Windows to a Microscopic World
Instructions
Activity 1. Learning to Use the Dissecting Microscope
All organisms, large and small, make valuable contributions to the functioning of the ecosystem. Many plants and
animals are large and easy to see, but many important organisms are too small to be seen without the assistance of
magnifying lenses.
Different types of microscopes can be used for different purposes. During this exercise, you’ll be learning to use a
dissecting microscope to examine larger objects and a compound microscope to view smaller specimens.
Materials
•
dissecting microscope
•
penny
Procedure
1.
Work in groups. Each group should get one of the dissecting microscopes from the back of the room. They
have green plastic covers over them. When carrying a microscope, always use two hands, as pictured below:
focusing knob
light control knob
stage
2.
Place the penny on the microscope stage. Locate the stage by referring to the labeled microscope. Turn the
penny so that the Lincoln Memorial is facing you. Turn the light control knob—the teardrop-shaped knob on
the base—one click. The penny should be illuminated from above (not beneath).
3.
Set the magnification control dial – the cylinder that hangs above the stage – to the lowest setting.
4.
Turn the focusing knob, located on the microscope arm, until the head is as close to the stage as possible. Look
through the ocular lenses (eyepiece) at the penny. Turn the focusing knob until the image of the penny is sharp
and clear (the head will be moving away from the stage).
5.
While looking through the eyepiece, gradually turn the magnification control dial. The change in image size
will resemble the zoom action of a camera.
Answer Question 1 on the Lab Report.
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Activity 2. Getting Familiar with the Compound Microscope
The compound microscope is the most commonly used type of microscope used in the biological sciences. It is called
compound because it uses two types of lenses to magnify the specimen. In this activity, you will take a tour of the
different parts of the standard compound microscope so that you can use it properly, as well as troubleshoot common
technical problems you may encounter.
Materials
•
compound microscope (one per student)
•
blunt dissecting probe
Procedure
1.
Work in groups. Get a compound microscope from the back of the room and a blunt probe.
2.
The power cord for your microscope is in the drawer next to your seat. Plug the microscope into an electrical
outlet and turn the light on. The light switch is located in the front of the microscope (facing the whiteboard
at the front of the room).
3.
The compound microscope consists of a system of optics (lenses and mirrors) and focusing controls. The base
and the arm support a body tube that houses the lenses that magnify the image.
4.
At the top of the microscope is the ocular lens (eyepiece). The ocular lens is only one of a series of lenses that
magnify the image. The ocular lens makes the image ten times larger than life size (abbreviated 10x).
5.
Look through the ocular lens. You should see a black pointer.
6.
At the bottom of the body tube is a revolving nosepiece. The lenses that screw into the nosepiece are called
objective lenses.
7.
Turn the nosepiece until you hear or feel one of the objective lenses quietly click into position. When an
objective lens clicks into position, it’s in the proper alignment for light to pass from the light source, through
the objective lens, through the ocular lens, and into the viewer’s eye. Turn the nosepiece again to bring a
different objective lens into position.
Answer Questions 2 & 3 on the Lab Report.
8.
Note that each objective lens is of a different length. Each of the lenses has a different magnifying power. The
shortest lens has the lowest magnifying power and the longest lens has the highest magnifying power.
Since light from the specimen pass through both an objective lens and the ocular lens, the total magnification
of the image is the result of the objective lens magnification multiplied by the ocular lens magnification:
Objective lens magnification X Ocular lens magnification = Total magnification of specimen
Using the information provided above, complete the magnification table (#4) on the Lab Report.
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9.
On both sides of the arm, you’ll see the focus knobs. The larger ring is the coarse focus knob and the smaller
ring is the fine focus knob.
Turn the microscope so that you’re looking at it from the side.
Position the blunt probe vertically at the side of the microscope so that the tip of the probe is approximately
level with the stage of the microscope.
Turn the coarse focus knob all the way in one direction and then reverse the process. Pay attention to what
happens to the position of the stage, relative to the tip of the probe.
10. Repeat the process with the probe, but this time, rapidly turn the fine focus knob all the way in one direction
and then reverse the process. Again, notice what happens to the position of the stage, relative to the tip of the
probe.
Answer Question 5 on the Lab Report.
