...» 



KOnig.] '>-> ii ict. i, 



of the borax has been volatilized ; this with the volatilized constituents of 

 the tesl substance, has now to be made up to 100 by addition of pulverized 

 boras glass. After remelting in the oxidizing flame and cooling it will be 

 found that the weigh! is still loo mgr. Now I examine the bead, [f too 

 deeply colored or even opaque, I remelt it, throw it from the wire 

 into a porcelain capsule, and crush it in the steel mortar. Of the powder 

 I weigh off 5-10-20 etc. mgrs. according to the depth of coloration, and 

 complement loo mgrs. by a head and pulverized glass, as before described, 

 [f after melting and cooling the color should he still too deep (in excep- 

 tional cases which will hereafter he described I, a second dilution is effected 

 in the same way. Of a substance containing 1..") p. c. Mn. 2 3 — and no 

 other coloring oxides— 5 mgrs. will just give the convenient depth of color 

 to the first head and no dilution is here necessary. Again melted and 

 thrown from the wire, the bead is ready for the next treatment. The 

 quantity of colored glass adhering to the wire matters not, if it does not 

 exceed 10 mgrs. 



2. Optical preparation of the bead. — In developing this method if was 

 soon found that with the spheroidal shape of the head no constant results 

 were obtainable. Acting as a lens it would concentrate the color if the 

 radius of curvature were smaller, and dilute it if larger, besides it did not 

 seem feasible to attach the bead in the chromometer so that the line of 

 greatest thickness should exactly fall into the line of vision. After trying 

 a number of contrivances unsuccessfully, I finally hit upon the simplest of 

 all and one that proved entirely satisfactory. I take a platinum cylinder 

 0.09 inch high, 0.145 inch inner, and 0.107 inch outer diameter, hold it with 

 a platinum tipped forceps into the flame until it is red hot, then press its 

 circumference upon the bead, so that the latter adheres firmly. If now 

 held again into the clear flame horizontally, bead downwards, until the 

 glass becomes liquid and the cylinder red hot, capillary attraction will 

 cause the -lass to flow up into the cylinder, without any overflow on the 

 outside of the platinum ring, and if turned properly while cooling, the 

 glass will equally protrude with convex surface on either end of the ring, 

 about 0.03 inch (fig. 3). After cool inn, the bead, thus mounted, must ap- 

 pear entirely free of air bubbles. By the next step the protruding convex- 

 ities are cut away, leaving two plane parallel faces and at the same time 

 bringing about the standard thickness in the glass. Fig. 4 represents in 

 natural size the serviceable contrivance which renders this operation both 

 rapid and accurate. 



A is a. brass plate r 3 ? inch thick and 2 inches in diameter. A central per 

 foration admits with easy friction the tube B, into one end of which the 

 platinum cylinder tits By slitting the tube has spring and holds the 

 cylinder sufficiently tight. A shoulder in the tube prevents the cylinder 

 from sliding in deeper when pressure is put on. The apparatus is placed 

 upon a plane glass plate with some tine corundum (or emery) and water, 

 the forefinger presses upon the knob of the tube, while thumb and middle 

 linger grasp the neck of the plate and move the bead over the glass plate. 



