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Name____________________________
Period______ Date______________
Laboratory Activity
#03 - Using a Compound Microscope
Background
Many objects are too small to be seen by the eye alone. They can be
seen, however, with the use of an instrument that magnifies, or visually
enlarges, the object. One such instrument, which is of great importance
to biologists and other scientists, is the compound light microscope.
A compound light microscope consists of a light source or mirror that
illuminates the object to be observed, an objective lens that magnifies
the image of the object, and an eyepiece (ocular lens) that further
magnifies the image of the object and projects it into the viewer’s
eye.
Objects, or specimens, to be observed under a microscope are generally
prepared in one of two ways. Prepared or permanent slides are made to
last a long time. They are usually purchased from biological supply
houses. Temporary or wet-mount slides are made to last only a short
time—usually one laboratory period.
The microscope is an expensive precision instrument that requires special
care and handling. In this investigation, you will learn the parts of
a compound light microscope, the functions of those parts, and the proper
use and care of the microscope. You will also learn the teclmique of
preparing wet-mount slides.
Objectives
1. To
learn how to use a compound light microscope.
Materials
(per group)
| compound light microscope |
microscope light |
coverslip |
| prepared slide |
lens paper |
dissecting probe |
| soft cloth (or cheesecloth) |
dropper pipette |
newspaper |
| scissors |
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Safety
Put on a laboratory apron. Always handle the microscope with extreme
care. You are responsible for its proper care and use. Use caution when
handling microscope slides, as they can break easily and cut you. Never
use direct sunlight as a light source for a compound light microscope.
The sunlight reflecting through the microscope could damage your eye.
Be careful when handling sharp instruments. Observe proper laboratory
procedures when using electrical equipment.
Procedures
Part A. Care of the Compound Light Microscope
1. Figure 1 shows the proper way to carry a microscope. Always
carry the microscope with both hands. Grasp the arm of the microscope
with one hand and place your other hand under the base. Always
hold the microscope in an upright position so that the eyepiece
cannot fall out. Place a microscope on your worktable or desk
at least 10 cm from the edge. Position the microscope with the
arm facing you.
2. Notice the numbers etched on the objectives and on the eyepiece.
Each number is followed by an “X” that means "times."
For example, the low-power objective may have the number “lOX”
on its side, as shown in Figure 2. That objective magnifies
an object 10 times its normal size. Record the magnifications
of your microscope in the Data Table. The total magnification
of a microscope is calculated by multiplying the magnification
of the objective by the magnification of the eyepiece.
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For example:
Use the formula to complete the data table.
4. Before you use the microscope, clean the lenses of the objectives
and eyepiece with lens paper. Note: To avoid scratching
the lenses, never clean or wipe them with anything other than lens paper.
Use a new piece of lens paper on each lens you clean. Never touch a
lens with your finger. The oils on your skin may attract dust or lint
that could scratch the lens.
Part B. Use of a Compound Light Microscope
| 1. Look at the microscope from
the side. Locate the coarse adjustment knob that moves the objectives
up and down. Practice moving the coarse adjustment knob to see
how it moves the objectives with each turn.
2. Turn the coarse adjustment so that the low-power
objective is positioned about 3 cm from the stage. Locate the
revolving nosepiece. Turn the nosepiece until you hear the high-power
objective click into position. See Figure 3. When an objective
clicks into position, it is in the proper alignment for light
to
pass from the light source through the objective into the viewer’s
eye. Now turn the nosepiece until the low-power objective clicks
back into position. Note:
Always look at the microscope from the
side when moving an objective so that the microscope does not
hit or damage the slide. |
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3. If your microscope has an electric light source,
plug in the cord and turn on the light. If your microscope has a mirror,
turn the mirror toward a light source such as a desk lamp or window.
CAUTION: Never use the sun as a direct source of
light. Look through the eyepiece. Adjust the diaphragm to permit
sufficient light to enter the microscope. The white circle of light
you see is the field of view. If your microscope has a mirror, move
the mirror until the field of view is evenly illuminated.
