How is the polariser of a polarising microscope calibrated?
In practice, the direction of vibration of the upper and lower polarising mirrors of the polarising microscope should be orthogonal to each other, or in the east-west and north-south directions, each being in line with the cross filament of the eyepiece in both the transverse and longitudinal directions. Sometimes only one lower polariser is used for observation, and the direction of vibration of the lower polariser must be determined, so the polariser must be corrected during operation.
(1) Inspection of the eyepiece cross filament
Generally check whether the eyepiece cross filament is orthogonal, and whether it is consistent with the direction of vibration of the upper and lower polarizers, and at the same time, select a piece of black mica with a very complete solving pole, move it to the centre of the eyepiece cross filament, place the solving slit parallel to one of the cross filaments and note down the number of scales of the carrier table, and then rotate the animal stage so that the solving slit is parallel to the other cross filament, and note down the number of scales of the carrier table, and the difference of the two scales is 90 °, which indicates that the cross filaments are orthogonal. The difference between the two scales is 90°, indicating that the cross filaments are orthogonal.
(2) Determination and correction of the direction of vibration of the lower polariser
Generally black mica is used to check the vibration direction of the lower polarizer, this is because black mica is a widely distributed transparent mineral that is very characteristic in single polarised light. First find a piece of solved and clear black mica, move it to the centre of the eyepiece cross filament, launch the upper polarizer, rotate the carrier table for one week, observe the change of black mica colour, because black mica absorbs the strongest vibration light in the direction of solving, so make the black mica colour reach the deepest when the direction of the solving slit is the vibration direction of the lower polarizer.
(3) Correction of orthogonality of upper and lower polarizers
After the direction of the lower polarizer is corrected, remove the sheet, push the upper polarizer, and observe whether the field of view is completely black, i.e., whether it is in an extinction state. If it is all black, it indicates that the vibration directions of the upper and lower polarisers are orthogonal to each other. Otherwise, the upper polariser must be corrected, i.e. rotate the upper polariser so that the field of view reaches the darkest. When turning, the stop screw of the upper polariser must be loosened first, and then tightened after correction.
