A section of a positive biaxial mineral with a small 2V cut perpendicular to Z, the acute bisectrix in positive minerals when viewed with the aid of polarizing microscope used in optical mineral mineralogy. The ray of light A passes through the section obliquely and is polarized into two rays, the vibration directions of which can be determined by the Biot-Fresnel law. Traces of the two planes each of which contains an optic axis and the normal to the effective section can be drawn to show the effective sections containing the two vibration directions and its normal, the parallel to ray A. When closely evaluated under the petrographic polarizing microscope used in optical mineralogy, the two vibration directions bisect the angles between these two planes, and in this case are parallel to the polarizer and analyzer vibration directions, and light is extinguished between crossed polars. The ray of light B, on the other hand,is polarized into two rays vibrating at an angle to the polarizer and analyzer directions, and light is not extinguished. It can be found that there are two curved bands of extinction called isogyres passing through the optic axes if the vibration directions of all other convergent rays of light are determined under polarizing microscopes. In the field of optical mineralogy, it has been known that the birefringence increases outwards from the optic axes, and if the birefringence or thickness of section are sufficiently high for interference color rings to be present, they usually form a figure-of-eight pattern. It has been also known in the field of optical mineralogy that unlike the uniaxial cross interference figure, the pattern of isogyres changes as the stage of polarizing microscope is rotated. The isogyres join to form a cross when the optical directions X and Y are parallel to the vibration directions of the polarizer and analyzer of polarizing microscopes, and open up again in the other quadrants on further rotations. The positions of the optic axes are always marked by the figure-of-eight pattern of interference color rings, if they are only found present under petrographic polarizing microscopes used in optical mineralogy.