4. Place a prepared slide on the stage so that it is centered over the
stage opening. Use the stage clips to hold the slide in position. Turn
the low-power objective into place. Look at the microscope from the
side and turn the coarse adjustment so that the low-power objective
is as close as possible to the stage without touching it.
5. Look through the eyepiece and turn the coarse adjustment to move
the low-power objective away from the stage until the object comes into
focus. To avoid eyestrain, keep both eyes open while looking through
a microscope. CAUTION: To avoid moving the objective
into the slide, never lower the objective toward the stage while looking
through the eyepiece.
6. Turn the fine adjustment to bring the object into sharp focus. You
may wish to adjust the diaphragm so that you can see the object more
clearly. In the appropriate space below, draw what you see through the
microscope. Record the magnification.
7. Look at the microscope from the side and rotate the nosepiece until
the high-power objective clicks into position. Look through the eyepiece.
Turn the fine adjustment to bring the object on the slide into focus.
CAUTION: Never use the coarse adjustment when focusing
the high-power objective lens. This could break your slide or damage
the lens. In the appropriate space below, draw what you see through
the microscope. Record the magnification.
8. Remove the slide. Move the low-power
objective into position.
Part C. Preparing a Wet Mount
1. Use a pair of scissors to cut a letter “e” from a piece of newspaper.
Cut out the smallest letter “e” you can find. Position the “e” upright
on the center of a clean glass slide as shown in Figure 4A.
2. Use a dropper pipette to place one drop of water on the cut piece
of newspaper. See Figure 4B.
3. Hold a clean coverslip in your fingers as shown in Figure 4C. Make
sure the bottom edge of the coverslip is in the drop of water. Use a
dissecting probe to slowly lower the coverslip onto the wet newspaper.
Slowly lowering the coverslip prevents air bubbles from being trapped
between the slide and the coverslip. The type of slide you have just
made is called a wet mount. Practice making a wet mount until you can
do so without trapping air bubbles on the slide.
4. Center the wet mount on the stage with the letter “e” in its normal
upright position. Note: Make sure the bottom of
the slide is dry before you place it on the stage. Turn the low-power
objective into position and bring the “e” into focus. In the appropriate
place below, draw the letter “e” as seen through the microscope. Record
the magnification.
5. While looking through the eyepiece, move the slide to the left. Notice
the way the letter seems to move. Now move the slide to the right. Again
notice the way the letter seems to move. Move the slide up and down
and observe the direction the letter moves.
6. Turn the high-power objective into position and bring the letter
“e” into focus. In the appropriate place below, draw the letter “e”
as seen through the microscope. Record the magnification.
7. Take apart the wet mount. Clean the slide and coverslip with soap
and water. Carefully dry the slide and coverslip with paper towels and
return them to their boxes.
8. Rotate the low-power objective into position and use the coarse adjustment
to place it as close to the stage as possible without touching. Carefully
pick up the microscope and return it to its storage area.
Analysis and Conclusions
1. Inferring Why do you place one hand under the base
of the microscope as you carry it?
2. Observing How is the image of an object seen through
the high- power objective different from the image seen through the
low-power objective?
3. Observing How does the position of the letter “e”
as seen through the microscope differ from the way an “e” normally appears?
4. Inferring Explain why a specimen to be viewed under
the microscope must be thin.
5. Inferring Why should you never use coarse adjustment
when focusing the high-power objective lens?
6. Drawing Conclusions Suppose you were observing an
organism through the microscope and noticed that it moved toward the
bottom of the slide and then it moved to the right. What does this tell
you about the actual movement of the organism?
Going Further
View some common objects, such as thread or a small piece of a color
photograph from a magazine, under the low-power and high-power objectives
of the microscope. Make a drawing for each object. Describe the appearance
of the objects when viewed under a microscope.
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