For the _nicol prism_ “a cleavage rhombohedron of calcite (the variety Iceland spar is universally used in consequence of its transparency) is obtained, having four large and two small rhombohedral faces opposite each other. In place of the latter planes, two new surfaces are cut, making angles of 68° (instead of 71°) with the obtuse vertical edges; these then form the terminal faces of the prism. In addition to this, the prism is cut through in the direction HH′, Fig. 3, the parts then polished and cemented together again with Canada balsam. A ray of light, _ab_, entering the prism, is divided into two rays polarized at right angles to each other. One of these, _bc_, on meeting the layer of balsam (whose refractive index is less than that of the ray _bc_), suffers total reflection, and is deflected against the blackened sides of the prism and extinguished. The other, _bd_, passes through and emerges at _e_, a completely polarized ray of light, that is, a ray with vibrations in one direction only, and that the direction of the shorter diagonal of the prism.”[18] The vertical plane through the shorter diagonal may be called the _plane_ of _vibration_[19] of the nicol. [Illustration: FIG. 2. ] [Illustration: FIG. 3. ] The polarizer must be below the stage _c_, and is generally adjusted so as to have its plane of vibration parallel to the N. and S. cross-wire in the eye-piece _o_. It is important to know the direction of the plane of vibration of the polarizer or lower nicol, as we can then determine, when absorption of light occurs in a mineral, the direction in this mineral parallel to which the absorbed rays are vibrating. The polarizer slides in an outer shell or frame, and, by means of a lever _d_, can easily be raised or lowered. A convenient test for the location of this plane of vibration of the polarizer is as follows: Make use of a section of biotite, cut at right angles to the basal plane, hence showing the basal cleavage cracks. Biotite has the property of absorbing to a marked extent the light vibrating parallel to these cleavage cracks. Rotate such a section on the stage of the microscope until the position of maximum darkness is reached and when such is the case the plane of vibration of the polarizer must be parallel to these cleavage cracks. On the top of the nicol is placed the _condensing lens_ for getting _convergent_ light, and the adjustments are so arranged that when the nicol is up as far as it will go, the condensing lens[20] is brought almost in contact with the lower surface of the transparent section resting on the stage.