Parts of a Petrological Microscope

A Petrological Microscope also called a Polarizing Microscope is an essential instrument in optical mineralogy and Petrology. It is used to observe thin sections of rocks under polarized light and to identify their physical and optical properties. The microscope is widely used to identify and classify rocks and minerals. Given below is a schematic diagram of a petrological microscope and its parts.

parts of  a petrological microscope

The function of each part are as follows,

Light Source - Provides light into the microscope for viewing. Usually an electric bulb or a two sided(plane and concave) mirror.

Polarizing unit 1 - This converts normal light into plane polarized light and is situated below the microscope stage.

Condenser System - This removes the effects of interference of light by changing the phase of light.

Diaphragm Lever - This lever is used to control the intensity of light.

Microscope Stage - This is a graduated, rotatable disk on which the thin sections are mounted for viewing purposes.

Objective - This contains several objective lenses of different magnifying power that can be selected. These lenses are of usually of three types. Low power objectives(3.5x) that provide a large coverage of the thin section where the micro-structures and grain percentages can be observed, Medium power objectives(10x) that shows several grains in considerable detail and can be used for identification purposes, and High power objectives(40x-50x) that show a single grain in very fine detail and used for advanced analysis.

Slot - the slot is used to insert accessory plates for different viewing purposes.

Analyzer (polarizing unit 2) - This is also a light polarizing unit. However the plane of polarization is perpendicular to that of the polarizing unit that is situated below the microscope stage. This can be set in either "in" or "out" positions.

Bertrand lens - This is a special lens system that is used to observe interference figures. This too can be ste in either "in or "out" positions. When the Bertrand lens is in the "in" position, the view seen from the eyepiece is smaller.

Eye piece - This is the lens through which the observer views the thin section. It has a circular view with centre cross-hairs to help viewing.

The petrological microscope can be set up in two arrangements depending on viewing purposes and optical properties of the minerals. They are,
1.  PPL arrangement (Plane Polarized Light) - analyzer is out, Bertrand lens is out
2.  CPL arrangement (Cross Polarized Light) - analyzer is in Bertrand lens is out.