Geology and microscopy have a long and complimentary history. Geologists use specialized petrographic / polarizing microscopes to examine carefully made thin sections of rocks, polished to precisely 30 microns thick - thin enough that they may typically be examined in transmitted light. The crystals within the thin section may be observed with brightfield or polarized light, and compensators such as gypsum plates (also known as full wave plates or first order red compensators), quarter wave compensators, and quartz wedges may be introduced. These samples may be observed "orthoscopically" observing the specimen directly, or "conoscopically" using a Bertrand lens. Stereomicroscopes are also routinely used by geologists for examining larger samples of rocks or fossils.