The polarizing microscope is a specialized optical instrument extensively used in crystallography, mineralogy, and economic geology for the identification and study of minerals in thin sections. It is designed to examine anisotropic properties of crystalline substances by using polarized light and consists of several distinct optical and mechanical parts arranged in a precise sequence. The source of illumination is usually a built-in electric lamp located at the base of the microscope. Light from the source passes through a collector lens and then through the polarizer, which is a Nicol prism or Polaroid sheet placed below the stage. The polarizer converts ordinary light into plane-polarized light vibrating in a single direction. Above the polarizer is the condenser, which focuses the polarized light uniformly onto the specimen. The condenser may be fitted with an iris diaphragm to control the intensity and angle of illumination, which is essential for clear observation of interference effects. The stage of the polarizing microscope is circular and rotatable, unlike the fixed stage of a biological microscope. It is graduated from 0° to 360°, allowing precise measurement of extinction angles and crystal orientation. The thin section of the mineral mounted on a glass slide is placed