Lab # ________: The Microscope
Date ____________
Introduction
The microscope, developed more than three centuries ago, is a basic tool of biologists. A microscope lets
us investigate living things and objects that are too small to be seen with the unaided eye. The
microscope magnifies objects (makes them appear larger). It also increases resolution. Resolution is the
ability of an optical instrument to show two close objects as separate. The type of microscope you will be
using is the light microscope. Some light microscopes can magnify objects up to 1000 times. Light
microscopes are good for studying living cells since they do not require complex preparation techniques
to prepare samples for observation and can therefore be used to observe cells that are still alive. They do
require a sample that is thin enough for light to pass through. We will usually use a wet mount slide for
this purpose.
Procedure
Part A: Observing a Wet Mount Slide
1. Remove your microscope from your cabinet and place it gently on the lab table about 10 cm from the
edge. Remember to always use two hands to carry your microscope (one on the arm and the other
under the base).
2. Prepare a wet mount of a small lower case letter e as demonstrated in class and on the web exercise.
You want to find the smallest e you can so that you can see the entire thing on low power.
3. Make sure the low power objective is in position over the opening of the stage.
4. Place the wet mount slide on the stage of the microscope so that the letter e looks right side up to you
with your naked eye.
5. Adjust the slide so that the letter is above the opening of the stage. Use the stage clips to hold the
slide in place.
6. Using the method demonstrated in class, use first the course and then the fine adjustment to focus on
the letter e.
7. Use the circle on your data sheet to sketch the letter e as it appears on low power.
8. The total magnification is equal to the product of the eyepiece magnification and the objective
magnification (eyepiece X objective). Your eyepiece gives 10X magnification. Find the number
engraved on the objective lens. Use these numbers to calculate the total magnification and write it in
the blank under your sketch. You should always label a sketch and include the total magnification.
9. While you are looking through the eyepiece, move the slide slowly in the different directions listed on
the data sheet. For each direction, record the direction in which the e appears to move.
10. Without touching the course or fine adjustment knobs, turn the nosepiece until the high power
objective clicks into place. Use only the fine adjustment knob to bring the specimen into focus.
11. Make a sketch of the e on high power and label it with the total magnification.
Part B: Measuring an Object Under the Microscope
1. Turn the nosepiece until the low power objective clicks into place.
2. Place the millimeter scale of a transparent plastic ruler over the center of the stage opening of the
microscope.
3. Focus first with the course adjustment and then with the fine adjustment until the image is clear.
4. The space between the red lines you see is 1 millimeter.
5. While looking through the eyepiece, move the ruler so that one of the millimeter lines is seen just at
one end of the field of view as shown in the diagram at the top of the next page.
Field of view
Edge of ruler
Millimeter lines
6. Remembering that the space between lines is one millimeter, estimate the diameter of the field of view
to the nearest tenth of a millimeter and record your estimate on your data sheet.
7. Since we are often studying things that are very small with the microscope, we often use a smaller unit
called a micron for estimating size. We use the symbol  to represent microns. A micron is 0.001
millimeters. Another way to express this relationship is to say that 1000  = 1 mm. Use this
conversion factor to convert your estimate of field diameter to microns.
8. Without touching the course or fine adjustment knobs, turn the nosepiece until the high power
objective clicks into place. Use only the fine adjustment knob to bring the ruler into focus. Record
your observations of what you see.
9. Because under high power the thickness of one millimeter line takes up practically the entire field of
view, it is difficult to estimate the diameter of the field of view under high power. We need to rely on
a mathematical relationship between magnification and field diameter to calculate diameter at high
power. Use the formula on your data sheet along with your estimate of the diameter at low power and
the total magnifications at high and low power to calculate the diameter of the high power field.
10. Return to low power. Place a prepared slide of the cross section of a leaf on the stage and position it
so that the leaf is above the center of the stage opening. Use the stage clips to hold the slide in place.
11. Focus using first the course then the fine adjustment. What you are seeing is the structure from the
top surface of the leaf to the bottom. Move the slide to a place where the thickness of the leaf is the
same from one end of the field of view to the other.
12. Position the slide so that the top edge of the leaf is just touching the top of the field of view. Now
estimate how many of these leaves you could stack, one on top of the other, from the top of the field
of view to the bottom. Record your estimate on your data sheet.
13. Now you can use this estimate along with your estimate of the field diameter at low power to calculate
the approximate thickness of this leaf. Remember that the diameter of the field from top to bottom is
the same as the diameter from side to side. Simply divide your estimated diameter (in ) by your
estimate of how many times you could stack the leaf on itself to fill the field. Show your calculation
on your data sheet and give your answer in microns.
Part C: Focusing on Different Depths
1. Obtain a prepared slide containing three crossed threads of different colors and observe them under
low power.
2. Notice that you can only focus on one depth at a time (only one thread will be in sharp focus, while
the others will be slightly blurred. Sketch the threads in the space provided on your data sheet and tell
the order of threads from top to bottom in the space provided.
Questions (You must answer questions with an * in complete sentences. Other questions are short
answer.)
1. How does the orientation of an object change when viewed under the microscope?
2. How do the differences you observed between the letter e at high power compared to low power
illustrate the resolving power of the microscope?*
3. How many times bigger does an object viewed at high power seem to be compared to the same object
viewed at low power?
4. What happens to the field of view when you go from low power to high power?*
Data – Use this sheet to record data as instructed in the procedure.
Part A: Observing a Wet Mount Slide
Letter e
Low Power (_________)
Letter e
High Power (_________)
How does the e appear to move when you move the slide in the following directions?
A) to the right _____________________
C) away from you _____________________
B) to the left ______________________
D) toward you ________________________
Observations: How does the appearance of the letter e change in going from low power to high power?
____________________________________________________________________________________
____________________________________________________________________________________
Part B: Measuring an Object Under the Microscope
Estimate of field diameter at low power: _______________ mm
Convert your estimate to  (1 mm = 1000 ) below: (Show your work!)
Observations: How does the appearance of the ruler change when you change to high power?
Calculate the estimated field diameter at high power in  using the equation below: (Show your work!)
(LP= low power; HP= high power)
(LP MAGNIFICATION) X (LP FIELD DIAMETER) = (HP MAGNIFICATION) X (HP FIELD DIAMETER)
Estimate of how many leaf cross-sections could fit (top to bottom) in low power field: ____________
Calculate the approximate thickness of leaf: (Show your work!)
Part C: Observing Depth with the Microscope
Prepared slide of 3 crossed threads
Total Magnification __________________
Order of threads:
Top _______________________________
Middle _____________________________
Bottom _____________________________