The resolving power of an ordinary microscope can be effectively doubled by the use of narrow pencils of highly inclined light. This illumination technique reveals many details in rock thin sections not previously reported in the literature. A mineral with a slightly higher index of refraction than its matrix can be made to exhibit tremendous positive relief. Orthoclase can easily be distinguished from quartz under plane polarized light thus making grain counting procedures much simpler.
Twinning in some feldspars can be observed under plane polarized light. Worm-shaped paths occur in quartz, feldspars and biotite. These are presumed to be paths along which material has migrated through the rocks.