12 ON THE COX ST RUCTION OF 



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

 scratehes, on account of grains of emery imbedding iu 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 wej-e likely 

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



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

 procured. The convex iron was laid out iu stpxares of j, of an inch on the side, 

 and each alternate one b(>ing touched with a thick alcoholic solution of Canada 

 balsam, a piece of wax of that size was put over it. Tliis was found afti-r many 

 trials to be tlie best method of protecting some sqiuires, 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 tlie iron, and a ])int of aqua regia ])oured 

 in. In a short time this corroded out tlie uncovered parts to a sufficient (lc|)tli. 

 leaving an appearance like a chess-board, except lliat the ])rojecting s(piares did not 

 touch at th(> adjoinhig angles {h. Fig. 6). T should have chipped the cavities out, 

 instead of dissolving them away, l)ut for fear of changing the radius of cur\ature 

 and breaking the thin ])late. However as soon as the iron was chnmed, it ])roved 

 to have become flatter, the radius of curxature having increased 7j' 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 r}^ of an inch thick, here and (here, from one surface, 

 causes so great a change. 



When the glass has been brought to the finest ])ossible grain on sucli a grinder, 

 a polishing tool has to be prepared by covering the convex iron with eitlier pitch 

 or rosin. These substances have very similar properties, but the rosin l)y being 

 clear affords an o])])ortunity of seeing Avhetlun- there are inqjurities, and therefore 

 has been fre([uently used, straining being luinecessary. It is, however, too hard as 

 it occurs in commer(-e, and requires to be softened with turpentine. 



A mass sufficientlv large to cover tlie iron \ of an inch thick is melted in a 

 porcelain or metal ca])side by a s])irit lanq). When thoroughly liquid the lanqj is 

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

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

 tlum immersed in water at tlie tciiipcrature of the room. After a minute or two it 

 is taken out, and tried with tlic tliiimh-nail. AN'hen the proper degi'ee of softness is 



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



_Jf^.J^ The iron having becm heated in hot water is then 



IH?1"tv ])ainted in stripes 1 of an inch deep with this resinous com- 



!S3^^\ ])osition. The glass concave to be jjolished being smeared 



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



^■■■■■■IjSBBg jjj {.j^gj^ p^^^. j,^ f^Qi^ water. With a narrow chisel straight 



"im"lslilH3flHH» grooves are made, dividing the surfixce into squares of one 



— — ^ inch, separated by intervals of one-ciuarter 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 ' f 1 1 p 



face irregularly here and there, to get an excess of action on 

 some particular portion of the mirror. 



