Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . inter-posed. In a section normal to the obtuse bisectrix these resultsare all reversed. * To determine the acute bisectrix it may be necessary to first measure the axialangle. Ordinarily the interference figure in a section normal to the obtuse bisectrix willnot come within the limits of the field. Fig. 367. OPTICAL CHARACTERS. 175 In crystals with a small axial angle the mica or gypsum platemay be used as described, p. 167. Determination of the Angle Between the Optic Axes. A section cut normal to t
Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . inter-posed. In a section normal to the obtuse bisectrix these resultsare all reversed. * To determine the acute bisectrix it may be necessary to first measure the axialangle. Ordinarily the interference figure in a section normal to the obtuse bisectrix willnot come within the limits of the field. Fig. 367. OPTICAL CHARACTERS. 175 In crystals with a small axial angle the mica or gypsum platemay be used as described, p. 167. Determination of the Angle Between the Optic Axes. A section cut normal to the acute bisectrix is held at P, , between the lenses of a horizontal polariscope. The vibrationdirections^of the nicols of the polariscope are crossed at 45 tothe horizon, so that when the line connecting the axial points ishorizontal the interference figure will show the hyperbola and notthe cross. The section must be centred so that a line in it is theaxis of revolution; and so that the axial points of the interferencefigure remain on the horizontal cross hair during Fig. 368. The crystal is then revolved by the horizontal plate until thetwo arms of the interference hyperbola are successively madetangent to the vertical cross-hair, the positions being read on theplate. The difference between the two readings is the apparentangle 2E, and this is frequently the angle recorded. It is alwayslarger than the true angle 2 V. 176 DESCRIPTIVE MINERALOGY. Fig. 368 shows the newer type of so called Universal Apparatusof Fuess. The centring device is precisely that described under theFuess goniometer and may be inserted either as in the figure or in-verted. In the latter position it forms with the telescopes F^ and7^ the equivalent of the Fuess goniometer, Fig. 273. When used for axial measurements, the crystal stand is replacedby the pincers P which clip the crystal plate. The optical por-tion inserted at A and A^ is the same Norremberg arrangement ofnicols and lens
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