12 ON THE CONSTRUCTION OF 



lead previously employed for fining they were found to be a fruitfid source of 

 scratches, on account of grains of emery imbedding in them, and gradually break- 

 ing loose as the lead wore away. Subsequently it appeared, that unless there was 

 some means of spreading water and the grinding powders evenly, rings were likely 

 to be produced on the mirror, and the iron was consequently treated as follows : — 

 A number of pieces of wax, such as is used in making artificial flowers, were 

 procured. The convex iron was laid out in squares of ^ of an inch on the side, 

 and each alternate one being touched with a thick alcoholic solution of Canada 

 balsam, a piece of wax of that size was put over it. This was found after many 

 trials to be the best method of protecting some squares, and yet leaving others in 

 the most suitable condition to be attacked. A rim of wax, melted with Canada 

 balsam, was raised around the edge of the iron, and a pint of aqua regia poured 

 in. In a short time this corroded out the uncovered parts to a sufficient depth, 

 leaving an appearance like a chess-board, except that the projecting squares did not 

 touch at the adjoining angles (6, Fig. 6). I should have chipped the cavities out, 

 instead of dissolving them away, but for fear of changing the radius of curvature 

 and breaking the thin plate. HoAvever as soon as the iron was cleaned, it proved 

 to have become flatter, the radius of curvature having increased 7f inches. This 

 shows what a state of tension and compression there must be in such a mass, when 

 the removal of a film of metal Jj of an inch thick, here and there, from one surface, 

 causes so great a change. 



When the glass has been brought to the finest possible grain on such a grinder, 

 a polishing tool has to be prepared by covering the convex iron Avith either pitch 

 or rosin. These substances have very similar properties, but the rosin by being 

 clear afl"ords an opportunity of seeing whether there are impurities, and therefore 

 has been frequently used, straining being unnecessary. It is, however, too hard us 

 it occurs in commerce, and requires to be softened with turpentine. 



A mass sufficiently large to cover the iron ^ of an inch thick is melted m a 

 porcelain or metal capsule by a spirit lamp. When thoroughly liquid the lamp is 

 blown out, and spirits of turpentine added, a drachm or two at a time. After each 

 addition a chisel or some similar piece of metal is dipped into the fluid rosin, and 

 then immersed in water at the temperature of the room. After a minute or two it 

 'is taken out, and tried Avith the thumb-nail. When the proper degree of softness is 

 obtained, an indentation can be made by a moderate pressure. 

 '^' The iron having been heated in hot water is then 



painted in stripes | of an inch deep with this resinous com- 

 position. The glass concave to be polished being smeared 

 with rouge, is pressed upon it to secure a fit, and the iron 

 is then put in cold water. With a narrow chisel straight 

 JWgHp grooves are made, dividing the surface into squares of one 



— ' inch, separated by intervals of one-quarter of an inch (Fig. 



7). Under certain circumstances it is also desirable to take 

 off" every other square, or perhaps reduce the polishing sur- 



The Polishing Tool. ^ 1 ' r I c ■ 



face irregidarly here and there, to get an excess or action on 

 some particular portion of the mirror. 



