Consider now the general question of the transmission of the doubly refracted rays through a plate. Whatever the angle of the parallel incident rays, each ray as AB, Fig. 17, is resolved, as just described, into two rays BC and BD, polarized at right angles to each other and following (usually) different paths in the plate. On emergence these follow parallel paths. Among the incident rays there are rays EG and FH, such that one component of FH will emerge at D with one of the components of AB, and one component of AB and one component of EG will emerge at C. Hence from every point of the upper surface of the plate there will emerge two rays and these rays will have travelled through different paths in the plate with different velocities and will have their vibrations at right angles to each other. When these doubly refracted rays come to the analyzer, whose plane of vibration is A′A, they cannot get through vibrating in their present directions, but components of these rays, such as _ot_ and _os_, can get through vibrating parallel to the plane A′A. Hence we have two series of rays coming to the eye, polarized in the same plane, but one set slightly in advance of the other. These rays will “interfere” and produce some _interference_ or _polarization_ color.[47] In using white light whenever one of two light rays of the same color has suffered a “retardation” of just one wave-length (or even multiple thereof) the color will be extinguished; and when the “retardation” is one half wave-length (or even multiple thereof) the color will be intensified. Therefore, some tints will be extinguished and others intensified, the combination resulting in the production of some definite interference color. Of course in the case of monochromatic light the thickness of the section may be such that “destructive interference” takes place producing no color (darkness). Now suppose the stage to be rotated until the section _cdef_ takes the position _c′d′e′f′_. The section will be found to be dark and no interference color will be seen. This is due to the fact that the directions of vibration in the section are parallel to the planes of vibration of the crossed nicols, consequently the light passes through the section still vibrating parallel to the plane PP′ of the polarizer and is all cut out by the analyzer. Darkness will occur every 90° and therefore four times during a complete rotation of the stage. The interference color is also observed to vary in intensity, but not in color, and to be at its maximum 45° from the positions of darkness. Sections of uniaxial crystals at right angles to the optic axis act like isotropic substances and remain dark during a complete rotation of the stage. In the biaxial crystals, a section at right angles to an “optic axis” shows uniform illumination[48] which does not change as the stage is rotated.