11. The mechanical stage is a movable platform designed to hold a microscope slide. Notice that one side of the
metal stage clip makes a 90° angle. When you place a slide on the stage, make sure that one corner of the slide
fits exactly into that angle. If not, you won’t be able to move the slide or focus properly.
open the clip
micro
scope
slide
Two control knobs move the mechanical stage. One moves the stage left and right; the other moves the stage
forward and back. Always move the stage using the control knobs. Don’t attempt to move the slide using your
fingers.
12. In the center of the stage, you’ll see the glass of the condenser lens, which focuses light on the specimen.
13. Directly above the light source, you’ll see a lever that moves from left to right. This lever is connected to the
iris diaphragm. Looking at the condenser lens, practice moving the iris diaphragm lever all the way to the left
and all the way to the right.
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Answer Question 6 on the Lab Report.
Activity 3. Parts of the Compound Microscope
Check your comprehension of the compound microscope by identifying the parts that you have just worked with.
Materials
•
compound microscope station
Procedure
1.
With your group, visit the station of labeled microscopes and identify the labeled parts.
Answer Questions 7, 8 & 9 on the Lab Report.
Activity 4. Learning to Use the Compound Microscope
Now that you are familiar with the parts and operation of a basic compound microscope, it is time to use one to view
a specimen.
Materials
•
compound microscope (from Activity 2)
•
letter “e” slide
Procedure
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1.
Complete the following steps to view the letter “e” with the compound microscope.
2.
Make sure the microscope is plugged in and the light source is on. Open the iris diaphragm so that you can
see the light shining through the condenser lens.
3.
Using the coarse focus knob, lower the stage as low as it can go.
4.
Clean the slide with lens paper and carefully position the slide so that it fits precisely into the 90° angle of the
metal clip on the stage. Position the slide on the stage so that the letter “e” is facing you right side up, in its
normal reading position.
5.
Rotate the lowest power objective lens into position.
6.
Using the control knobs on the mechanical stage, position the slide so that light is shining on the letter “e.”
7.
While looking through the eyepiece, use the coarse focus knob to raise the stage slowly. Continue until the “e”
pops into focus.
8.
Once again, use the control knobs on the mechanical stage to center the letter “e” in your field of view.
9.
Using the fine focus knob, adjust the focus until the “e” is sharp and clear.
Answer Questions 10–17 on the Lab Report.
10. Follow these steps to view a specimen at a higher magnification:
•
Center the letter “e” in your field of view.
•
Rotate the 10x objective lens into position. DON’T adjust the stage at this point!
•
DON’T use the coarse focus knob! Using the fine focus knob, adjust the focus slowly until the letter “e”
is sharp and clear.
•
Repeat the above steps to change to other objective lenses.
11. Centering the specimen is absolutely necessary before you can change to a higher-power objective lens. Do
you know why? The reason is simple. The more powerful the magnifying lens, the smaller the area you see, but
you see that area in greater detail. The area you can see at one time is called the field of view. So you could say
that the higher the power of the objective, the smaller the field of view.
If the specimen you’re trying to view isn’t centered before you switch to a higher-power lens, it may no longer
be within the field of view, and you’ll think your specimen has disappeared!
12. Looking through the eyepiece, can you still see the letter “e”?
Is the letter “e” exactly in the center of the field of view?
If not, move the slide slightly to center the image.
13. Is the letter “e” sharp and clear?
If not, gradually adjust the fine focus knob until the problem is corrected.
14. Do you have enough light?
If not, gradually adjust the iris diaphragm.
Answer Question 18–22 on the Lab Report.
Activity 5. Preparing Temporary Slides – Wet Mounts
A wet mount is a method of preparing a slide that will only be used for a short time. Unlike the letter “e,” which was
permanently attached to the slide, a wet mount is made by placing the specimen into a drop of liquid on a slide. The
specimen and water droplet are held in place by a coverslip.
Materials
•
•
•
compound microscope (from Activity 2)
two clean microscope slides
two clean coverslips
•
•
•
iodine solution
toothpick
KimWipe or paper towel
Procedure
1.
Work in groups. Your group will make two different wet mounts of cheek cells. One will be made with a drop
of stain. The second will be made by substituting a drop of water for the stain.
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2.
Visit the wet mount station at the front lab bench.
3.
Get two clean microscope slides and lay them on the bench. Place a small drop of iodine near the center of
one slide and a small drop of water near the center of the other slide.
4.
Gently scrape the inside of your cheek with the end of a toothpick.
Caution! If you scrape too hard, you’ll be examining blood cells instead of epithelial cells!
