this care, it must not be supposed that a mass of perfect glass is the result. On the contrary, it is full of flaws and imperfections, and only a part of it can be used. It is customary to grind and polish parallel faces on the
