The refractive index of the cementing material should not be very much lower than that of either of the thin sections, as the result would then be to disperse the emerging rays too much and dim the effect. In the case of distinguishing between minerals of high refractive power, such as augite and garnet, methyliodide is recommended instead of balsam. In the case of doubly refracting crystals, the lower nicol (polarizer) must be retained, if it is desired to obtain the relative refractive index of one of the two rays, either with respect to the balsam or to a ray in an adjoining crystal with a parallel vibration direction. For determining vibration directions of rays and the faster and slower rays in two adjoining sections see later, pp. 31 and 33. The method of _Schrœder van der Kolk_[41] can also be employed for the same purpose as the Becke test. The ordinary objective (Seibert No. II) is used, and the method essentially consists of darkening a portion of the field by means of the finger or a plate inserted below the polarizer. The condenser lens must be lowered beyond the sharp focus of the edge of the shadow (on raising the condenser the following phenomena are _reversed_). As the shadow approaches the contact between two minerals a bright line will appear on the edge of the “far” mineral, if its refractive index is the higher, and a dark line on this same edge if its index is lower than that of the “near” mineral. The indices of the two rays in doubly refracting minerals must be determined by the aid of the polarizer. [Illustration: FIG. 11.—Biotite, showing perfect cleavage, in rhyolite. ] (_d_) ▄Cleavage▄,[42] which appears as more or less distinct and regular lines or cracks, see Figs. 11 and 12. These cleavage cracks may be parallel or intersect, depending on the position of the section relative to the cleavage planes of the crystal. [Illustration: FIG. 12.—Augite _a_, showing good cleavage, and plagioclase _p_ in diabase. The plagioclase shows “polysynthetic” twinning between crossed nicols. ] [Illustration: FIG. 13.—Garnet in mica schist, showing fracture. Franconia, N. Y. ] Cleavage is sometimes best observed by slightly lowering the condensing lens under the section. When sections show intersecting cleavage cracks it is often possible to recognize the mineral by its known cleavage angle, as in the case of amphibole and pyroxene. (_e_) ▄Fracture▄, which appears as irregular and non-parallel cracks, see Fig. 13. [Illustration: FIG. 14.—Apatite in feldspar _a_. Garnets in quartz, Branchville, Ct., _b_. Liquid inclusions of CO_{2}, some showing gas bubbles, in quartz _c_. ] [Illustration: FIG. 15.—Enstatite showing “Schiller” Structure. ]