In the Seibert microscope eye-piece No. 0 is used for most purposes. Other eye-pieces, Nos. 1 and 2, with cross-wires, are used for different degrees of magnification, and one eye-piece, No. 3, without cross-wires, is provided to be used in connection with an eye-piece micrometer. Footnote 27: With the Seibert microscope, Fig. 2, the No. 0 eye-piece and the No. II objective will prove most satisfactory for the following tests. With the Fuess microscope use No. 4 objective. With an English microscope use an ordinary eye-piece and a 1″ or ¾″ objective. Footnote 28: Twins may be recognized just as in macroscopic specimens, and zonal structure noticed if the zones differ in color. When a colorless mineral is surrounded by other colorless minerals, of about the same index of refraction, its outline is often best brought out by observation between crossed nicols. Footnote 29: The term “Anhedron,” meaning without planes, has been suggested by L. V. Pirsson to describe in rocks the crystal fragments which have no plane faces, as, for example, the augites of augitic rocks. _Science_, Jan. 10, 1896, p. 49. Footnote 30: Partial resorption and recrystallization may produce a border of secondary minerals, surrounding the original crystal. Footnote 31: The surfaces of all minerals in sections are more or less rough, but this roughness is only made visible when there is a marked difference between the indices of refraction of the minerals, and the index of refraction of the balsam in which the minerals are embedded. The index of refraction of balsam is about 1.54, so it is only when the mineral has a higher or lower index of refraction that its surface appears rough. When internal structure is to be studied, the crystal should be surrounded by a fluid of nearly the same index of refraction as that of the crystal, and when the exterior of the crystal is to be studied, then a fluid should be used with a very different index of refraction. Footnote 32: The Seibert microscope has a very convenient and quick lowering adjustment, by means of the lever _d_, for making this test. For convenience the lower nicol and condensing lens are generally left in place below the stage of the microscope, as polarized light serves as well for these investigations as ordinary light. An additional advantage in this arrangement is that the condensing lens is always ready for the “relief” test and the lower nicol for the pleochroism test; but it must be remembered that the polarizer or lower nicol cuts out one half of the light, which comes to it from the reflector, and this loss is important when high power objectives are to be used. When very low power objectives are used, the condensing lens must be removed. Footnote 33: _Mem. de l’Acad._, Paris, 1767–68. Footnote 34: _Min. Mag._, Vol. I., p. 193; Vol. II., p. 1. Footnote 35: