Sulphate of Iodo-quinine for Optical Purposes. 349 



its attachment to the horizontal string ; this is best done by 

 holding the perpendicular twine in the left finger and thumb, at 

 the same time cutting the upper end of it with a pair of scissors to 

 avoid all disturbance. It will now swing easily and steadily, and 

 may be carried and gently deposited upon its gallipot support. 

 Then attach the edge of one of the circular glass discs to the end 

 of the glass rod by a little of the wax or marine glue, and let it, 

 when cold, be carried flatly down the neck of the flask, which 

 should be very gently inclined, as nearly horizontally as possible, 

 to admit of this being easily accomplished. Having selected the 

 largest crystalline plate, pass the glass circle gently beneath it, 

 raise the plate by depressing the hand, and the little crystalline 

 gem is at once caught on its surface. 



If this operation be neatly accomplished, it appears spread out 

 as a thin uniformly-coloured film upon the glass ; if any black 

 patches appear, they are occasioned by the accidental crossing of 

 some interposed crystals, or from some on the under surface of 

 the glass disc. These last must be at once wiped off by the 

 blotting-paper, the others will sometimes float out upon raising 

 the edge of the glass disc to a perpendicular position ; if they 

 are near the edge, they may sometimes be gently drawn out 

 from under the large crystal by a little dexterity on the part of 

 the operator ; frequently there are no such precautions necessary. 



Now rapidly dry the plate by imbibing all the fluid most 

 scrupulously by blotting-paper. This must be done without 

 touching the crystalline surface, for the least contact destroys its 

 beauty, symmetrical arrangement, and optical usefulness. Having 

 done so, let it dry by exposure to the air in a cool room, say at 

 40° to 50°; this is to prevent resolution and disintegration 

 of the crystal in its own mother-water, a little of which must 

 remain attached after all our care. 



It is sometimes necessary to float them on, or dip them for an 

 instant only in a little cold distilled water somewhat imbued with 

 iodine. This serves two purposes ; it removes all mother-liquid, 

 and prevents those crystals of sulphate of quinine subsequently 

 forming and interfering with the perfect polarization of the new 

 tourmaline, as every crystal of this substance interposed between 

 the plates would of course rotate the polarized beam as far as 

 its influence extended, and depolarize it. The iodine acts also 

 in preventing the solution of the new crystals in the water. 

 They must again be dried by imbibition and by exposure to air, 

 as before, and then placed under a cupping-glass, having a 

 watch-glass with a few drops of tincture of iodine in it. This 

 gives a decidedly black tone to the field j and if the crystal were 

 before too thin to obstruct all the light, and thus give a red or 

 purplish violet-tint, its power of polarization will be very mate- 

 rially improved by following the above simple directions. 



