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.

Read about the contribution of van Leeuwenhoek and Hooke

Magnification: the increase of an object's apparent size.

Field of view: the area visible through the microscope lenses. Field of view decreases as magnificaiton increases.

Depth of field: the thickness of the viewing area. Even small objects have depth. This will affect the way you focus the microscope.

Resolution: the ability to show details clearly. Resolution allows the viewer to see two objects that are very close together as two objects rather than as one.

Resolution is determined by the quality of the microscope lenses - the better the lenses, the better the resoultion.

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. Most high school 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 of the Lab Microscopes

• 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.
   
• DO NOT slide the microscope back and forth on the lab table.
   
• 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.
   
• Cleanup:
   
  • 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.
     
  • Always rotate the nosepiece so that the LOW POWER objective lens is in line with the eyepiece.
     
  • Return the microscope to storage.

Viewing Specimens Under a Microscope

• Because the microscope uses lenses and mirrors, the image you see through most light microscopes will be upside down and backward.

  On the stage	Through the lens

• 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.
   
• Making a wet-mount slide:
  
  
  • Place a clean slide on the lab table. Handle slides at the ends, not the center, to avoid getting fingerprints in the viewing area of the slide.
     
  • Add specimen to the slide.
     
    • For liquid samples, place one small drop 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 stain.
     
  • Hold the coverslip by the edges to avoid fingerprints. 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 view a slide without a coverslip. The coverslip protects the objective lens from the liquid on the slide.
   
• Many objects do not have distinct, contrasting colors. This makes seeing details difficult. Observation may be improved by staining with a biological stain.

  Natural cheek cells	Stained cheek cells

• Observing Protozoans and Crustaceans:
   
  • Small animals are often difficult to observe because of their motion. They must be slowed if any but the lowest magnification is to be used.

    Several things can be done to help in this reguard.

    • Methyl cellulose 1.5% - a viscous material that can be mixed with the water of a wet mount to slow the animals without harming them. Use a toothpick to mix the two on the slide before adding a coverslip.
       
    • Cotton fibers - the natural cotton fibers will provide barriers to block some of the movement of the animals. Using a very small piece of natural cotton, pull the fibers apart. Lay the cotton in the center of the slide before adding the water and coverslip.
       
    • Well slides - microscope slides that have a depression in the center of the slide. This depression may hold a small drop of water, keeping it from spreading like on a flat slide. The disadvantage is that the animals now have more room to move up and down, instead of from side to side.
   
• Drawing Objects:

  When making drawings on your lab report, do the following:

  • For drawings only, use a pencil - you can erase and shade areas.
     
  • Begin by identifying the area viewed through the microscope with a circle.
     
  • Specimens should be drawn to scale. If the specimen takes up the whole viewing field, make sure your drawing shows this.
     
  • Drawings should be labeled with the specimen name and magnification.
     
  • Drawings should be large enough to view details and have significant details labeled.
     
  • All labels should be written on the outside of the circle.

Electron Microscopes

Light microscopes are limited to about 2000X by the properties of light. The 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 live specimens.

Two types of electron microscopes:

• Transmission electron microscope - TEM: transmits a beam of electrons through a very thinly sliced specimen. TEMs can magnify objects over 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.