Microscope and Cell Observation Lab
PURPOSE: To learn how to use the microscope, and to observe some cells.
You will need to record data and answer questions from each part of this lab. Blank paper will be available if you prefer
it for drawings. All drawings must be labeled with what it is and with the magnification used.
INTRODUCTION:
Because biological objects can be very small, a microscope is often used to view them. We have two kinds of
light microscopes available in the lab. Light microscopes use light rays that are magnified and focused by
means of lenses. The binocular dissecting microscope is designed to study entire objects in three
dimensions at low magnification. The compound light microscope is used for examining small or thinly
sliced sections of objects under magnification that is higher than that of the dissecting light microscope.
Illumination is from below, and the light passes through clear sections but does not pass through opaque
sections. To improve contrast, the microscopist uses stains or dyes that bind to cellular structures and absorb
light.
THE COMPOUND LIGHT MICROSCOPE
Always observe the following rules for microscope use:
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The lowest power objective must be in position at the beginning and end of microscope use.
Use only lens paper for cleaning lenses.
Do not tilt the microscope when viewing a wet mount.
Keep the stage clean and dry to prevent rust and corrosion.
Do not remove parts of the microscope.
Keep the microscope dust-free by covering it after use.
Report any malfunctioning.
Identification of Parts
Your teacher will explain how to set up and move a microscope. Identify the following parts on your
microscope, and record the answers to the questions in Table 1 below.
1. Eyepiece (ocular lens): Topmost series of lenses through which
an object is viewed. What is the magnifying power of the ocular
lens on your microscope?
2. Body tube: holds nosepiece at one end and eyepiece at the
other end; conducts light rays.
3. Arm: Supports upper parts and provides carrying handle.
4. Objectives (objective lenses):
a. Scanning power objective: Holds scanning lens used
to view the whole slide. What is its magnifying power?
b. Low-power objective: Holds low power lens used to
view the object in greater detail. What is the magnifying
power of this lens on your microscope?
c. High-power objective: Holds high power lens used to
view the object in even greater detail. What is the
magnifying power of this lens on your microscope?
d. Oil immersion objective: This lens will not be used in
this course.
5. Coarse-adjustment knob: Knob used to bring object into approximate focus; used only with lowpower objective.
6. Fine-adjustment knob: Knob used to bring object into final focus.
7. Diaphragm or diaphragm control lever: Controls amount of illumination used to view the object.
8. Light source: An attached lamp that directs a beam of light up through the object.
9. Base: The flat surface of the microscope that rests on the table.
10. Stage: Holds and supports microscope slides.
11. Mechanical stage: A moveable stage that aids in the accurate positioning of the slide. Does your
microscope have a mechanical stage?
12. Mechanical stage control knobs: Two knobs that are usually located below the stage. One knob
controls forward/reverse movement, and the other controls right/left movement.
Total Magnification
Total magnification is calculated by multiplying the magnification of the ocular lens (eyepiece) by the
magnification of the objective lens.
Table 1: Magnification
Eyepiece magnifying power:
Scanning objective power:
Low-power objective power:
High-power objective power:
Oil immersion objective power:
Total magnification:
Total magnification:
Total magnification:
Total magnification:
Focusing the Microscope
Lowest Power
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Turn the nosepiece so that the lowest power lens is in straight alignment over the stage.
Always begin focusing with the lowest power objective lens.
With the coarse-adjustment knob, lower the stage until it stops.
Place a slide of the letter e on the stage, and stabilize it with the clips (instructor will demonstrate if
necessary).
Again, be sure that the lowest power objective is in place. Then, as you look from the side, decrease the
distance between the stage and the tip of the objective lens until the lens comes to an automatic stop or
is no closer than 3 mm above the slide.
While looking into the eyepiece, rotate the diaphragm (or diaphragm lever) to give the maximum
amount of light.
Slowly increase the distance between the stage and the objective lens, using the coarse-adjustment
knob, until the object - in this case the letter e - comes into view, or focus.
Once the object is seen, you may need to adjust the amount of light. To increase or decrease the
contrast, rotate the diaphragm slightly.
Use the fine-adjustment knob to sharpen the focus if necessary.
Practice having both eyes open when looking through the eyepieces, as it greatly reduces eyestrain.
Exercise 1
1. Draw the letter e as it appears on the slide (look from the side, not through the eyepieces).
2. Draw the letter as it appears when you look through the eyepieces.
3. Move the slide to the right. Which way does the image appear to move?
This phenomenon is known as inversion.
Higher Powers
Compound microscopes are parfocal; that is, once the object is in focus with lowest power, it should
also be almost in focus with the higher power.
1. Bring the object into focus under the lowest power by following the instructions in the previous
section.
2. Make sure that the letter e is centered in the field of the lowest objective.
3. Move to the next higher objective (low power, 10x) by turning the nosepiece until you hear or
feel it click into place. Do not change the focus; parfocal microscope objectives will not hit
normal slides when changing the focus if the lowest objective is intially in focus.
