G L A

G L A

( 502 )

Scratches can be perceived. In tkis ftate theyare again examined, and if any material defeftsftill remain below the ground furface, they arecut up with the diamond into {mailer plateswith the greateft economy poffible, the diamondnow .dividing them with much greater eafethan before, .both on account of the quantity offubftance of glafs removed and the uniformityof the furface.

The next procefs is that of polifhing both.furfaces to that perfect brightnefs which is feenin finifhed mirrors, fo that the rays of light maypafs through unimpaired to the filvering on thepofterior furface and be reflected again fromthence according to the laws-of .catoptrics. Thefubftance ufed to give this daft polifh is colcothar,imported from this country and called rougetf Angleterre. or Potee. It is the refidue left in theretorts of the aqua-fortis makers, and whenwell wafhed and levigated conflfts of little elfethan a red and perfect oxyd of iron.

The poliflring inftrument is a block of wood.covered with .feveral folds of black cloth withcarded wool between each fold fo as to makea firm elaftic culhion. This block has a handlefor the workman to hold ; for the whole of thispart is done by band and not by machinery, asthe latter would work too uniformly, and notallow of that variation of preflure and tliofe.finifliing touches which ar.e required to bring.every part of the glafs -to exactly the fameheight of polifh. But to encreafe .the pref-fure of the polilher without fatiguing the work-man, the- handle is lengthened by a woodenfpring bent to a bow and three or four leetlong, which at the other extremity refts againfta fixed point in a beam placed above. Theplate being fixed on the table by plafter he.then moiftens the paliftier with a wet brufh,covers it with colcothar and begins his opera-tion by working it backwards and forwardsover the furface of the plate. Much practicalikill and dexterity is required to give an uni-form and high degree of polifh over the fur-face of a large plate, as it muft be done byfeparate portions and the finifliing touches givenwith great , care. The glades of moderate fizeare compleated in four portions from cornerto corner, the centers of which interminglefo as to leave no part untouched, but thelarger riaffes require additional polifhing in thecenter. When one,fide is completed and thereverfe is about to be done, the polifhed fide,no.w the undermoft, is entirely covered withthe red colcothar to prevent the dazzle re-flefted from the white plafter which would

prevent the workman from judging fo accu-rately of the ftate of the furfaces oil which heis employed. When both tides of the glafs arethus brought to the fame perfection of polifh, theoperation is finifhed by infpecting the glafs,firft cleaning both furfaces, and laying it, eachfide alternately upwards, upon a dark blue orblack cloth admitting only a moderate light, andif any part appear lefs highly finifhed than thereft it is retouched by a fmall hand-polifher andcolcothar as before.

When a number of fmaller pieces of glafs,fuch as are ufed only for chamber or fimilarmirrors are to be polilhed, they are laid togetheron the table and feveral of them polifhed at atime. But as thefe confift of pieces often ofunequal thicknefs, though their furfaces havebeen rendered perfeftly flat by the previousgrinding, if they were Amply placed fide byfide and fixed on the table by plafter as ufual,the polilher would not work well over fuch avariety of heights, and would aft chiefly on theedges of each piece of plate. Therefore theyare all firft arranged on a large fmooth plate,finifhed all but the polifhing, and previouflywetted, and plafter is poured upon them bywhich they are fixed together, and then whentaken off, the furfaces which were in contaftwith the plate are perfeftly level with eachother, and the polifhing goes on with the fameoafe as on an entire plate.

What is termed filtering of mirrors, is apply-ing to the pofterior furface a coating of quick-filver, which metal when perfeftly bright andbrilliant reflefts the rays of light with greataccuracy and beauty. But as this fluid metalcould not be alone applied without great incon-venience, it is firft made to adhere by a partialamalgamation to the furface of a fheet of tin-loaf, and then by the help of preflure is appliedclafely to the glafs in a very thin lamina. Itis therefore properly a thin fheet of tin fullyimpregnated with mercury which is the re-flefting furface.

The management of the filvering is extremelyAmple. A perfeftly flat flab of fmoothed free-ftone (or fometimes of thick wood) a littlelarger than the largeft plate, is inclofedi n afquare wooden frame or box open at top, andwith a ledge rifing a few inches on three fidesand cut down even with the ftone on the fourth.A fmall channel or gutter is cut at bottom ofthe wooden frame, ferving to convey the waftemercury down into a veffel below, let to catchit. The flab is alfo fixed on a centre pivot, fo.that one end may be raifed by wedges (and of