Microscope Lab
Background:
In almost every type of biological research, microscopes play a fundamental role. Scientists rely on it
to study the fine structures of cells and tissues – things too small to be seen with the unaided eye.
Making detailed observations and measurements is an important part of the scientific process.
Compound light microscopes are very useful in the lab and in the field for viewing a wide range of
living and non-living samples. Some modern microscopes feature fluorescent illumination and a
calibrated pointer that makes measuring easier. Because direct measurement of microscopic organisms
and objects is not practical, it is necessary to estimate their sizes. This can be done by comparing the
size of an object as it appears under the microscope with the size of the field of view.
Purpose:
To determine how to use a microscope properly
To learn how to make a wet mount slide
To determine the size of the field of view of a microscope
Material:
* Microscope
* Coverslip
* Eye dropper
* Small piece of newspaper
* Clear Plastic Ruler
* Microscope slide
* Water
* Scissors
* Forceps (tweezers)
Procedure:
Part I – Making a Wet Mount Slide
1) Using a pair of scissors, cut a small, lowercase letter “e” from the newspaper print.
2) Place the letter “e” upright on the microscope slide. Using the eye dropper, place enough drops of
water to fully cover the letter “e”. (Watch to not apply too much water…2-3 drops is usually enough)
3) Holding the coverslip by the edges at a 45o angle to the microscope slide, lower the coverslip slowly
to avoid trapping air bubbles.
4) Place the slide on the stage of the microscope. Move the slide so the letter “e” is directly over the
hole in the stage.
4) View the letter “e” under the low power objective. Turn the coarse adjustment knob so that the low
power objective is near the coverslip.
5) Use the fine adjustment knob to bring the letter “e” into sharp focus. CAUTION: NEVER LOWER
THE BODY TUBE WHILE LOOKING THROUGH THE EYEPIECE. THIS CAN BREAK THE
LENSE.
6) On a separate sheet of paper, make a drawing of the letter “e”. Record the total magnification under
the drawing.
To calculate the total magnification, multiply the number on the eyepiece by the number
on the objective lense.
example: If the eyepiece magnification is 10x and the high power objective
magnification is 40 x, then the total magnification is 400x (10x X 40 x = 400x).
7) Recognize spatial relationships: Move the slide to the left, to the right, toward you, and away
from you. Note the direction in which the letter “e” appears to move.
8) Draw the letter “e” as it appears under medium and high power. Record the total magnification
under the drawing.
Part II – Measuring the Field of View
9) Measure the diameter of the field of view. Be sure the low-power objective is in place. 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.
Field of view with a
diameter of 3.7 mm
10) Count the number of whole millimeters to the right edge of the field and estimate any fractions to
determine the diameter of the field of view. Record the estimated diameter of the low-power field of
the view in millimeters in Table 1. Express a fraction of a millimeter in decimal form.
Table 1
Objectives
Diameter of the Field of View
Millimeters (mm)
Area of the Field of View
in square mm (mm2)
Micrometers (μm)
Low-power
Medium-power
High-power (calculated)
11) Move the medium-power objective into place and repeat this procedure for the medium-power
field of view. Record the estimated diameter of the medium-power objective in millimeters in Table 1.
12) 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 1/1,000 of a
mm. Find the diameter of the objective lens in μm by using the following formula.
# of millimeters (mm) X 1,000 = ________ micrometers (μm)
Record the estimated diameter of each field of view in micrometers in Table 1.
13) Calculate the diameter of the high-power field of view using the diameter of the low-power field of
view. The calculated value is more accurate than the estimated value and should be used whenever the
diameter of the high-power field of view is required. Calculate the diameter of the high-power field of
view in millimeters and micrometers with the following formula:
Diameter of high-power field of view =
Magnification of low-power objective X Diameter of low-power FOV
Magnification of high-power objective
For example, a microscope has a low-power objective with a magnification of 10X and a high-power
objective with a magnification of 40X. If the low-power field of view (FOV) is 3 mm wide, the
diameter of the high-power field of view is (10 X 3 mm)/40 = 0.75 mm.
14) Record the calculated diameter of the high-power field of view in millimeters and
micrometers in Table 1.
15) Now that you know the diameters of the fields of vision in micrometers, you can estimate the sizes
of objects under the microscope by comparing them with the diameter of the field of vision. For
example, assume a field of vision diameter of 350 μm under high-power. A tiny shrimp takes up
approximately one-half the field of vision under high-power. To find its size in micrometers, use the
following formula:
proportion of field of view X diameter of power objective = size of object
16) Calculate the area of the low-power, medium-power, and high-power fields of view. To determine
the area of the field of view, use the formula:
A = pi ( r2 )
where A is the area, the value of pi is 3.14, and r is the radius, or one-half the diameter of the field of
view. For example, a field of view with a diameter of 4 mm would have a radius of 2 mm and an area
of 3.14 X (2 mm)2 , or 12.56 mm2. Calculate the area for the low-power, medium-power, and highpower fields of view of your microscope, and record the area of each field of view in Table 1.
Part III - Extension
17) View other objects, such as hair or thread, under the microscope. Make a drawing of what you
see. Record the total magnification under the drawing.
Discussion:
1) When you move the slide toward you, how does the letter “e” appear to move under the
microscope?
2) Describe the appearance of the letter “e” under high power as compared to low power.
3) What is the purpose of the coverslip?
4) Why should the coverslip be held by the edges?
5) How do you calculate the total magnification by which an object was viewed under?
6) Why must a specimen be very thin to be viewed under the microscope?
7) Which objective lens provides the larger field of view?
8) Why are micrometers, rather than millimeters, used for microscopic measurements?
9) What is the importance of knowing the diameter and area of the field of view?
10) Calculate the estimated size of the tiny shrimp in Step 15 of the lab procedure. Show your work.
Name ___________________________ Period ____________
Date ___________________
Drawings of the Letter “e”
________________________
total magnification
_____________________
total magnification
__________________
total magnification
Table 1
Objectives
Diameter of the Field of View
Millimeters (mm)
Area of the Field of
View in square mm
(mm2)
Micrometers (μm)
Low-power
Medium-power
High-power
(calculated)
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Extension
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object being viewed
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object being viewed
____________________________
total magnification
__________________________
total magnification