Alignment Procedure for Polarizers of the Polarizing Microscope
In practical operation, the vibration directions of the upper and lower polarizers of a polarizing microscope must be mutually perpendicular (aligned along the east-west and north-south directions), and consistent with the crosshairs of the eyepiece. When only the lower polarizer is used for observation, its vibration direction must be determined. Therefore, the polarizers must be calibrated before use.
(1) Inspection of eyepiece crosshairsIt is generally necessary to check whether the eyepiece crosshairs are perpendicular to each other and whether they are consistent with the vibration directions of the upper and lower polarizers. Meanwhile, select a biotite grain with perfect cleavage, move it to the center of the eyepiece crosshairs, align the cleavage parallel to one crosshair line, and record the reading of the stage. Rotate the stage to align the cleavage parallel to the other crosshair line, and record the new stage reading. A difference of 90° between the two readings indicates that the crosshairs are perpendicular.
(2) Determination and calibration of the vibration direction of the lower polarizerBiotite is commonly used to determine the vibration direction of the lower polarizer, as it is a widely distributed transparent mineral and exhibits distinctive features under plane-polarized light. First, select a biotite grain with distinct and clear cleavage, center it in the eyepiece crosshairs, and withdraw the upper polarizer. Rotate the stage through one full revolution and observe the color change of the biotite. Since biotite strongly absorbs light vibrating parallel to its cleavage, the direction of the cleavage corresponds to the vibration direction of the lower polarizer when the biotite reaches its darkest tint.
(3) Calibration of the upper and lower polarizersAfter the lower polarizer is properly aligned, remove the thin section and insert the upper polarizer. Check whether the field of view is completely dark (i.e., in the extinction position). If the field is fully dark, the vibration directions of the upper and lower polarizers are mutually perpendicular. If not, the upper polarizer must be calibrated: rotate the upper polarizer until the field reaches maximum extinction. Before rotation, the set screw of the upper polarizer must be loosened; retighten it once calibration is finished.
