Hi I’m Sara. In this video I will show you how to use a compound microscope for our lab
activity. We'll talk about proper handling of the microscope and how to use it to examine
opaque and wet specimens. You will need this skill to study living and nonliving things
and to explore some specimens in the biodiversity lab.
Let's get started by grabbing a microscope from the cabinet.
Grasp the arm of the microscope
with one hand and place your other hand under the base. Set it on the bare table with the
stage facing you. Unwind the cord and plug it in to the outlet at your table. If the
light does not work, check to be sure that the table is plugged into the wall outlet.
The let's review the parts of the microscope. The lenses at the top are the oculars, or
eyepieces. You will always use the slanted one when you are seated. The upright ocular
is useful when you are standing at a low table. The magnification of the ocular is printed
on the rim of the lens. There are three objective lenses attached
to the revolving nosepiece. As the nosepiece is rotated, you can hear the objectives click
into place. This is why the microscope is a compound microscope. The total magnification
is the product of the ocular and the objective lens that is in exact alignment. The magnification
of each objective is shown on the side of the lens. The longest one is the high power
objective. The shortest lens is the scan objective. The one of medium length is the low power
objective. The longer the lens the more powerful the magnification.
Clean the lenses with lens paper before you begin using the microscope. Never use kimwipes
to clean the microscope lenses because they will scratch them.
You will place specimen slides on the stage of the microscope. Although you have clips
on the stage, you do not need to use them, unless you will be moving the microscope with
the slide on the stage. The opening in the stage allows light to pass through your specimen.
If your slide is too dark and the specimen isn't visible, try adjusting the diaphragm.
The diaphragm controls the amount of light coming through the specimen. It is attached
to a condenser lens, which should be visible through the opening in the stage. You can
adjust the amount of light coming through the condenser lens by moving the diaphragm
lever. Use as little light as possible. Too much light can wash out or distort your specimen.
The right amount of light improves the clarity of the specimen.
Now we will make a wet mount, or temporary specimen of a magazine punch. Grab a clean
plain glass slide from the blue box in your tray. Place a magazine punch on the slide
with the colored side up. To prepare a wet mount, the specimen must be placed in some
type of liquid, usually water. Place one drop of water on top of the magazine punch, using
the dropping bottle provided. Obtain a clear plastic coverslip from your tray. Handle the
coverslip by the edges to avoid fingerprints. Place one edge of the coverslip in the drop
of water and lower the coverslip on the slide at an angle to prevent air bubbles.
To focus on the wet mount, place the scan objective (4x) in exact alignment with the
ocular lens. Place the slide on the microscope and put the magazine punch over the center
of the stage opening. As you look at the microscope from the side, turn the coarse adjustment
knob to raise the stage until it stops. This makes the working distance, which is the distance
between the objective and the slide, as small as possible. A built-in stop will prevent
the scan and low power objectives from breaking the slide.
Now, while looking through the ocular, and try this with both eyes open to avoid fatigue,
lower the stage with the coarse adjustment until the specimen comes into view. Switch
to the low power objective and use the coarse adjustment to bring the specimen into clear
focus. Sharpen the image with the fine adjustment. You will notice the magazine punch looks quite
different under the microscope, because of the microscope’s ability to clearly separate
details. This is called resolving power, or resolution. If you see some unrecognizable
or odd-looking object, you may be viewing an artifact. No matter how hard you try, artifacts
are a fact of life with wet mounts. Artifacts include air bubbles, which have dark and regular
borders, dust particles, trash, fingerprints, and scratches. Learn to recognize these as
you work with the microscope so that you will not mistake them for your specimen.
Now we’re going to look at a transparent specimen. Obtain a drop of pond water from
the bottom of the culture dish where there is a concentration of organisms. Place the
drop in the center of a clean glass slide. Lower a coverslip over the drop of water at
an angle. Place the slide on the stage of the microscope and focus it using scan power,
with the diaphragm closed to its smallest diameter. Scan the slide by grasping each
side of the slide with your thumb and index finger. Move the slide slowly up and down
and from side to side while observing the sample through the ocular. This is called
scanning, and it allows you to rapidly explore the entire area under the coverslip. Once
you have located a live organism, try to center it and then switch to low power. If the organism
is small enough, you may also wish to switch to high power after focusing under low power.
Let’s talk about using the high power objective or the most powerful lens on the microscope.
Remember the specimen must be clearly focused under low power before you switch to high
power because the microscope is parfocal. Otherwise, the high power objective may be
damaged as it comes into contact with the slide. Now sharpen the image with the fine
adjustment. When using the high power objective, you cannot use the coarse focus knob. If you
cannot see your specimen, go back and focus it under low power. Once you switch to high
power, you may have to adjust the light with the diaphragm lever, since the high power
objective lens has a smaller opening.
You may need to focus continuously with the fine adjustment because live organisms tend
to move not only from side to side but also from top to bottom, which is the vertical
distance an organism can move up and down under a coverslip.
It isn’t possible to use the high power objective lens in this example because our
specimen is opaque. However, it will be appropriate to use high power for viewing some specimens
in the future.
When you have finished with your wet mounts, wipe them off with a Kimwipe and discard the
Kimwipe and the coverslip in the trash. Place the clean slide back in your blue box. When
you are finished with your microscope, turn the nosepiece so the high power objective
is not in alignment, wind the cord around the brackets on the back of the arm, place
the plastic cover on the microscope, and put it in the cabinet with the arm facing out.
OK, so now you're ready to use the microscopes in your lab activity. See you in the lab!