398

DIAMOND MICROSCOPES.

by 48 the square of this weight, and they call the product in francs the value of thediamond. Thus, for example, a cut diamond of 10 carats would be worth (10 X 2) 5X 48=19,200 francs, or 7681., allowing only 25 francs to the pound sterling.

The diamond mines of Brazil have brought to its government, from the year 1730till 1814, 3,023,000 carats; being at the average rate annually of 36,000 carats, or alittle more than 16 lbs. weight. They have not been so productive in the lateryears of that period; for, according to Mr. Mawe, between 1801 and 1806, only115,675 carats were obtained, being 19,279 a year. The actual expenses incurred bythe government, during this interval, was 4,419,700 francs; and, deducting the pro-duction in gold from the washings of the diamond gravel, or cascalho, it is found thatthe rough diamonds cost in exploration, per carat, 38 francs 20 c., or nearly 31s.British money. The contraband is supposed to amount to one third of the abovelegitimate trade. Brazil is almost the only country where diamonds are mined at thepresent day; it sends annually to Europe from 25 to 30 thousand carats, or from 10 to16J lbs.

DIAMONDS, cutting of. Although the diamond is the hardest of all known sub-stances, yet it may be split by a steel tool, provided a blow be applied; but this requiresa perfect knowledge of the structure, because it will only yield to such means in certaindirections. This circumstance prevents the workman from forming facettes or planesgenerally, by the process of splitting; he is therefore obliged to resort to the processof abrasion, which is technically called cutting. The process of cutting is effected byfixing ‘the diamond to be cut on the end of a stick, or handle, in a small ball ofcement, that part which is to be reduced being left to project. Another diamond isalso fixed in a similar manner; and the two stones being rubbed against each otherwith considerable force, they are mutually abraded, flat surfaces, or facettes, beingthereby produced. Other facettes are formed by shifting the diamonds into freshpositions in the cement, and when a sufficient number are produced, they are fit fo rpolishing. The stones, when cut, are fixed for this purpose, by imbedding them in softsolder, contained in a small copper cup, the part, or facette, to be polished, being left toprotrude.

A fiat circular plate of cast-iron is then charged with the powder produced duringthe abrasion of the diamonds; and by this means a tool is formed which is capable ofproducing the exquisite lustre so much admired on a finely-polished gem. Thosediamonds that are unfit for working, on account of the imperfection of their lustre « rcolor, are sold, for various purposes, under the technical name of Bort. Stones of thikind are frequently broken in a steel mortar, by repeated blows, until they are reduceto a fine powder, which is used to charge metal plates, of various kinds, for the use ojewellers, lapidaries, and others. Bol t, in this state of preparation, is incapable opolishing any gems; but it is used to produce flat surfaces on rubies and other precxoustones.

Fine drills are made of small splinters of bort, which are used for drilling small hol&in rubies, and other hard stones, for the use of watch-jewellers, gold and silver wit®drawers, and others, who require very fine holes drilled in such substances. These dri ^are also used to pierce holes in china, where rivets are to be inserted; also for piercWoholes in artificial enamel teeth, or any vitreous substances, however hard. „

DIAMOND MICROSCOPES were first suggested by Dr. Goring, and havewell executed by Mr. Pritchard. Previous to grinding a diamond into a spher> ^figure, it should be ground flat and parallel upon both sides, that by looking throng ’as opticians try flint glass, we may see whether it has a double or triple refra tpower, as many have, which would render it useless as a lens. Among the 14 < “*^ nescrystalline forms of the diamond, probably the octahedron and the cube are ' m0 nd

that will give single vision. It will, in many cases, be advisable to grind am a[J( jlenses, plano-convex, both because this figure gives a low spherical aberration,because it saves the troubleVif grinding one side of the gem. A concave tool waSiron, paved with diamond powder, hammered into it by a hardened steel P unC ( j’ oU bleemployed by Mr. Pritchard. This ingenious artist succeeded in completing a ^ aaconvex of equal radii, of about of an inch focus, bearing an aperture of ■{ ,y oneS ,inch with distinctness upon opaque objects, and its entire diameter upon transpare® Dr-This lens gives vision with a trifling chromatic aberration; in other respects, - 0I . toGoring’s Amician reflector, but without its darkness, its light is said to s “!J e s am ethat of any compound microscope whatever, acting with the same power, an inter-angle of aperture. The advantage of seeing an object without aberration by 1 ^ eye-position of only a single magnifier, instead of looking at a picture of it wit moreglass, is evident. We thus have a simple direct view, whereby we shall saccurately and minutely the real texture of objects. r . known

DIAPER is the name of a kind of cloth, used chiefly for table linen. « eXte nsiv eamong the French by the name of toile fourrt, and is ornamented with the mos