SELECTION OF GLASS.
The glass of which lenses are made is purchased in the form of rough plates (sec Fig. 528) from -various British and dustry, suitable machinery and tools can not be purchased, as looms for weaving may be. Opticians have either to content themselves with relatively crude appli ances, or to design and make more perfect ones for themselves. A reference to the well-known Leicester works of Taylor, Taylor, and Hobson will give the reader a good idea of what has to be done. At these works the making of tools receives considerable attention, and a separate de partment known as the tool-room (see Fig. 526) is devoted to the construction and re pair of tools. Another distinct branch of manufacture has arisen in the making of a machine, originally designed for en graving on lenses, but now also used in other industries for requirements as widely different as the construction of moulds for biscuits and the engraving of Maxim guns. Fig. 527 shows the shop in Continental makers. Good quality glass is expensive, a plate four inches costing sometimes as much as fifty shil lings. Both faces of these raw plates are
ground and polished to facilitate a thorough inspection of the material before it is converted into lenses, and in the course of the examination a large pro-1 portion of the glass is marked to be cut out and thrown away. It might be thought that so expensive a material should be free from all defects ; but the difficulties in its manufacture are so serious as to make this almost impossible. In order that the various glasses shall possess the necessary refractive and dis persive powers, a large variety of sub-, stances are employed by the makers to modify or temper the fusible earths which form the chief constituents of the glass. The perfect incorporation of all these in gredients by inciting and stirring them in a crucible, and the avoidance of dis coloration and dirt, is an extremely delicate task. When a mass of glass has been prepared as described, it is allowed to cool and then broken into fragments. The cleanest and most perfect of these arc next selected for remelting.