The _rotating_ circular _stage e_ is supported on a suitable frame _c_, and arranged so that its axis of rotation coincides with the line of sight of the microscope. The stage is graduated, and, by means of an index fixed to the frame, the angular rotation can always be obtained. It is also provided with two adjusting screws[21] _f_, by means of which the axis of rotation can be accurately centered. The method of centering is as follows: Bring some prominent mark in the section exactly in coincidence with the intersection of the cross-wires in the eye-piece. Rotate the stage 180°, and correct one half the error by means of the centering screws, and the other half by moving the section on the stage. Check the result by rotating the stage 180° again, and if necessary repeat the corrections in the same way until the adjustment is satisfactory. The _objective_[22] _g_ screws into the collar _h_, which has a slot _k_, in the upper portion, for the introduction of a sensitive color plate, a one quarter undulation mica plate or a quartz wedge.[23] The slot _k_ is so arranged that, when the sensitive color plates are introduced, the vibration directions of these plates will make an angle of 45° with the planes of vibration of the crossed nicols, and the interference color will thus be at its maximum intensity. A revolving _nose-piece_ is sometimes used which can be attached to the collar _h_ and arranged to carry two or three objectives, which can thus be very quickly brought into position for use. This is convenient in passing rapidly from observations with parallel light to observations with convergent light, which must be made with a high power objective. The difficulty is that recentering is generally required. The modern microscopes are provided with a clip for holding the objectives, instead of the screw-thread collar _h_. The _analyzer_[24] or upper nicol is contained in the frame _l_, which is arranged so as to slide in and out of the tube of the microscope. The plane of vibration of the analyzer is fixed by the instrument maker so as to be at right angles to the plane of vibration of the polarizer, hence in the Seibert microscope parallel to the E. and W. cross-wire in the eye-piece. Consequently when the frame _l_ is pushed into the tube, the analyzer is introduced in the line of sight between the objective and the observer’s eye, with its plane of vibration at right angles to the plane of vibration of the polarizer; that is the nicols are crossed. When the nicols are crossed, if they are properly adjusted, no light can pass through to the eye and the field of view should be dark.[25] The _eye-piece_[26] _o_ fits into the top of the tube, and, by means of a little projecting piece fitting into a slot in the frame, can always be adjusted so as to have its cross-wires parallel to the planes of vibration of the two nicols.[25]