(_f_) ▄Inclusions▄, which may be _solid_ (either distinct crystals or glass), _fluid_ or _gas_, see Fig. 14. These inclusions are distinguished by the fact that the solid inclusions generally have sharp contours, the fluid inclusions distinct dark borders, and the gas inclusions broad dark borders more distinctly marked than those of the fluid inclusions. The fluid inclusions often contain a bubble of gas. The inclusions may have a definite or indefinite position in the crystal in which they occur, and can sometimes be more distinctly seen by using convergent light or a “spot” lens. (_g_) ▄Schiller Structure.▄ “Is that in which cavities of definite form and orientation (‘negative crystals’) are developed along certain planes and filled or partially filled by material dissolved out of the enclosing crystal,”[43] see Fig. 15. Characters Observed by Polarized Light. The polarized light is obtained by passing the beam of light from the reflector through the polarizer or lower nicol,[44] which must be in place below the stage of the microscope. White light is supposed to be used. ▄Pleochroism▄, that property which all anisotropic minerals have, to a greater or less extent, of absorbing certain colored rays in certain directions, thereby showing different colors in different directions by transmitted light. Uniaxial minerals are dichroic showing two differences in color, produced by the rays which vibrate parallel to the direction of the vertical axis _ć_ and the plane of the basal axes. Biaxial crystals are trichroic showing theoretically three differences of color produced by rays with vibration directions corresponding very nearly to those of the three principal vibration directions.[45] Any given section of a biaxial crystal will, of course, appear only dichroic. The practical way of testing for _pleochroism_ is as follows: Revolve the stage, carrying the section, when a change in the color of the mineral will be noticed, if it is pleochroic. This pleochroism may appear as an actual change in color or simply as a change in the shade of the same color. At times it may be so weak as hardly to be noticed, when it is best to make the test with the condensing lens in position immediately under the section, or by rotating the lower nicol instead of revolving the stage. The color of the light, vibrating parallel to certain definite crystallographic directions in minerals, is often very characteristic. In some cases there may be such strong _absorption_ of one of the rays, that, when the vibration direction of this ray is over the plane of vibration of the polarizer, practically no light is transmitted and the section appears dark. Strong absorption is characteristic of certain minerals, such as biotite, amphibole, tourmaline and allanite and takes place parallel to definite directions in these minerals. Pleochroism may often be noticed with ordinary light in the case of hand specimens.