Some mineral sections have no power to produce any illumination. They are singly refracting and consequently are quite dark between crossed nicols of petrographic polarizing microscopes. They remain in that condition during a complete rotation of the stage of the polarizing microscope, and are said to be isotropic. All sections of the minerals crystallizing in the cubic system, and basal sections of minerals crystallizing in either of the diametric systems, are isotropic. All other sections are anisotropic, that is, they are alternately dark and illuminated when the stage of the polarizing microscope for geologists is rotated.