# Part I. Learning About the Microscope

PART I. LEARNING ABOUT THE MICROSCOPE

1. Compare your microscope with Figure 2 below. Identify each part on your microscope.

2. Look at the number followed by an “X” on the side of each objective or the number after DIN. This number is the objective’s magnifying power. The “X” stands for “times.” Thus the number tells how many times an object is magnified by this lens.

1. How many objective lenses does your microscope have?
2. What is the magnifying power of each?

3 . If the lenses look dirty or smudged, carefully wipe them with lens paper. Use only lens paper because other kinds of paper candamage the lenses.

4. The ocular lens also has a magnifying power. The total magnifying power of the microscope is easy to calculate. Simply multiply the magnifying power of the ocular by the magnifying power of the objective. For example, if the ocular is 5X and the objective is lOX, the total magnification of the object being viewed is 5X x lOX = 50X.Look carefully at the engraving of the ocular lens.

C. What is its magnifying power?

D . What is the total magnification for each of your objective lenses? Show your calculations.

PART II. PREPARING AND EXAMINING A WET MOUNT

Cut out a 1-cm square of a colored newspaper cartoon or a colored picture from a magazine printed on thin paper. Choose a square that has both light and dark tones, but not black

1.Place the colored newspaper or magazinein the middle of a clean slide. With a pipette,put 1 drop of water on the square. Drop the water from about1 cm above the slide. Do not touch the pipette to the paper orthe paper will stick to the pipette.

2. Now cover the mount with a clean cover slip. One way to do this is shown in Figure 3-a. Hold the cover slip at about a 45° angle to the slide and move it toward the drop. As the water touches the cover slip, it will spread along the edge. Gently lower the cover slip intoplace. Do not press on the cover slip—it should rest onthe top of the water. A good wet mount is free of bubbles. If yourmount has too many bubbles, take off the cover slip and absorbthe water with a paper towel. Then repeat Steps 2 and 3.

3. Click the low-power objective into place. Make sure you have a good light source and that the diaphragm is at the largest opening.

4. Check to be sure the bottom of the slide is dry before placing it on the stage of the microscope.

5. Look at the microscope from the side. Using the course focus knob to position the low-power objective about 1/2 to 1 cmabove the slide, or until you feel an automatic stop.

6. Look through the ocular, keeping both eyes open. Keeping botheyes open is difficult at first, but it helps to prevent eyestrain. It will become easier with practice.

7. Slowly lower the stage by turning the coarse adjustment until the letters come into focus. Use the fine adjustment to sharpen the focus.

1. The space on your answer sheet (like the one below) represents your field of view. This is the lit circle you observe when looking through the ocular. Draw what you see.

8. Move the slide to the left.

B. Which way does the image move?

9. Move the slide to the right.

C. Which way does the image move?

10. Move the slide away from you.

D. Which way does the image move?

11. Observe the wet mount as you change the diaphragm to each of its settings. Adjust it to give good contrast and illumination without glare.

E. What does the diaphragm control?

Before using high power, the specimen must be in sharp focus in the center of the low-power field of view. Note: All focusing under high power is done with the fine adjustment knobs. There is no automatic stop for the high-power objective.

12. Watching from the side, carefully switch to the medium power objective. Center the object and focus.

13. Now without changing anything, switch to the high-power objective. Make sure that the objective does not hit the slide, butexpect it to be very close.

14. Focus on the paper. Only a slight turn of the fine adjustment knob will be needed to do this.

F. In the space on your answer sheet, draw what you see under high power.

G. Is the field of view (how much area you see)larger under high power or low power?

H. Compare the brightness of the field under high power and low power.

15. Prepare another wet mount, this time using two hairs of different colors. Cross them on the slide, then add a drop of water and thecover slip.

16. View the slide under low power. Focus directly on the point where the hairs cross.

D Are both hairs in focus under low power?

17. Switch to high power and observe the hairs.

E.Are both hairs in focus under high power? Explain.

Part III: Measuring with the Microscope

Background

It is interesting and informative to observe specimens under the microscope, but it is often difficult to know the actual size of the object being observed. Magnification causes us to lose the idea of actual size. You cannot hold a ruler up to a paramecium or a plant cell while it is under a microscope. Therefore, size must be measured indirectly—that is, it must be compared with the size of something you already know. The diameter of the microscope field seen through the ocular is a convenient standard to use.

Two metric units are useful when measuring small objects:

1 meter (m) = 1000 millimeters (mm) 1 mm = 1000 micrometers (μ m)

Procedures and Observations

1. Examine the markings on a transparent metric ruler. Determine which marks indicate millimeter lengths. Then place the ruler on the stage so that it covers half of the stage opening as shown in Figure 1.

2. Prepare your microscope for low-power observation of the ruler.

3. Look through the ocular. Focus on the edge of the ruler, using the coarse adjustment. Adjust the position of the ruler so that the view in the low-power field is similar to

4. Place the center of one mark at the left side of the field of view. Make sure that the edge of the ruler is exactly across the center of the field. If the ruler sticks to your fingers, use the eraser end of a pencil to arrange it.

5. Note that 1 millimeter is the distance from the middle of one mark to the middle of the next mark. For example, in the picture above, the diameter of the low-power field measures 3 millimeter plus a fraction of another.

A. Record the measurement of the low-power field diameter in millimeters, expressing the length to the nearest tenth of a millimeter.

B. Record the measurement of the low-power field diameter in micrometers.(Convert the measurement in millimeters to micrometers by multiplying by 1000).

