The cement is made by mixing sifted wood ashes with melted pitch, the essential oil of which is absorbed by the wood ashes, and the adhesiveness of the pitch is thereby reduced. The proportions are somewhat dependent on the temperature of the weather, and the quality of the pitch; but generally about 4lbs. of wood ashes to 141bs. of pitch are employed, and the cement if too hard and brittle is softened with hog's lard, or tallow.
The glasses are in all cases rough ground separately within the shell, fig. 1127, either with river sand and water, or coarse emery and water, until the surfaces are brought nearly to the curve of the shell. The glasses are rubbed with large circular strokes, and the shell is usually placed within a shallow tray, to catch the loose sand or emery thrown off in the grinding. The second side is rough ground in the same manner, the glass being warmed for the removal of the cement handle, which is transferred to the other side. The parallelism of the two sides is obtained by observing that the edge of the glass is left of equal thickness all round.
So far the lenses whether large or small, and of the best or common quality, are treated alike, but for grinding the glasses to the correct form in the brass tool, and also for polishing, they are operated upon either singly or several together, according to the size and degree of accuracy required in the lenses. The best lenses for the object glasses of telescopes being ground and polished singly, while on the other hand as many as four dozen of common spectacle glasses are sometimes cemented upon a runner and ground and polished at the same time. When several lenses are to be ground and polished together, the number must be such as admits of being arranged symmetrically around a central lens, as 7, 13 or 21, at other times a group of four forms the nucleus, and the numbers run 4, 14, 30. Lenses of medium quality and size are however generally ground true and polished seven at a time.
The cement at the back of the lenses is first flattened with a heated iron, and the seven lenses are then arranged with the cemented sides upwards in the concave brass tool, one lens being placed in the center, and the other six at equal distances around it, very near together but without touching. The cast iron runner is then heated just sufficiently to melt the cement, and carefully placed upon the cemented backs of the lenses. As soon as the cement is sufficiently softened to adhere firmly to the runner, the latter is cooled with a wet sponge, as the cement must be only so far fused as to fill up the spaces nearly, but not quite, level with the surface of the lenses.
The block of lenses, shown in fig. 1129, is now mounted upon the post, and ground with the concave brass tool, fig. 1128, in exactly the same manner as explained for correcting the forms of the tools themselves. About six sizes of washed emery progressively finer are employed for grinding the lenses to the true figure, or as it is called trueing the lens, the last size of emery being the fine powder collected after one hour's subsidence as explained at page 1055-6, and which leaves so smooth a surface, that when the lens is held between the eye and the light, it shows a semi-polish.
Of course the grinding is continued with every size of emery until all the marks made with the previous size are removed, and between every change, the brass tool, hands and block of lenses, are thoroughly washed, and wiped first with damp and afterwards with dry cloths, to remove every particle of the previous emery, which without the greatest possible care would be especially liable to lodge in the spaces between the lenses, and might near the conclusion of the work become detached, and make a scratch that would render it necessary to recommence the grinding.
The lenses have next to be polished, for ordinary lenses of medium size, the polisher is made by warming a cast iron shell, and coating it uniformly about one quarter of an inch thick with melted cement. A piece of thick woollen cloth, such as was formerly used for watchmen's coats, is cut to the size of the polisher, and unless the cloth is old and the nap worn off, it is seared with a heated iron. The cloth is placed on the cement in the polisher, and pressed into form by working the brass convex tool within it; the pores of the cloth are then filled up with putty powder prepared as explained on page 1088. The putty powder is mostly sifted through lawn, and enclosed in a box having a lid perforated with small holes. The putty powder is shook uniformly over the cloth, and moistened by sprinkling a few drops of water over it; the powder is then worked into the pores of the cloth with the brass convex tool, additional powder being applied until the surface is made quite level, and it is worked quite smooth with the tool; from two to three hours being generally required for making up a polisher of 8 or 9 inches diameter. Kerseymere is sometimes used for small lenses instead of the thick cloth, principally because the face of the cloth being finer it is sooner filled up with the putty powder.
The polisher when completed is placed upon the block of lenses, still fixed on the post, and worked with wide and narrow elliptical strokes, the operator continually walking around the post the same as for grinding. The point requiring the principal attention is the degree of moisture of the putty powder, which should be only moderate; if too wet the putty is apt to run loose upon the polisher, which produces a curdled surface so difficult to remove that when once produced it is generally necessary to return to the fine grinding. If upon the other hand the polisher is allowed to become too dry, it is indicated by the edges of the lenses cutting up the surface of the putty powder, which then works with an unpleasant scratching action that will be immediately detected.
The proper degree of moisture of the putty powder, is indicated by its being in a rather stiff saponaceous state, and during the principal portion of the polishing, the surface should present a partially glazed appearance. When the surface becomes almost entirely glazed, a little more water is sprinkled on it; but towards the conclusion of the polishing less moisture is used, and the polisher is allowed to become as nearly dry as is consistent with safety, the glazed appearance then covers almost the whole surface.