_Microlites_, more or less completely defined microscopic crystals, which usually show double refraction, but cannot be always specifically determined, see Fig. 8, _a_. _Skeleton Forms_, resulting from rapid cooling of the magma occur in some lavas, see Fig. 8, _b_. (_b_) ▄Color.▄ It must be remembered that the colors observed are always due to transmitted light and may be called “_absorption tints_.” Minerals which in hand specimens are opaque are often colored in sections; and minerals which are commonly colored may appear colorless in sections. At times color may be given to a section simply by the presence of a great number of minute inclusions. [Illustration: FIG. 9.—Olivine crystal in basalt showing “high relief” and cleavage. ] (_c_) ▄Index of Refraction.▄ _n_ = sin _i_/sin _r_. This can be approximately determined by the appearance of the surface and outline of a mineral, that is by its _relief_. The descriptive terms are of course relative, but in common practice minerals are said to have _medium_ refraction when the refractive index is between that of balsam (1.54) and that of calcite (1.60), _strong_ refraction when higher than 1.60 and _weak_ refraction when lower than 1.54. In the case of uniaxial and biaxial minerals the mean value of the indices of refraction is used. A mineral which has _strong refraction_ appears to have _high relief_, _i. e._, distinct dark contours and a rough or “shagreened” surface,[31] which has bright illumination and appears to stand out above the surfaces of the surrounding minerals with weaker refraction, see Fig. 9. A mineral with _medium refraction_ does not show any _relief_, hence has a smooth surface and no dark contours, see Fig. 5. A mineral with _weak refraction_ may also appear to have a rough surface, but not so marked as in the case of a mineral with very strong refraction, due to the smaller contrast in indices between the mineral and balsam. The practical way of testing for the approximate index of refraction or the _relief_ of a mineral is to make use of the lens for convergent light, which is placed on top of the lower nicol immediately below the section. By lowering[32] the lens the character of the relief of a mineral section is made very apparent. In order to become familiar with the way in which the “relief” or appearance of the surface indicates the strength of the refractive index, the student may use a long glass slide on which are embedded in balsam (1.54) small fragments of different minerals, for example: Sodalite (1.483), orthoclase (1.523), quartz (1.547), topaz (1.620), hornblende (1.631), augite (1.70), epidote (1.751), zircon (1.95) and rutile (2.712).