4.2 Parts of a Petrographic Microscope

This section provides an overview of the parts of a petrographic microscope and what each part does.  It is also a good review for a student who has used a petrographic microscope in a previous class but needs a refresher on the anatomy of a microscope.

Learning Objectives

Identify and describe the purpose of each part of a petrographic microscope.

Prior Knowledge and Skills


Key Terms

  • Eyepiece
  • Viewing tube
  • Bertrand lens
  • Analyzer
  • Accessory plate
  • Revolving nosepiece
  • Objective
  • Rotating stage with goniometer
  • Vernier
  • Mechanical stage
  • Focus (fine and coarse)
  • Condenser
  • Condenser aperture diaphragm
  • Substage centering screw
  • Polarizer
  • Field diaphragm
  • Base of microscope
  • Illumination intensity controller
  • Illuminator

Guided Inquiry

Look at the diagrams of the petrographic microscope and its parts in Figures 4.2.1-4.2.4.

Insert interactive diagrams by Christie Liu and Mark Peale here:

Figure 4.2.1 Front view of microscope

Figure 4.2.2 Side view of microscope

Figure 4.2.3 Top view of microscope

Figure 4.2.4 View of substage

4.2.1 Which of the parts of the microscope can you define the purpose of without looking anything up?  List all that you feel you completely understand how to use here:

4.2.2 Which three parts of the microscope are the most confusing to you, either from experience or from looking at these diagrams?

Concept Check 4.2.1

In how many places is the thin section image magnified in a petrographic microscope?  How do you determine the magnification of the image?


Descriptions of the microscope parts, from top to bottom, are found in the list below or in these references:

Eyepiece: https://en.wikipedia.org/wiki/Eyepiece

Viewing tube: The tube which connects the eyepiece to the microscope

Bertrand lens: https://www.photonics.com/EDU/Bertrand_lens/d2582

Analyzer: The polarizer located above the sample and objective lens.  The polarization direction is oriented 90 degrees (perpendicular) to the polarization direction of the bottom polarizer.

Accessory plate: The microscope includes a slot above the objective lenses in which an accessory plate can be inserted.  Accessory plates help determine optical properties of minerals, and include a quartz wedge, a gypsum plate, and a mica plate (Nesse 1991)

Revolving nosepiece: https://sciencing.com/revolving-nosepiece-microscope-8715601.html One should always use the nosepiece to move the objectives- never grab the objective lens itself.

Objective: The objective lenses (as well as the eyepieces) provide the magnification for the microscope.  Typical microscopes contain three objectives: low, medium, and high magnification.  Our microscopes have 4x, 10x, and 63x objective lenses.  The magnification should be written on the objective.

Rotating stage with goniometer: The stage is where the thin section is placed.  Most petrographic microscopes have a circular rotating stage with 360 degrees marked in 1 degree increments around the edge of the stage.  This aids in optical measurements.

Vernier: https://en.wikipedia.org/wiki/Vernier_scale

Mechanical stage: https://youtu.be/9-1AQtMCezI

Focus (fine and coarse): The focus knobs move the stage closer to or farther away from the objective lens.

Condenser: https://en.wikipedia.org/wiki/Condenser_(optics)#Microscope_condenser

Condenser aperture diaphragm: Because it is near the rear of the condenser lens, closing the condenser aperture reduces the illumination across the entire field of view (https://physics.stackexchange.com/questions/197163/whats-the-difference-between-two-types-of-diaphragms)

Substage centering screw: Adjusts the substage (condenser) horizontally for centering the light beam.

Polarizer: https://en.wikipedia.org/wiki/Polarizer This is located below the stage and sample. The polarization direction is oriented 90 degrees (perpendicular) to the polarization direction of the analyzer.

