_Orthorhombic crystals_ show the figures always in two of the pinacoids and in white light the color distribution will be symmetrical to the trace of the axial plane and the line through the centre at right angles to this trace and also to the central point. _Monoclinic crystals_ show the figures in the clino pinacoid or in sections at right angles to this. In white light the color distribution is never symmetrical to two lines, but is symmetrical either to the trace of the axial plane (_inclined dispersion_[79]), or to the line through the centre at right angles to this trace (_horizontal dispersion_), or to the central point (_crossed dispersion_). _Triclinic crystals_ show in white light figures with distribution of color unsymmetrical to any line or point. In white light the “color fringes” of the hyperbola are due to the “_dispersion_”[79] of the optic axes and bisectrices. That is, for each color (for light of each wave-length) there is a particular interference figure; the overlapping of these superposed figures producing the color fringes. When the axial angle is larger for red light than for violet, the dispersion is said to be ρ > ν and the interference figure, in the position of Fig. 31, will show the hyperbolic curves fringed with red towards the centre (inside). In general the color with the larger axial angle is nearer the centre of the field. This is due to the extinguishing of light of each color at the axial points, the resulting colors at these points being produced by white light minus the absorbed color. When the dispersion is ν > ρ the reverse distribution of color fringes will take place. By measuring the axial angle in red and blue light, this dispersion of the optic axes can also be obtained. RESUMÉ OF THE USES OF PARALLEL AND CONVERGENT POLARIZED LIGHT. _Parallel_ light is used to detect pleochroism, to distinguish between isotropic and anisotropic substances, to study interference colors, to determine the strength of the double refraction, to locate directions of vibration, to measure extinction angles, to find the directions of vibration of the faster and slower rays, to determine the relative value of the indices of refraction of the two rays, and to investigate the crystal structure in general. _Convergent_ light is used to distinguish between uniaxial and biaxial crystals, to determine whether a section that appears to be isotropic is really so or only perpendicular to an optic axis and to determine the optical character, grade of symmetry (system), axial angle and dispersion. CHAPTER IV. THE MICROSCOPIC AND OPTICAL CHARACTERS OF MINERALS. OPAL. ISOTROPIC. AMORPHOUS. COMPOSITION: SiO_{2}.nH_{2}O, generally soluble in caustic alkalies.