Petrographic microscope nowadays comes into numerous designs. Although they can be different in detail, they are fundamentally the same construction and function. Almost all Petrographic Microscopes are composed of illuminator, substage, assembly, stage, objective lenses, upper polar, Bertrand lens, and ocular lens. Most modern Petrographic Microscope is well equipped with prism positioned between the upper polar and the ocular. This actually allows the microscope tube to be tilted from the vertical, and all have wonderful focusing mechanism.

The Illuminator

Most modern petrographic polarizing microscope nowadays is equipped with an incandescent bulb that is usually mounted in the base of the microscope. The light exhibited by this light source is directed upward with a combination of mirrors and lenses. This light is to be diffused by passing through a piece of ground glass. Then it is filtered with a piece of blue glass so that the color balance of the light will more closely approximate the natural light of the sun. A rheostat control or a filter adjusts the intensity of the light. Some polarizing microscopes are also equipped with the iris diaphragm or the field diaphragm that are usually mounted at the base of the equipment. The size of the area on the sample that is being illuminated is controlled by the field diaphragm.

Some microscopes may simply use mirror as illuminator. With this design, an external source of light like the sun for instance is directed at the mirror, which is adjusted to direct the light beam upward. This mirror illuminator is also used when an external source of monochromatic light, like the sodium vapor lamp, is needed or when a built-in illuminator is not provided.