A gypsum test-plate, so prepared as to give some definite interference color, as red of the 1° order, may be introduced between the two nicols, and the section revolved until this color is exactly matched. This perfect matching of color is only possible when the directions of vibration of the section are exactly parallel to the planes of vibration of the nicols, as otherwise some interference color would be produced by the section and the true color of the test-plate would not appear. The most favorable condition is when the section is colorless and only covers a part of the field of view, as then the rest of the field shows the color of the test-plate and the exact matching of color is an easy matter. This method of testing can be employed to recognize the very weak double refraction of some minerals, whose interference colors are such dark grays as not to be noticed without this test. After the test-plate has been introduced, some slight variation in the color will be observed when the stage carrying the doubly refracting section is rotated. There are other methods[53] for locating these directions of vibration. They depend generally on substituting changes of color for degrees of darkness or on changes in interference figures, and consist in the use of color test-plates or Klein’s quartz-plate, or of special oculars prepared by Bertrand, Calderon, etc. METHOD OF TESTING FOR THE VIBRATION DIRECTIONS OF THE FASTER AND SLOWER RAYS, _A′_ AND _C′_, IN A MINERAL SECTION. A mica or gypsum plate can be used to make this test, the directions of vibration of the faster and slower rays on these plates being known and marked.[54] The crystal section to be tested is placed on the rotating stage, between crossed nicols, and turned to the position of extinction, when these directions of vibration will be parallel to the cross-wires in the eye-piece. The section is then turned 45°, when the interference color will be at its maximum, and these directions of vibration will make angles of 45° with the planes of vibration of the nicols and the cross-wires in the eye-piece. Now introduce[55] the test-plate, between the section and the analyzer, so that its known directions of vibration also make angles of 45° with the cross-wires in the eye-piece. When the test-plate is introduced a new interference color will be noticed which is either higher or lower in the color scale[56] than the original interference color of the mineral section. When the known directions of vibration of the test-plate are superposed over corresponding directions in the mineral section, the effect is to thicken the section and the interference color rises in the scale. When the directions of vibration of the test-plate are superposed over directions of vibration in the mineral section which are not corresponding, the effect is to thin the section and the interference color sinks in the scale.