The optical character of the mineral is _positive_ when the ellipses, surrounding the points of emergence of the two optic axes on the convex sides of the hyperbola, appear to expand or open out towards the centre when the quartz wedge is pushed in with its axis parallel to the plane of the optic axes. The optical character is _negative_ when the ellipses appear to expand or open out when the wedge is pushed in with its axis at right angles to the plane of the optic axes. As the ellipses expand they move from the points of emergence of the optic axes towards the centre of the interference figure, and finally open into lemniscates which move outward from the plane of the optic axes. Even when the section is very thin and the double refraction very weak, only the black hyperbolas without ellipses being seen, the test can be made; and colored ellipses will appear, after the pushing in of the quartz wedge, which will act in the same way as the ellipses of the interference figure. In a section at right angles to the _obtuse bisectric_ these results are all reversed. When the section is perpendicular to one optic axis, rotate the section until the plane of its optic axes is 45° to the planes of vibration of the nicols. The interference figure will now have the appearance as shown in Fig. 29, the hyperbola being convex towards the acute bisectrix. Insert the ¼ undulation mica plate, so that its direction c is parallel to the plane of the optic axes. If the optical character is positive the hyperbola will move towards the acute bisectrix and if negative away from it. When the gypsum plate is used the blue color will appear on the convex side for (+) and on the concave side for (−) minerals. ▄Determination of the Axial Angle.▄[78] This can be approximately determined with a petrographical microscope, if equipped with a micrometer eye-piece. Have the axial plane of the crystal section in the diagonal position, Fig. 31; and measure the distance _d_ from the centre to either hyperbola with a micrometer (or average the distance to both). Then sin _E = d/C_, in which _C_ is a constant for the same combination of lenses and is obtained by using a crystal section (mica cleavage) of known axial angle. For example, in a mica with 2_E_ = 91° 50′ and _d_ = 41.5 divisions on the micrometer scale, _C = d_/sin _E_ = 57.78 for that special combination of lenses. The true axial angle can be obtained from the equation sin _V = d/βC_. ▄Optical Distinctions between Orthorhombic, Monoclinic, and Triclinic Crystal Sections (perpendicular to acute and obtuse bisectrices).▄ The interference figures are always symmetrical in shape and distribution of color to the planes and axes of symmetry of the crystal system; hence are most symmetrical in the orthorhombic, less so in the monoclinic and still less so in the triclinic system.