Using a Compound Light Microscope
INTRODUCTION
Many organisms are too small to be seen by the eye alone. They can be seen, however, with a microscope that magnifies, or visually
enlarges, the organism. The microscope is a precision instrument that requires special care and handling. In this lab activity you
will be learning how to use a microscope.
OBJECTIVES
1. What are the parts of a compound light microscope?
2. How do you focus a compound light microscope?
3. How do you calculate the magnification of a compound light microscope?
4. How do you make a make a slide on a compound light microscope?
5. How do you measure the diameter of field of view?
6. How do you calculate the size of an organism on a compound light microscope?
EQUIPMENT
microscope, calculator, Q-tip, lens cleaner, prepared slide, newspaper, scissors, glass slide, cover slip, eye dropper
plastic ruler, toothpick, methylene blue stain
PROCEDURE
Part A: CARE
1.
2.
3.
4.
5.
The illustration at right shows the proper way to carry a microscope.
Always carry the microscope with two hands.
Grasp the arm of the microscope with one hand and put your other hand under the base of the microscope.
Position the microscope with the arm facing you.
Complete question 1.
Part B: PARTS
1. Compare the labeled diagram on the next page with your microscope.
2. Complete question 2.
Part C: MAGNIFICATION
1.
2.
3.
4.
Find the magnification numbers on the objective lenses and the eyepiece.
There are several numbers on each objective lens. The magnification number is printed in larger size on the lens.
Magnification of the objective lens x magnification of the eyepiece = total magnification
Complete question 3.
Part D: CLEANING
1.
2.
3.
4.
Spray a little lens cleaner on the end of a Q-tip.
Use the Q-tip to gently clean each objective lens.
NOTE: go gently with the Q-tip and avoid pushing overly hard on the lens
Complete question 4.
Part E: FOCUSING
1. Practice moving the coarse adjustment knob. Complete question 5a.
2. Look at the microscope from the side and rotate the low-power objective lens into place. Complete question 5b.
3. Practice rotating the diaphragm. Complete question 5c.
4. Place a prepared slide on the stage: center it over the opening and use the stage clips to hold it in place.
5. Look at the microscope from the side: use the coarse adjustment knob to move the low-power objective lens as close to the
stage as possible without actually touching the stage.
6. Turn on the light and look through the eyepiece: use the coarse adjustment knob to move the low-power objective lens away
from the stage until the prepared slide comes into focus. Complete question 5d.
7. Make a detailed drawing, in pencil, of your prepared slide under the low-power objective lens. Complete question 5e.
8. Look at the microscope from the side and rotate the medium-power objective lens into place.
9. Use the fine adjustment knob to place the prepared slide in sharp focus.
10. Look at the microscope from the side and rotate the high-power objective lens into place.
11. Use the fine adjustment knob to place the prepared slide in sharp focus.
12. Make a detailed drawing, in pencil, of your prepared slide under the high-power objective lens. Complete question 5f.
Part F: WET MOUNT
1. Cut a lower case letter “e” from the newspaper. Cut out the smallest “e” you can find.
2. Position the “e” on the center of a glass slide.
3. Use a medicine dropper to place one drop of water on the “e”.
4. Hold a cover slip in your fingers: place one edge of the cover slip in the drop of water.
5. Slowly lower the cover slip onto the wet “e”.
6. NOTE: lowering the cover slip slowly prevents the formation of air bubbles.
7. Following the directions from Part E, focus the “e” under the low-power objective lens.
8. While looking through the eyepiece, move the slide to the left . . . . and then to the right. Complete question 6a.
9. While looking through the eyepiece, move the slide up . . . and then down. Complete question 6b.
10. Make a detailed drawing, in pencil, of your prepared slide under the low-power objective lens. Complete question 6c.
Part G: FIELD OF VIEW
1. Adjust the microscope for viewing under the low-power objective lens.
2. Place a plastic ruler on the microscope stage: position the edge of the ruler over the center of the hole in the stage.
3. Move the ruler so that a scale line is just visible on the left.
4. Determine the diameter of the field of view by counting the number of millimeters across the lighted, circular area.
5. Complete question 7a.
scale line is just visible on the left
center of the hole in the stage
6. Organisms are often so tiny that we must use a tiny unit called a micron or micrometer (µm). There are 1000 µm
in every millimeter. Convert your field of view in mm to µm. Complete question 7b.
7. The diameter of the field of view under the high-power objective lens cannot be calculated using a plastic ruler.
We must instead use a formula to determine the field of view under the high-power objective lens.
Look over the formula below and complete question 7c.
magnification of low-power objective lens x low-power field of view in mm = high-power field of view in mm
magnification of high-power objective lens
8. Convert the high-power field of view in mm into µm: complete question 7d.
Part G: PRACTICE
1. Place a drop of water on a microscope slide.
2. Use the blunt end of a toothpick to gently scrape the inner lining of your cheek.
NOTE: Go gently. Only a few cells are needed. You may see nothing on the end of the toothpick but saliva.
3. Swish the end of the toothpick covered with cheek cells in the drop of water on the microscope slide.
4. Throw the used toothpick in the garbage!
5. Place a cover slip over the water drop as you learned in Part F.
6. Stain the cheek cells: place one drop of methylene blue next to the cover slip.
7. Draw the stain under the cover slip: hold a small piece of paper toweling at the opposite end of the cover slip.
8. Make a detailed drawing, in pencil, of your cheek cell under the high-power objective lens.
Label the cell membrane, cytoplasm, and nucleus. Complete question 8a.
9. Estimate the size of the cheek cell by using the formula below. Complete question 8b.
diameter of the high-power field of view (in µm)
the number of cheek cells that would fit across the diameter of the field of view
QUESTIONS
1. Explain how to carry a microscope.
________________________________________________________________________________________________________
2. List the parts and functions of the microscope.
part
function
A
B
C
D
E
F
G
H
I
J
K
3. Calculate the total magnification of each objective lens.
eyepiece magnification objective lens magnification total magnification
low power
medium power
high power
4. Why do you suppose that you should not use your finger to clean an objective lens?
________________________________________________________________________________________________________
5. Focusing
a. How does the function of the coarse adjustment knob differ from the function of the fine adjustment knob?
__________________________________________________________________________________________________
b. Why is it a wise to look at the microscope from the side when rotating the nosepiece?
__________________________________________________________________________________________________
c. When might you need to adjust the diaphragm to let in more light? ____________________________________________
d. Why is it wise to move the low-power objective lens away from the prepared slide when focusing?
__________________________________________________________________________________________________
e. Why is it wise to draw in pencil? _______________________________________________________________________
f. What is the advantage of looking at the prepared slide under the high-power objective lens? _______________________
_____________________________________________________________________________ And the disadvantage?
__________________________________________________________________________________________________
low-power objective lens
high-power objective lens
6. Creating a wet mount
a. When you move the slide to the left, the “e” appears to move
And what happens when you move the slide to the right?
_____________
________________
b. When you move the slide up, the “e” appears to move ___________________
And what happens when you move the slide down? ______________________
c. The “e” definitely does not appear in normal position under the microscope!
List two ways in which the position of the “e” has changed.
________________________ and ________________________
7. Field of view
low-power objective lens
a. Low-power diameter of the field of view in mm = ______
b. Low-power diameter of the field of view in µm = ______
c. High-power diameter of the field of view in mm = ______
x
Show your substitutions below.
= ______
d. High-power diameter of the field of view in µm = ______
8a. Draw and label one cheek cell under high-power.
8b.
high-power diameter of the field of view in µm
number of cheek cells that fit across the diameter
estimated cheek cell size in µm = _________
=