The Compound Microscope
The microscope used in most biology classes, the compound microscope, contains a combination of lenses. The eyepiece lens is located in the top portion of the microscope. This lens usually has a magnification of 10×. Other lenses, called objective lenses, are at the bottom of the body tube on the revolving nosepiece. By rotating the nosepiece, you can select the objective through which you will view your specimen.
The shortest objective is a low-power magnifier, usually 10×. The longer ones are of high power, usually up to 40× or 43×. The magnification is marked on the objective. To determine the total magnification, multiply the magnifying power of the eyepiece by the magnifying power of the objective. For example, with a 10× eyepiece and a 40× objective, the total magnification is 10 × 40 = 400×.
Learning the name, function, and location of each of the microscope's parts is necessary for proper use. Use the following procedures when working with the microscope.
Carry the microscope by placing one hand beneath the base and grasping the arm of the microscope with the other hand.
Gently place the microscope on the lab table with the arm facing you. The microscope's base should be resting evenly on the table, approximately 10 cm from the table's edge.
Raise the body tube by turning the coarse adjustment knob until the objective lens is about 2 cm above the opening of the stage.
Rotate the nosepiece so that the low-power objective (10×) is directly in line with the body tube. A click indicates that the lens is in line with the opening of the stage.
Look through the eyepiece and switch on the lamp or adjust the mirror so that a circle of light can be seen. This is the field of view. Moving the lever of the diaphragm permits a greater or smaller amount of light to come through the opening of the stage.
Place a prepared slide on the stage so that the specimen is over the center of the opening. Use the stage clips to hold the slide in place.
Look at the microscope from the side. Carefully turn the coarse adjustment knob to lower the body tube until the low-power objective almost touches the slide or until the body tube can no longer be moved. Do not allow the objective to touch the slide.
Look through the eyepiece and observe the specimen. If the field of view is out of focus, use the coarse adjustment knob to raise the body tube while looking through the eyepiece. CAUTION: To prevent damage to the slide and the objective, do not lower the body tube using the coarse adjustment while looking through the eyepiece. Focus the image as best you can with the coarse adjustment knob. Then use the fine adjustment knob to focus the image more sharply. Keep both eyes open when viewing a specimen. This helps prevent eyestrain.
Adjust the lever of the diaphragm to allow the right amount of light to enter.
To change the magnification, rotate the nosepiece until the desired objective is in line with the body tube and clicks into place.
Look through the eyepiece and use the fine adjustment knob to bring the image into focus.
After every use, remove the slide. Return the low-power objective into place in line with the body tube. Clean the stage of the microscope and the lenses with lens paper. Do not use other types of paper to clean the lenses; they may scratch the lenses.
Parts of the Microscope and their Function
Eyepiece Contains a magnifying lens
Arm Supports the body tube
Stage Supports the slide being observed
Opening of the stage Permits light to pass up to the eyepiece
Fine adjustment knob Moves the body tube slightly to sharpen the image
Coarse adjustment knob Moves the body tube to focus the image
Base Supports the microscope
Illuminator Produces light or reflects light up toward the eyepiece
Diaphragm Regulates the amount of light passing up toward the eyepiece
Diaphragm lever Opens and closes the diaphragm
Stage clips Hold the slide in place
Low-power objective Provides a magnification of 10× and is the shortest objective
High-power objective Provides a magnification of 40× and is the longest objective
Nosepiece Holds the objectives and can be rotated to change the magnification
Body tube Maintains the proper distance between the eyepiece and the objectives
Preparing a Wet-Mount Slide
Obtain a clean microscope slide and a coverslip. A coverslip is very thin, permitting the objective lens to be lowered very close to the specimen.
Place the specimen in the middle of the microscope slide. The specimen must be thin enough for light to pass through it.
Using a dropper pipette, place a drop of water on the specimen.
Lower one edge of the coverslip so that it touches the side of the drop of water at about a 45° angle. The water will spread evenly along the edge of the coverslip. Using a dissecting needle or probe, slowly lower the coverslip over the specimen and water as shown in the drawing. Try not to trap any air bubbles under the coverslip. If air bubbles are present, gently tap the surface of the coverslip over the air bubble with a pencil eraser.
Remove any excess water at the edge of the coverslip with a paper towel. If the specimen begins to dry out, add a drop of water at the edge of the coverslip.
Staining Techniques
Obtain a clean microscope slide and coverslip.
Place the specimen in the middle of the microscope slide.
Using a dropper pipette, place a drop of water on the specimen. Place the coverslip so that its edge touches the drop of water at a 45° angle. After the water spreads along the edge of the coverslip, use a dissecting needle or probe to lower the coverslip over the specimen.
Add a drop of stain at the edge of the coverslip. Using forceps, touch a small piece of lens paper or paper towel to the opposite edge of the coverslip, as shown in the drawing. The paper causes the stain to be drawn under the coverslip and to stain the cells in the specimen.