In the field of optical mineralogy, conoscopic is the term used to describe the lens combination in the polarized microscope in obtaining interference figures. The usual arrangements produces interference figures visible in the field of the ocular of the polarizing microscope used in optical mineralogy. Such figures are particularly useful for determining the optical directions in the crystals of the mineral. Their interpretation involves the principles of polarized light as described in the field of optical mineralogy combined with an understanding of the crystallization of minerals.

 

            It is necessary to exercise particular care of the small crystals when determining its its interference figure and make sure to put that all the elements in the optical train are in exact alignment and are properly centered when viewed under petrographic polarizing microscope used in optical mineralogy. For easier manipulation and satisfactory results, it is better to use moderately high magnification for polarizing microscope lenses. The auxiliary condenser should be inserted across the axis of the petrographic polarizing microscope used in optical mineralogy, just below the stage. It has been known in optical mineralogy studies that the front lens of the condenser throws a concentrated convergent beam against the mineral plate. Most polarizing microscopes today are provided with a diaphragm between the polarizer and the lower component of the condenser. As discussed in optical mineralogy, the diaphragm limits the field of view and helps to improve the outer portion of the interference figure. A Bertrand lens is then inserted as needed in the tube of the polarizing microscope just above the analyzer. It has been known that the lens brings the image of the interference figure into focus on the focal spot of the ocular of the polarizing microscope. As studied in optical mineralogy, good figures of small size can be obtained by removing the ocular and not using the Bertrand lens of the polarized microscope.

 

            There have been two types of interference figures known in optical mineralogy given by anisotropic minerals. These are uniaxial and biaxial interference figures. It has been known also in the field of optical mineralogy that minerals crystallizing in the hexagonal and tetragonal systems are uniaxial. On the other hand, those that are crystallizing in the orthorhombic, monoclinic, and triclinic systems are known to be biaxial. Occasionally biaxial crystals have such a small axial angle as to appear uniaxial, and conversely on certain occasions normally uniaxial crystals may become biaxial due to strain. These variations are most commonly given thorough examination and with much caution when encountered.