A change, which cannot be called pleochroism, takes place when sections of the mineral Calcite are rotated over the polarizer of the petrographic polarizing microscope. Calcite is doubly refracting when evaluated under geological polarizing microscopes, and, consequently, most of its sections possess the two rectangular vibration directions for transmitted light. When the rotation takes place, […]

We are now in a position to consider the microscopic examination of minerals in thin sections when the lower nicol, the polarizer, is in position in the axis of the polarizing microscope underneath the stage.
 
            The light reaching the section has previously passed through the polarizer, and consequently performing its vibration parallel to the principal […]

Certain minerals, otherwise colorless in thin section, show curious circular brown areas when viewed in polarized light of the petrographic polarizing microscope. Each of which contains at its center an included fragment of some other mineral. These brown patches change color when the section is rotated above the polarizer of the polarizing microscopes. They are […]

Having completed a survey of the characters employed in mineral identification when the polarizing microscope is used without either nicol, the attention of the viewer may be directed to the characters to be observed when the lower nicol or the polarizer is inserted in the position in the axis of the polarizing microscope. The upper […]