4. If any adjustment is needed use only the fine-adjustment knob. Always use only the fineadjustment knob with higher powers.
On your drawing of the letter e that you made for exercise 1, draw a circle around the portion of
the letter that you are now seeing with the higher-power magnification.
5. Repeat steps 2-4 with the 40x power objective lens.
6. When you have finished your observations of this slide (or any slide), rotate the nosepiece until
the lowest power objective clicks into place, and then remove the slide.
Diameter of Field
The diameter of the field (the circle visible through the lens) is the length of the field across the center.
Exercise 2:
LOW POWER (10x)
1. Place a clear plastic ruler across the stage so that the edge of the ruler is visible as a horizontal lilne
along the diameter of the low-power (not scanning) field. Be sure that you are looking at the millimeter
side of the ruler. You will use the ruler to measure the diameter of the field of view under various
magnifications.
2. Estimate and record the number of millimeters (mm), to tenths, that you see along the field: (Hint:
Start with one of the millimeter markers at the edge of the field.)
3. Convert the figure to micrometers (mm) and record. This is your low-power diameter of field (LPD). To
convert from millimeters to micrometers, multiply the millimeter value by 1,000.
HIGH POWER (40x)
1. Record the following data for your microscope:
1. LPD=low-power diameter of field (in micrometers).
2. LPM=low-power total magnification (not scanning).
3. HPM=high-power total magnification.
2. Compute the high-power diameter of field (HPD) by substituting the preceeding data into the following
formula: HPD = LPD x (LPM / HPM)
To locate small objects that are seen on low power, place these objects in the center of the field before rotating
to high power.
LPD:
LPM:
HPM:
HPD:
Questions:
1. Does low power or high power have a larger field of view and allow you to see more of the object?
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2. Which has a smaller field but magnifies to a greater extent?
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Microscopic Observations
When a specimen must be prepared for observation, the object should always be viewed as a wet mount. A
wet mount is prepared by placing a drop of liquid on a slide or, if the material is dry, by placing it directly on
the slide and adding a drop of water or stain. The mount is then covered with a coverslip. The instructor will
demonstrate how to make a wet mount.
Exercise 3:
HUMAN EPITHELIAL CELLS
Epithelial cells cover the body's surface and line its cavities.
1. Wash the underside of a wrist that will be sampled for epidermal cells with soap and water.
2. Stick a clean piece of clear tape on the underside of the washed wrist.
3. Gently remove the piece of tape from the wrist being careful to avoid getting fingerprints on the tape. A
forceps might help to remove the tape and avoid fingerprinting the tape.
4. Place the tape, sticky-side up, on a clean microscope slide.
5. Stain the top, sticky side of the tape with 2 or 3 drops of 1% methylene blue solution.
6. Use a dissecting needle to gently place a cover slip over the sticky tape. Lower the coverslip down onto the
tape and then remove the dissecting needle. This should help prevent staining your fingers. Caution: Use
methylene blue carefully. It will stain most items including skin, clothing, and table tops.
7. Examine the slide under a microscope. Look for cells with low power first, and then switch to high power for
details.
8. Record your observations of epidermal cells by making drawings. Label your drawings with appropriate
magnifications. Use your knowledge of the size of the microscopic field to estimate the size of the cells.
Exercise 4:
PLANT CELLS
1. With a scalpel or your fingers, strip a thin, transparent layer of cells from a piece of onion.
2. Place it gently on a clean, dry slide.
3. Add a drop of methylene blue stain and cover with a coverslip.
4. Observe under the microscope and draw what you see. Be sure to label your drawing.
5. Locate the cell wall. Is a nucleus visible?
6. Count and record the number of cells across the diameter of the high power field, both lengthwise and
side to side.
7. Using the number you calculated in exercise 3, calculate and record the length and width of an onion
cell in micrometers.
8. Record some obvious differences between the human cheek cells and the onion cells.
9. Record any similarities between the two types of cells.
10. If there is time, obtain another plant sample, make a slide (without methylene blue, use water) and
observe under the microscope.
11. Draw what you see. Be sure to label your drawing.
12. Record some differences and similarities between the different plant cells you observed.
Exercise 5:
PROTISTS
1. Obtain a sample of a protist from one of the containers at the front of the room. The instructor will help
you obtain this sample.
2. Place a drop containing one or more protists on a slide and cover with a cover slip.
3. Scan the slide for organisms: Start at the upper left-hand corner, and move the slide forward and back
as you work across the slide from left to right.
4. Experiment by using all available lenses (NOT the oil immersion!), by focusing up and down with the
fine-adjustment knob, and by adjusting the light so that it is not too bright.
5. Draw any organisms you see and label your drawings.
Conclusion Questions:
1. Why is it helpful for a microscope to be parfocal?
2. What is inversion?
3. Why is locating an object more difficult if you start with the higher power objectives than with the
scanning objective?