A microscope is an instrument that produces an enlarged image of an object. Biologists use microscopes to study things that are too small to be seen with the unaided eye. Most microscopes are called light microscopes because they accomplish their task by using lenses to bend light rays.
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Observing an object:
Because the light rays from an object cross before reaching your eye, the image you see through most microscopes will be inverted and upside down.
Magnification: the increase of an object's apparent size.
Resolution: the power to show details clearly. Resolution allows the viewer to see two objects that are very close together as two objects rather than as one.
Compound microscopes use multiple lenses to produce an increase in magnification. If the eyepiece lens enlarges by a factor of 10 (10X) and the objective lens enlarges by a factor of 40 (40X), the total magnification is the product of the two - 400X. Resolution is controlled by the quality of the lenses being used - the better the lenses, the better the resoultion.
Basic parts of a compound light microscope:
- Eyepiece (Ocular): Usually contains a 10X lens.
- Arm: contains the housing for the fine and coarse adjustments and connects the base of the microscope to the nosepiece and ocular.
- Nosepiece: A rotating head that has the objective lenses attached to it. The lens to be used should "click" into position when the wheel is gently turned so that it is directly over the speciman slide.
- Objective: Basically a housing for a lens. Our microscopes have three objective lenses - 4X, 10X, and 40X.
- Stage: The speciman slides rests on this part of the microscope.
- Coarse adjustment knobs: The larger of two sets of knobs located on either side of the arm, just above the base. This adjustment is used to make large adjustments in focusing by moving the lenses up and down. Never use this adjustment when using the 40X objective.
- Fine adjustment knobs: The smaller of two sets of knobs located on either side of the arm. This adjustment is used to make small adjustments in focusing. It has a limited amount of movement and is most efficiently used after focusing with the 4X objective and coarse focus, then increasing magnification and making final adjustments with the fine focus knob.
- Light source: Located directly under the stage.
- Adjustable diaphragm: This rotating wheel on the underside of the stage allows the user to adjust the amount of light that passes through the specimen. As a general rule, the lowest intensity of light that allows you to resolve the structure of the object you are viewing should be used.
Care and handling:
A microscope is a delicate piece of equipment and should be treated with care.
- Use two hands when carrying the microscope. Place one hand around the arm of the microscope and the other under the base for support.
- Carry the microscope upright and close to the body.
- Place the microscope flat on the table, but not too near the edge where it might be knocked off.
- If it becomes necessary to clean the lenses on the microscope, ask your facilitator for a piece of lens paper. Other materials, such as paper towel, can scratch the surface of the lens.
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Preparing specimens for viewing:
- In most instances, light must pass through any object to be viewed with a light microscope. For this reason, the object must be fairly thin. Thick objects must be sliced into thin sections for viewing.
- Many objects do not have distinct, contrasting colors. This makes it difficult to see details. To improve the viewing of these objects, they are stained. Staining is the use of a biological dye to make the details visible.
- Making a slide:
- Place a clean slide on the table.
- For liquid samples, place one or two drops in the center of the slide. For solid samples, place the sample in the center of the slide and add one drop of water or staining solution.
- Hold the plastic coverslip by the edges. Do not get fingerprints on the coverslip. Set one edge against the slide and lower it until it contacts the liquid. The liquid should spread across the whole area of the coverslip.
- Never use a slide under the microscope without a coverslip. Its major purpose is to protect the objective lens for the liquid on the slide.
- Unless otherwise instructed, wipe the sample and coverslip off the slide with a paper towel when finished. Throw the paper towel and its contents away. Return the microscope slide to its container.
Light microscopes are limited to about 2000X by the properties of light. Another type of microscope called an electron microscope uses a beam of electrons instead of light and magnets instead of lenses. Because of the high-energy particles involved, these microscopes cannot be used to view living specimens.
There are two types of electron microscopes:
- Transmission electron microscope - TEM: transmits a beam of electrons through a very thinly sliced specimen. TEMs can magnify objects up to 1,000,000 times.
- Scanning electron microscope - SEM: specimens are not sliced. The surface of the specimen is sprayed with a fine metal coating and a beam of electrons is passed over the specimen. Electrons from the metal coating are projected onto a screen or photographic plate. SEMs can magnify objects up to 300,000 times.