A polarizing microscope is used to observe rocks and minerals in thin sections. It is a typical optical microscope equipped with a circular bank, rotating 360°, special objective lenses, a polarizer for the production of polarized light, and a second polarizer, called "analyzer" in the path of the light between the objective lens and the eyepiece. Some petrographic microscopes can also be equipped with the "Bertrand lens", which focuses and enlarges the interference pattern depicted on the back of the lens. Petrographic microscopes usually have special oriented filters located within the optical path between the polarizers to determine the birefringence and the order of the minerals. Most crystalline materials and minerals change the orientation of the polarizing light, allowing part of the changing light to pass through the analyzer to the eyepieces. Using a polarizer makes it possible to view in plain polarized light. Using the two polarizers, it allows the analysis in a so-called "cross polarization" state.