6. You cannot measure the diameter of the high-power field using the process you have just completed. Viewing a ruler under high power presents light and focusing problems. Also, the high-power field diameter is less than 1 millimeter. But you can indirectly obtain the high-power field diameter. You know the low-power field diameter and the magnifying power of both objectives. The magnification of the objectives is inversely proportional to the field size. You can use this formula:

C. Record the high-power field diameter in micrometersby substituting the values you know in this formula to calculate the high-power field diameter. Show your calculations.

The measurements of the low-power and high-power field diameters can be used to measure other things indirectly.

7. Under low power, focus on a prepared cross section of prepared paramecium slide (a paramecium is a single-celled organism in the protist kingdom).

D. Estimate how many of these paramecia would fit end to end across the diameter of the field of view.

To calculate the length of a single paramecium, divide the diameter of the low-power field by the number of cells given for Question D

E. What the length of a paramecium in micrometers.

11. Switch to high power and focus with the fine adjustment.

F. How many paramecia would fit across the diameter of the high- power field?

12. To calculate the diameter of a pith cell as seen under high power, divide the diameter of the high-power field by the number of cells given for Question F

G. What is the length of a paramecium, as measured under high power.

H. Compare the measurement of the diameter of a cell under low and high power. Which do you think is more accurate? Why?

I. Find the diameter of the high-power field of a microscope with an ocular marked lOX, a low-power objective marked 1OX, a high-power objective marked 40X, and a low-power field diameter of 1600 micrometers.

Part IV: How PlantandAnimal Cells Differ

Introduction

Although plant and animal cells have many structures in common,they also have basic differences. Plant cells have a rigid cell wall, and ifthey are green, they also have chloroplasts. Animal cells lack both a cell wall and chloroplasts. They also lack the central vacuole common to plant cells.

You will observe and compare animal cells and plant cells. You will first examine epithelial cells from the inside of your cheek. Epithelium is a type of tissue that covers the surfaces of many organs and cavities of the body.

You will then examine cells from a leaf of the freshwater plant elodea. Elodea is often used in home fish tanks. The cells of this plant are green because they contain the pigment, chlorophyll. Chlorophyll, which is found in chloroplasts within each cell, enables plants to manufacture their own food.

Finally you will observe the skin of an onion and compare to the elodea.

HUMAN EPITHELIAL CELLS

1. Place a SMALL drop of water on a clean slide. Obtain epithelial cells by gently scraping the inside of your cheek with a clean toothpick as shown in Figure 1. CAUTION: Never reuse a toothpick or put any thing in your mouth which may not be clean. Stir the material from the toothpick in the drop of water on the slide. Then immediately break the toothpick in half and throw it away.

2. Allow the cheek cells to dry and heat fix the slide by quickly passing the slide through the flame 4-5 times. Add a small drop of methylene blue stain to the slide. CAUTION: Stain can damage clothing and discolor skin. Let the stain sit for 1 minute and then rinse the stain off the slide. Carefully place a cover slip on the slide. Examine the slide under low power.

Now focus on the highest power using the emersion oil. To do this, focus first (in sequence) with the 3 lower power objectives. Next place the nosepiece so that the highest power is not yet straight down and you can access the slide. Take the dropper out of the emersion oil bottle and drop ONE drop (small) on the top of the cover slip. Now lower the high power objective so that it is immersed in the oil. Use the fine adjustment only to focus the cells.

a. Make a drawing of two or three cells as they appear under high power. Label the nucleus, cytoplasm, and cellmembrane of one of the cells. Indicate the total magnification below the circle

b. What is the shape of the cells?

c. Describe the appearance of the cytoplasm.

ELODEA LEAF CELLS

1. Break off a small leaf near the tip of an elodea plant. With a forceps place the entire leaf in a drop of water on a clean slide. Add a coverslip. See Figure 2.

2. Examine the leaf under low power.

D. What is the shape of the cells?

E. Estimate the size of a single cell.

The boundary that you see around each cell is the cell wall. Thenumerous small, green bodies in the cells are the chloroplasts.

3. Look for an area in the leaf where you can see the cells most clearly. Examine these cells under high power, carefully focusing up anddown with the fine adjustment.

F. Describe the shape and location of the chloroplasts.

4. As you examine the cells, you may see the chloroplasts moving around. If they are not moving, warm the slide in your hand or under a bright lamp for a few minutes. Do not allow the slide to dry out. Then examine again under high power.

G. Make a drawing of an elodea cell. Label the cell wall, chloroplasts, and any other structures you see.Be sure to indicate the total magnification.

The cell membrane is pressed tightly against the inside of the cell wall and is difficult to see. Furthermore, the numerous chloroplasts often make it difficult to observe other cell structures in the elodea leaf cells. In order to see the nucleus, nucleoli, and vacuole more clearly, you are going to use a stain.

6. Break off another elodea leaf and place it in a drop of Lugol’s iodine solution on a clean slide. Add a cover slip. Wait a minute or so for the stain to penetrate into the cells. Then examine the stained elodea cells under low and high power.

H. Make a drawing of a stained cell. Label the cell wall, cell membrane (if visible), chloroplasts, nucleus, nucleolus, and the largevacuole. Be sure to indicate the total magnification.

I. What structures can you see more clearly after staining?

ONION CELLS

Remove a thin “skin” of onion using the forceps. Place the onion onto a drop of water on a clean slide. Observe the cells on high power.

J. Compare the two unstained plant cells you have observed and give a logical reason for the differences you see.

Part V: Prokaryotic cells

Look at the prepared slide of prokaryotic cells under oil emersion.

1. Sketch what you have seen.
2. Estimate the size of a single cell. Show your work.

Part VI: Pond water

Take a drop of pond water and place it on a clean slide. Look at it under low power, scanning the entire area for something alive. Go slowly and look for something that may be moving. It may take some time and patience to locate a living organism.

1. Sketch what you have seen.
2. Is your organism likely to be prokaryotic or eukaryotic? Explain why.

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