The optical character may also be determined in parallel light by proving _ć_ = c(+), _ć_ = a(−). The optical character of the principal zone or the sign of the elongation is often given in tables. This optical character or sign is (+) when the principal zone axis or the direction of elongation is parallel to c and (−) when parallel to a. Footnote 73: Iddings’ _Rock Minerals_, 1911, p. 172. Footnote 74: This assumes the optic axes for different colors to emerge about at the same points. If there is marked “dispersion” the black bands and hyperbolas may be rainbow-hued, as with titanite. Footnote 75: The interference figure, perpendicular to the _obtuse bisectrix_, would be of the same type with a larger axial angle. Ordinarily this figure would not come within the limits of the field of view of the microscope. Confusion may arise, however, but in a section perpendicular to the acute bisectrix the cross dissolves more slowly into the hyperbolas than in the case of a section perpendicular to the obtuse bisectrix. At times it may be necessary to measure the axial angle to be sure. When, however, the mineral is known, the section perpendicular to the acute bisectrix can be recognized, because if the mineral is optically positive the trace of the axial plane is parallel to a and if negative parallel to c. Footnote 76: For construction of quartz wedge, see p. 34. Footnote 77: The wedge can be introduced in either of the several ways described for the introduction of the test-plates on p. 33. Footnote 78: For methods of measuring the axial angle, see _Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, p. 147. For convenience in many cases only 2_E_ is recorded, as then an indication is given as to whether the axial angle is visible with an ordinary microscope (arranged for observation with convergent light for interference figures). If 2_E_ is very large the axial angle can only be observed by covering the section with some transparent, strongly refracting fluid. For the Seibert microscope with objective V the limit for good results is about 2_E_ = 90°–100°. Footnote 79: For dispersion, etc., see A. J. Moses’ _Characters of Crystals_, p. 140. Footnote 80: The system of crystallization of leucite has been the subject of much discussion. Its habit is isometric. The consensus of opinion seems to be that leucite crystallizes in the isometric system, but that the isometric molecular arrangement, at least of the larger crystals, cannot exist for the temperature and pressure at the earth’s surface. Hence molecular displacement takes place, giving rise to a more or less complicated apparent twinning, and optical anomalies are noticed. The isotropic character returns if the section is heated to 500° C. Iddings’ _Rock Minerals_, p. 249, 1911. Footnote 81: C. W. Knight, _Canad. Rec. of Sci._, IX, No. 5. 265. Footnote 82: