MIRRORS 357 



raised successively to different elevations by means of a cord passing over a pulley 

 in the ceiling of the room. Thus the mirror has its slope graduated from day to 

 day, till it finally arrives at a vertical position. About a month is required for 

 draining out the superfluous mercury from large mirrors ; and from 18 to 20 days 

 from those of moderate size. The sheets of tin -foil being always somewhat larger than 

 the glass-plate, their edges must be pared smooth off, before the plate is lifted off the 

 marble table. 



Process for Silvering Concave Mirrors. Having prepared some very fine Paris plaster 

 by passing it through a silk sieve, and some a little coarser passed through hair-cloth, 

 the first is to be made into a creamy liquor -with water, and after smearing the concave 

 surface of the glass with a film of olive-oil, the fine plaster is to bo poured into it, and 

 spread by turning about, till a layer of plaster be formed about jth of an inch thick. 

 The second or coarse plaster, being now made into a thin paste, poured over the first, 

 and moved about, readily incorporates with it, in its imperfectly-hardened state. Thus 

 an exact mould is obtained of the concave surface of the glass, which lies about f of 

 an inch thick upon it, biit is not allowed to rise above its outer edge. 



The mould, being perfectly dried, must be marked with a point of coincidence on 

 the glass, in order to permit of its being exactly replaced in the same position, after 

 it has been lifted out. The mould is now removed, and a round sheet of tin-foil is 

 applied to it, so large that an inch of its edge may project beyond the plaster all 

 round ; this border being necessary for fixing the tin to the contour of the mould by 

 pellets of white wax, softened a little -with some Venice turpentine. Before fixing the 

 tin-foil, however, it nmst be properly spread over the mould, so as to remove every 

 wrinkle ; which the pliancy of the foil easily admits of, by uniform and well-directed 

 pressure with the fingers. 



The glass being placed in the hollow bed of a tight sack filled with fine sand, set in 

 a well-jointed box capable of retaining quicksilver, its concave surface must be dusted 

 with sifted wood-ashes, or Spanish-white contained in a small cotton bag, and then 

 well wiped with clean linen rags to free it from all adhering impurity, and particularly 

 the moisture of the breath. The concavity must be now filled with quicksilver to the 

 very lip, and the mould being dipped a little way into it is withdrawn, and the adher- 

 ing mercury is spread over the tin with a soft flannel roll, so as to amalgamate and 

 brighten its whole surface, taking every precaution against breathing on it. When- 

 ever this brightening seems complete, the mould is to bo immersed, not vertically, but 

 one edge first, and thus obliquely downwards till the centres coincide ; the mercury, 

 meanwhile, being slowly displaced, and the mark on the mould being brought finally 

 into coincidence with the mark on the glass. The mould is now left to operate by its 

 own weight in expelling the superfluous mercury, which runs out upon the sand-bag, 

 and thence into a groove in the bottom of the box, whence it overflows by a spout 

 into a leather bag of reception. After half an hour's repose the whole is cautiously 

 inverted, to drain off the quicksilver more completely. For this purpose a box like 

 the first is provided, with a central support rising an inch above its edges ; the upper 

 surface of the support being nearly equal in diameter to that of the mould. Two 

 workmen are required to execute the following operation. Each steadies, the mould 

 with the one hand, and raises the box with the other, taking care not to let the mould 

 be deranged, which they rest on the (convex) support of the second box. Before in- 

 verting the first apparatus, however, the reception-bag must be removed, for fear of 

 spilling the mercury. The redundant quicksilver now drains off; and, if the weight 

 of the sand-bag is not thought sufficient, supplementary weights are added at pleasure. 

 The whole is left in this position for two or three days. Before separating the mirror 

 from its mould the border of tin-foil, fixed to it with wax, must be pared off with a 

 knife. Then the weight and sand-bag being removed, the glass is lifted up with its 

 interior coating of tin-amalgam. 



For Silvering a Convex Surface. A concave plaster mould is made on the convex 

 glass, and their points of coincidence are defined by marks. The mould is to be lined 

 with tin-foil, with the precautions above described ; and, the tin surface being first 

 brightened with a little mercury, the mould is then filled with the liquid metal. The 

 glass is to be well cleaned, and immersed in the quicksilver-bath, which will expel the 

 greater part of the metal. A sand-bag is now to be laid on the glass, and the whole 

 is to be inverted, as in the former case, on a support; when weights are to be applied 

 to the mould, and the mercury is left to drain off for several days. 



If the glass be of large dimensions, 30 or 40 inches for example, another method 

 is adopted. A circular frame, or hollow ring of wood or iron, is prepared, of twice 

 the diameter of the mirror, supported on three foet. A circular piece of new linen 

 cloth of close texture is cut out, of equal diameter to the ring, which is hemmed 

 stoutly at the border, and furnished round the edge with a row of small holes,. for 

 lacing the cloth to the ring, so as to leave no folds in it, but without bracing it so 



