Some instruments are provided with an additional slot in the tube between the analyzer and the eye-piece for the introduction of a Bertrand lens, which is used to magnify the interference figures produced by convergent light. The first approximate focusing is made by the screw _m_, and the fine adjustment by the screw _n_. In focusing always start with the objective very near the section, and move it away until the right focus is obtained. Never move down towards the section in obtaining the focus, as there is danger then of striking the objective against the section. For cleaning the microscope xylol can be used, as it will not injure the lacquer. To lubricate any of the parts use a small quantity of soft tallow, good clock oil or paraffin oil. CHAPTER III. INVESTIGATION OF MICROSCOPIC AND OPTICAL CHARACTERS OF MINERALS. ▄Characters of Opaque Minerals▄, observed by _reflected_ light. The minerals which remain perfectly opaque in thin rock sections have usually metallic lustre and are very minute in size, as is the case with the iron ores. When the metallic minerals of a rock specimen are distinctly seen with the unaided eye, it is rarely necessary to make a section for their determination, as this can more easily be done by any of the well known blowpipe methods. Form, Lustre, Color, Cleavage, etc., are recognized in the same way as in the case of macroscopic specimens. In order to make the observations by reflected light alone, the beam of light from the reflector-mirror must be cut off by holding the hand over the mirror or by moving the mirror. ▄Characters of Transparent Minerals▄, observed by _transmitted_ light. The petrographical microscope for these observations is supposed to be in the condition of an ordinary microscope, both nicols being out of the field and a strong beam of light coming up through the transparent section from the reflector below. The characters are considered in the order in which they would most naturally appear to the observer during a complete investigation with the microscope.[27] [Illustration: FIG. 4.—Idiomorphic augite crystals in camptonite. The section is about at right angles to the vertical axis _ć_, and shows the intersecting cleavages parallel to the prism of 87° 06′. Keene Valley, N. Y. ] [Illustration: FIG. 5.—Allotriomorphic quartz _q_, showing no “relief,” plagioclase _p_ and decomposed feldspar _o_ in granite. As seen with crossed nicols. ]