The test-plates are generally introduced in the slot _k_, in a microscope of the Seibert type, or if a cap-nicol is used in a slot below this. In case no provision is made by the instrument maker for these test-plates, the regular analyzer is left out of the tube, and a simple nicol prism is used as an analyzer and is held by the observer over the eye-piece. Care must be taken to have the plane of vibration of this nicol at right angles to that of the polarizer, and to leave sufficient room for the introduction, by hand, of the test-plate between the eye-piece and the nicol. With care the plates can be introduced with sufficient accuracy to make the test practical. Footnote 56: A scale or chart of interference colors, or the interference color diagram, should be before the observer in order to avoid any mistakes as to whether the new color is higher or lower in the scale. Footnote 57: The quartz wedge is cut so that one of its faces is exactly parallel to the _ć_ axis (hence also parallel to the _c_ vibration direction) while the other face makes a very small angle with it. The direction _c_ is marked on the wedge. Footnote 58: Described under next test. Footnote 59: In applying this rule count the 1° order white as green and the 1° order gray as blue. Footnote 60: A. J. Moses, _Trans. N. Y. Acad. Sci._, Vol. XVI., p. 55, Jan., 1897. Footnote 61: E. von Federow, _Zeit. f. Kryst., etc._, Vol. XXXV., p. 340, 1895. Footnote 62: After the first rough determination of the phase difference by the mica wedge, the more exact phase difference can be obtained by the aid of a good color chart or diagram, see end of book. Footnote 63: It is not safe to use minerals near the edge of the section, as the thicknesses are apt to be unequal. Footnote 64: See at end of appendix. Footnote 65: In this way eliminate, so far as possible, the effect of the orientation of the mineral section. Footnote 66: The different mineral sections are all supposed to have the same thickness throughout the rock section. Footnote 67: Harker’s _Petrology for Students_, 1895, p. 14. Footnote 68: Iddings’ _Rock Minerals_, pp. 153, 173. Moses’ _Characters of Crystals_, p. 115. Footnote 69: In the Seibert microscope use No. V objective, in Fuess microscope No. 7 objective, and in English microscopes a ¼″ or ⅕″ objective. Footnote 70: Each convergent ray will have its vibration direction either in or at 90° to the plane through the ray and the optic axis. Hence all rays vibrating parallel to the vibration planes of both nicols will be completely cut out. As the section is rotated new rays successively come into these positions, so the same effect is maintained. Footnote 71: In the Seibert microscope there is a little slot _k_ for this purpose just above the objective. Footnote 72: