The lower nicol is generally so adjusted that its plane of vibration is parallel to the north and south cross-wire in the eye-piece. This adjustment can be tested by means of a section of biotite, showing cleavage cracks. When the plane of vibration of the polarizer is parallel to the N. and S. cross-wire in the eye-piece, the biotite section becomes almost dark when its cleavage cracks are parallel to the same cross-wire. The upper nicol, or analyzer, must, of course, be removed during this test. This method is more convenient than taking the nicol out of its frame, in order to ascertain its plane of vibration (the direction of its shorter diagonal). Footnote 45: Although the “absorption directions” may not necessarily coincide with the principal vibration directions in Monoclinic and Triclinic crystals; still for convenience the absorption colors are usually given for the light rays vibrating parallel to these principal vibration directions. Footnote 46: In the Fuess and Seibert microscopes the analyzer or upper nicol is so fitted that it slides in and out of the tube of the microscope with its plane of vibration always at right angles to the plane of vibration of the polarizer or lower nicol. Footnote 47: Moses’ _Characters of Crystals_, p. 106. Moses and Parsons’ _Min. Cryst._ and _B. P. Analysis_, p. 163. Footnote 48: Iddings’ _Rock Minerals_, 1911, p. 172. Footnote 49: These sections always contain the principal vibration directions _a_ and _c_. Footnote 50: _Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, p. 101. Footnote 51: A chart of interference colors can be obtained from Baudry et Cie, Paris, and is also published in _Les Minéraux des Roches_, by Lévy and Lacroix, _Rock Minerals_ (1911), by Iddings and in Rosenbusch’s _Mikroskopische Physiographie_. Footnote 52: Iddings’ _Rock Minerals_, 1911, pp. 141, 183. Footnote 53: _Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, p. 132. Footnote 54: The ¼ undulation mica plate consists of a thin cleavage of mica on which is marked _c_, the vibration direction of the slower ray, which in mica is the line joining the “optic axes.” The thickness is such that the slower ray is ¼ wave-length behind the faster and the interference color is a bluish-gray. The gypsum plate is a thin cleavage of gypsum, on which is usually marked a, the vibration direction of the faster ray. The chosen thickness is such as to produce the red interference color of the 1° order. Footnote 55: