Before students new to the world of microscopy are able to view microscopic specimens, they have to first learn their way around a microscope. Below are descriptions of the key parts of a compound microscope. Helpful photos corresponding to these descriptions appear at the bottom of the article.
What Is a Compound Light Microscope?
A compound light microscope (Photo #1) uses visible light to illuminate a specimen and has two sets of lenses for magnification; the ocular lenses and the objective lenses.
Ocular Lens
The ocular lens, or lenses, are closest to the eye when one looks through a microscope. This lens can be monocular, with one lens in a tube, or binocular, with two ocular lenses. The microscope pictured is binocular (Photo #2). The ocular lenses generally magnify objects 10x. The level of magnification should be written somewhere on the lens tube.
Objective Lenses
The objective lenses of a compound microscope are the lenses closest to the stage (where the slide sits). These lenses are mounted on a revolving nosepiece, which can be rotated to select one of typically three or four objective lenses (Photo #3):
- Scanning lens > has a red band > objective magnification 4x
- Low power lens > has a yellow band > objective magnification 10x
- High dry power lens > has a blue band > objective magnification 40x
- Oil immersion lens > has a black and white band > objective magnification 100x
The total magnification (xTM) is calculated by multiplying the magnification of the ocular lens by the magnification of the objective lens in use.
Mechanical Stage
Beneath the objective lens is the stage, a flat area where the slide / specimen is placed. Some stages are mechanical, meaning that they have a metal apparatus where the slide is inserted (Photo #4), and knobs hanging off the side of the stage (coaxial knobs) for moving the slide.
Light Controls
Most compound microscopes have two ways in which the light level can be adjusted. There is a dial on the base of the microscope that can increase and decrease the brightness of the light shining up through the stage from the base. There is also an iris diaphragm, located directly beneath the stage, that can be turned to open and close the aperture, increasing or decreasing the amount of light that is able to shine through the hole in the stage.
Focus Knobs
At the base of the microscope, there are two knobs used to adjust the height of the stage. When the specimen on the stage is moved closer or further away from the objective lenses, this affects the focus — how clear and crisp the image is. The smaller fine focus knob is often located or inset within the larger coarse focus knob (Photo #5). Coarse focus should only be adjusted at lower magnifications (when the scanning and low power objective lenses are in place). When at higher magnifications (when the high dry or oil immersion objective lenses are in place) only use fine focus, to avoid smashing the lens into the specimen.
More Free Information on Microscopy
For more information on using a compound light microscope, see the Suite 101 articles:
Access printable quizzes for identifying parts of a compound microscope, or for more information on the microscope and microbiology in general, see the Virtual Microbiology Classroom.
Tami Port - Tami Port is a college professor of cell and microbiology and creator of ScienceProfOnline.com, a free science education website.
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