5.
You’ve removed some of the cells that form a protective covering for the inside of your mouth. Like other
epithelial cells, these are constantly being worn off and replaced with new cells of the same type. Spread the
material from the toothpick into the drop of iodine or water. Add a coverslip.
coverslip
microscope slide
specimen
6.
Put two microscopes together (yours and a partner’s) on your lab bench so that you can view and compare the
stained and unstained slides side-by-side.
7.
View both slides. Begin with the 4x objective. Continue until you’ve located the cells using all three objective
lenses.
Hint: Unstained cells are clear. They’re only visible with very low light levels. If you think there are no cells on your
slide, adjust the iris diaphragm.
8.
You should be able to identify the following structures in your cells:
Structure
Function
cytoplasm
jellylike fluid that makes up the volume of the cell; site of numerous chemical reactions that
are important to life
cell membrane
serves as the boundary of the cell; allows materials into and out of the cell
nucleus
contains the cell’s DNA
nucleolus
darkly-staining particle inside nucleus; produces molecules needed for protein synthesis
Answer the remaining questions on the Lab Report.
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Windows to a Microscopic World
Lab Report
Name ________________________________________
Activity 1. Learning to Use the Dissecting Microscope
1.
Is Lincoln sitting in the Lincoln Memorial in your penny? ____________________
Activity 2. Getting Familiar with the Compound Microscope
2.
If you look through the eyepiece while rotating it, what happens to the pointer?
3.
How do you know when you’ve placed an objective lens in the proper position?
4.
Complete the magnification table below:
Length of
Objective Lens
Magnification of
Objective Lens
Short
4x
Medium
10x
Long
40x
Magnification of
Ocular Lens
Total Magnification
of Specimen
5.
How does the movement of the stage differ when using the coarse vs. fine focus knobs?
6.
What happens when you move the iris diaphragm lever…
…to the left?
__________________________________________________
…to the right?
__________________________________________________
Activity 3. Parts of the Compound Microscope
7.
Identify the labeled parts on the microscopes at the two stations.
A
F
B
G
C
H
D
I
E
J
8.
At what total magnification is Microscope #1 set? ______________________
9.
At what total magnification is Microscope #2 set? ______________________
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Activity 4. Learning to Use the Compound Microscope
10. If the letter “e” isn’t lighted brightly enough for you, what should you do?
11. If the light is too bright, causing glare and eyestrain, what should you do?
12. Draw the letter “e” exactly as it appears under low power.
13. What is the total magnification of the image of the letter “e”? ______________________
14. How is the orientation of the letter “e” as seen through the microscope different from the way an “e” normally
appears?
15. While looking through the eyepiece, move the stage to the left.
In what direction does the image appear to move?
16. While looking through the eyepiece, move the stage away from you.
In what direction does the image appear to move?
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17. If you wanted to center the letter “e” in this drawing, in which two directions would you have to move the
stage?
e
_____________________________ and _______________________________
18. Now that you are using the 10x objective lens, how has the image of the letter “e” changed from the way it
looked using the 4x objective lens?
19. When using the 40x objective lens, can you see the entire letter “e” through the eyepiece? __________
20. (Circle your answer.) Your field of view on high power is larger / smaller than the field of view on 10x.
21. How does the size of the field of view determine how much of the letter “e” you can see?
22. When using the high-power objective, what is the total magnification of the image of the “e”? ____________
Activity 5. Preparing Temporary Slides – Wet Mounts
23. Draw a picture of one cheek cell, viewed on high power. Make it large and clear.
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24. Label the following structures in your drawing of the cheek cell:
•
cytoplasm
•
nucleus
•
•
cell membrane
nucleolus
Thinking Critically about Today’s Lab
25. A student is looking at a specimen on a microscope and finds that only half of
the field of view is illuminated (see example on right). What is the probable
cause of this? (See if you can replicate this with your microscope. Hint: it has
to do with a problem with the light path.)
26. A student is viewing a very faintly-stained cheek cell slide under the microscope. What should she do to
improve the image? (Again, try to replicate this yourself with your microscope.)
27. Is there an advantage to using a stain to view cells microscopically? ________ If so, how is the stain helpful?
28. Why do we use coverslips when we make wet mounts? (What might happen if we didn’t use them?)
29. Extra credit: Give an example of a medical procedure in which cells are scraped from an area of the body and
examined microscopically.
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