INVESTIGATION A2.2
The human eye cannot distinguish objects much smaller than
0.1 mm in size. The microscope is a biological tool that
extends vision and allows observation of much smaller
objects. The most commonly used compound microscope is
monocular (one eyepiece). Light reaches the eye after being
passed through the objects to be examined. In this
investigation, you will learn how to use and care for a
microscope.
Procedure
PART A: Care of the Microscope
1. The microscope is a precision instrument that requires
proper care. Always carry the microscope with both
hands, one hand under its base. The other on its arm.
2. When setting the microscope on a table, keep it away
from the edge. Keep everything not needed for microscope studies off your lab table.
Never plug it into an outlet that is not next to your table.
3. Avoid tilting the microscope when you are using it.
4. The lenses of the microscope cost as much as all the other parts put together. Never
clean lenses with anything other than the lens paper or microfiber cloths designed for this
task. Ask your teacher if you need some.
5. Before putting the microscope away, always return it to the low-power setting. The
high-power objective reaches too near the stage to be left in place safely. Turn it off and
then unplug it. If any liquid has spilled onto the stage OR gotten on your lens, make sure
to clean it with the appropriate cloth from your teacher. Wrap the cord around the cord
holder very loosely so it won’t start to fray. If there are any slides that you prepared clean
them at the sink and let them air dry in your lab tray. Always carry the microscope with
two hands and put it back in the cupboard in the spot for your group.
PART B: Setting Up the Microscope
6. Rotate the low-power objective into place if it is not already there. When you change from
one objective to another you will be able to feel the objective click into position.
7. Adjust the amount of light entering your microscope using a diaphragm. Look at the
numbers on the diaphragm and notice the size of the holes under the stage. Some
materials are best viewed in dim light others in bright light.
8. Make sure the lenses are dry and free of fingerprints and debris. Wipe lenses with lens
paper or special microfiber cloths only.
PART C: Using the Microscope
9. Cut a lowercase letter o from a piece of newspaper. Place it right side up on a clean
slide. With a dropping pipet, place one drop of water on the letter. This type of slide is
called a wet mount.
10. Wait until the paper is soaked before adding a
cover slip. Hold the cover slip at about a 450
angle to the slide and then slowly lower it. This
avoids air bubbles on your slide.
11. Place the slide on the microscope stage and
clamp it down. Move the slide so the letter is in
the middle of the hole in the stage – you should
still be on low power. Use the coarse-adjustment
knob to lower the low-power objective to the lowest position.
12. Look through the eyepiece and use the coarse-adjustment knob to raise the objective
slowly until the letter o is in view. Use the fine-adjustment knob to sharpen the focus.
Position the diaphragm for the best light. Compare the way the letter looks though the
microscope with the way it looks to the naked eye. Record this on your worksheet.
13. Notice the numbers etched on the objectives and on the lower eyepiece. Each number is
followed by an “x” that means “times”. For example, the low-power objective may have
the number “10x” on its side. The low power objective magnifies an object 10 times its
normal size. The total magnification of a microscope is calculated by multiplying the
magnification of the objective by the magnification of the eyepiece. For example:
eyepiece magnification (10X) x objective magnification (10X) = total magnification (100X).
14. Follow the same procedure with a lowercase c. On your worksheet, describe how the
letter appears when viewed through a microscope. Does it still look like a normal c?
15. Make a wet mount of the letter e or the letter r. describes how the letter appears when
viewed though the microscope. What new information (not reveled by the letter c) is
revealed by the e or r?
16. Look through the eyepiece at the letter as you use your thumbs and forefingers to move
the slide slowly away from you. Which way does your view of the letter move? Move the
slide to the right. Which way does the image move?
17. On your worksheet, make a pencil sketch of the letter as you see it under the
microscope. Label the changes in image and movement that occur under the
microscope.
18. Make a wet mount of two different-colored hairs, one light and one dark. Cross one hair
over the other. Position the slide so the hairs cross in the center of the field. Sketch the
hairs under low power; then go to Part D.
Part D using high power
19. With the crossed hairs centered under low power, adjust the diaphragm for the best light.
20. Turn the high-power objective into viewing position, do not change the focus.
21. Sharpen the focus with the fine-adjustments knob only. Do not focus under high power
with the coarse-adjustment knob.
22. Readjust the diaphragm to get the best light. If you are not successful in finding the object
under high power the first time, return to step 20 and repeat the whole procedure
carefully.
23. Using the fine-adjustment knob focus on the hairs is the point where they cross. Can you
see both hairs sharply at the same focus level? How can you use the fine adjustment
knob to determine which hair is crossed over the other? Sketch the hairs under high
power.
Part E Measuring with a Microscope
24. Measure the diameter of the field of view. To do this, make sure the low-power objective
is in place once again. Put a clear plastic ruler on the microscope stage. Using low
power, focus on the millimeter marks of the ruler. Move the ruler so that one of the
millimeter marks is at the left edge of the field of view, as shown below. Knowing the
diameter of the field of view can help you estimate the actual size of objects seen through
the microscope.
25. Count the number of whole millimeters to the right edge of the field and estimate the
fractions to determine the diameter of the field of view. Record the estimated diameter of
the low-power field of view in millimeters below then repeat with medium and high power.
Record your answers in the data table on your worksheet.
26. Since microscopic dimensions are very small, they are usually measured in micrometers
(µm) rather than millimeters. There are 1,000 micrometers in a millimeter or a micrometer
is 1/1000 of a mm. Find the diameter of the objective lens in (µm) by using the following
formula.
a. # of millimeters (mm) X 1000 = __________ micrometers (µm)
27. Now we can estimate the sizes of objects by comparing them with the diameter of the
field of vision. For example, a tiny shrimp takes up approximately one-half the field of
view under high-power. To find its size in micrometers, use the following formula:
b. (Proportion of field of view) X (diameter of power objective) = size of object.
28. Calculate the area of the fields of view using the formula:
c. A = π r2
d. A = area and π = 3.14 and r = radius (one half the diameter of the field of view)