Field diaphragm: Closing the field diaphragm reduces the size of the light beam that passes upwards through the sample, by blocking the edges of view.  When the field diaphragm is closed, you can see the edges of the diaphragm in the field of view. (Nesse 1991 and https://physics.stackexchange.com/questions/197163/whats-the-difference-between-two-types-of-diaphragms)

Base of microscope: The heavy bottom part of the microscope which contains the illuminator

Illumination intensity controller: A dial which adjusts the light intensity from the illuminator

Illuminator: the light source, located in the base of the microscope (for transmitted light) https://www.amscope.com/microscope-parts-and-functions/?medium=tsa&gclid=Cj0KCQiAxZPgBRCmARIsAOrTHSaHLQ2o-9q9OKkPq2zc_vK528OlsKYtxViwdKvv01cIRXQsw8nsdKkaAvwAEALw_wcB&gclsrc=aw.ds


AmScope (ret. 12/3/2018) Microscope Parts and Functions. https://www.amscope.com/microscope-parts-and-functions/?medium=tsa&gclid=Cj0KCQiAxZPgBRCmARIsAOrTHSaHLQ2o-9q9OKkPq2zc_vK528OlsKYtxViwdKvv01cIRXQsw8nsdKkaAvwAEALw_wcB&gclsrc=aw.ds

Microscope World (3/12/2014) Microscope Mechanical Stage. https://youtu.be/9-1AQtMCezI

MyScope (ret. 12/1/2018) Microscope Components. https://myscope.training/#/M101level_1_5

Nesse, W.D. (1991) Introduction to Optical Mineralogy, 2nd edition. Oxford University Press, New York, 335p.

Olympus: Spring, K.R. and Davidson, M.W. (ret. 12/1/2018) Microscope Optical Components Introduction. https://www.olympus-lifescience.com/en/microscope-resource/primer/anatomy/components/

Olympus: Spring, K.R., Long, J.C and Davidson, M.W. (ret. 12/1/2018) Condenser Image Planes. https://www.olympus-lifescience.com/en/microscope-resource/primer/java/components/condenser/

Olympus: Abramowitz, M. and Davidson, M.W. (ret. 12/1/2018) Light Sources. https://www.olympus-lifescience.com/en/microscope-resource/primer/anatomy/sources/

Olympus: Abramowitz, M. and Davidson, M.W. (ret. 12/1/2018) Eyepieces (Oculars) https://www.olympus-lifescience.com/en/microscope-resource/primer/anatomy/oculars/

Photonics Media (2018) Bertrand Lens. https://www.photonics.com/EDU/Bertrand_lens/d2582

Stack Exchange Inc, User Contributions. (ret 2018) What’s the principal difference between aperture iris diaphragm and field iris diaphragm? https://physics.stackexchange.com/questions/197163/whats-the-difference-between-two-types-of-diaphragms

Wikipedia contributors. (2018, November 21). Condenser (optics). In Wikipedia, The Free Encyclopedia. Retrieved 01:06, December 4, 2018, from https://en.wikipedia.org/w/index.php?title=Condenser_(optics)&oldid=869957949

Wikipedia contributors. (2018, October 25). Eyepiece. In Wikipedia, The Free Encyclopedia. Retrieved 00:38, December 2, 2018, from https://en.wikipedia.org/w/index.php?title=Eyepiece&oldid=865744166

Wikipedia contributors. (2018, December 2). Polarizer. In Wikipedia, The Free Encyclopedia. Retrieved 01:04, December 4, 2018, from https://en.wikipedia.org/w/index.php?title=Polarizer&oldid=871569059

Wikipedia contributors. (2018, November 18). Vernier scale. In Wikipedia, The Free Encyclopedia. Retrieved 00:51, December 4, 2018, from https://en.wikipedia.org/w/index.php?title=Vernier_scale&oldid=869424168

Licenses and Attributions

Text written by Elizabeth A. Johnson, James Madison University (JMU).
Figures created by Mark Peale, Elizabeth Johnson, and Juhong Christie Liu, JMU.
CC-by License