20 GLASS lishment, that the glass has been taken from the furnace, cast, and put into the annealing oven in less than five minutes. From the annealing oven the plates are taken to the warehouse, where they are carefully examined to see how they may be cut to the best advantage. In dif- ferent manufactories and at different times va- rious processes have been in use for grinding and smoothing the surface of plate glass, but the principle has been the same in all, viz.: rubbing the surface to be smoothed with an- other surface either of glass or iron, and at the same time applying sand or emery of different degrees of fineness and water between the two impinging surfaces. One of the most approved methods of grinding and smoothing the plates was introduced into England in 1856, and adopted in the British plate-glass works. This apparatus consists of a revolving table, 20 ft. in diameter, fixed upon a strong cast-iron spin- dle, and capable of running at an average speed of 25 revolutions a minute. Above the table frames are arranged to hold the plates of glass, which are laid in a bed of plaster of Paris, with the face to be polished resting upon the table. These frames also revolve on their centres by the friction of the table upon the glass, slowly, but so as to present each side of the plates they hold to an equal amount of rubbing as they are moved nearer to the centre of the table or further from it. Sand and water are applied to facilitate grinding down the glass. The grind- ing by this process is found to be even and equal, and the machinery to work smoothly and steadily from the facility with which the plates accommodate themselves to the power applied. After grinding they are smoothed with emery powder of finer and finer quali- ties, and are thus prepared for polishing. By the process above described the grinding and smoothing are done by the same machine ; but formerly two sets of apparatus were required for this purpose. By grinding the surface of the plate is made true, but presents a rough appearance which is removed by the process of smoothing, At this stage it is somewhat opaque, and this defect disappears after the final process of polishing. This is performed chiefly by machinery. The plate of glass hav- ing been fixed upon the table by means of plaster of Paris, the surface is subjected to the action of a series of wooden blocks covered with felt and attached to a frame by which they are made to move over the surface of the glass. At the same time a polishing powder, generally red oxide of iron, is applied, while the friction may be increased by adding weight to the rubbers. Polishing sometimes brings out defects which were before concealed ; the plates are consequently again assorted, and, if need be, reduced to smaller sizes. (For the methods of silvering them, see MIRROR.) Bend- ing the large plates or the smaller sheets of glass for the purpose of fitting them for bow windows, &c., is an especial branch of the manufacture. A core of refractory material and suitable shape is introduced upon the floor of the furnace ; and upon this is laid the sheet to be bent, which as it softens by gravity conforms itself to the shape of the bed upon which it is laid. The value of plate glass varies greatly with the size. In the United States the price of a plate of standard British or French glass, 5x3 ft., is about $35 ; but when the dimensions are double, the plate being 10x6 ft., the price is increased to about $175. A plate 14 x 8 ft. is valued at about $500. No glass is of such importance in the arts as that of which the lenses of optical instru- ments are made. Both flint and crown glass are applied to this use, but each of them has its defects. The former, from the great differ- ence in the densities of its ingredients, is with much difficulty obtained of homogeneous struc- ture, an essential requisite in all glass used for optical purposes ; and the latter is difficult to procure of uniform composition and texture, from the high temperature required for its fu- sion and the consequent tendency to devitrify in cooling; or if this is obviated by an in- creased proportion of alkali in the composition, the excess of this causes attraction of moisture from the air and a damp surface to the lens. The best flint glass is subject to defects, chief among which are undulatory appearances called striaa, resulting from a want of uniform density in the glass, and tending to refract and disperse in different directions the rays of light passing through it. These defects are of great impor- tance when the glass is to be used for optical purposes. In 1753 John Dollond, an English optician, first began the construction of achro- matic object glasses, formed of two kinds of glass of different density, in accordance with the theory announced not long before by Euler. For this purpose Dollond used fragments of flint and of crown glass, but did not succeed in making object glasses with a larger aperture than 2 or 3 in. in diameter ; and when the need of telescopes of greater magnifying power was strongly felt, it was difficult to produce flint glass sufficiently free from striae for a lens 4 in. in diameter. The scientific bodies of France and England offered prizes for the at- tainment of this result, and the most renowned glass manufacturers at the end of the last and the beginning of the present century endeav- ored to solve the problem. This was done by Guinand of Switzerland, a man not con- versant with science, nor even a glass man- ufacturer, but an optician. By methods of his own he made the furnaces, crucibles, and mix- tures he employed, and produced the glass, which he shaped and polished, giving without knowledge of mathematics the requisite pro- portion to the curves of its surface, and com- pleted lenses of flint glass of great perfection of structure, 9 in. in diameter. The secret of his success in making the glass is believed to have consisted in keeping the mixture agitated by stirring when at its greatest liquidity, and then j suffering it to cool and anneal in the pot. From
