CARE AND USE OF A COMPOUND MICROSCOPE
MATERIALS:compound microscope microscope slides
cover slips (2)paper towel
lens paperpiece of newspaper
piece of coloured paper from a magazineclear plastic ruler
forceps (tweezers) or dissecting needle medicine dropper
PROCEDURE A: (Read discussion questions first)
a)Obtain a microscope from the front of the room. Always grasp the ARM firmly with one hand. Place the other hand under the base. This is the ONLY way to lift and carry a microscope.
b)Place the microscope gently on the desk. The arm should point towards you.
PROCEDURE B:
a)Ensure that the low power objective is above the hole in the centre of the stage. You will hear a click when the lens is moved and is exactly in the right position.
b)Use the COARSE ADJUSTMENT KNOB to raise or lower the STAGE.
c)Close the diaphragm to the smallest opening and see what happens to the amount of light entering the lens.
d)Adjust the diaphragm until the field of view is the proper intensity. Generally bright but not blaring.
PROCEDURE C:
a)Hold a MICROSCOPE SLIDE by the edges and wet it with water. Wipe both sides dry with a paper towel. Hold the slides by the edges or you will get fingerprints on it.
b)Obtain a clean COVERSLIP.
c)Using the MEDICINE DROPPER, place one drop of water near the centre of the microscope slide.
d)Use the FORCEPS to place a small piece of newspaper containing a small letter "a", "b", "d", "e"' "g"' or "h" in a drop of water. The smaller the letter the better.
e)Hold a coverslip by the edges to lower one edge so that it touches one side of the drop of water at an angle of about 45. Now slowly lower the coverslip onto the slide. This will prevent the trapping of air bubbles under the coverslip.
NOTE: If the mount begins to dry out at any time, you can add water to it by placing a drop of water at one end of the coverslip.
f)Place the prepared slide on the stage so that the letter is in a reading position and in the center of the opening of the stage. Use the STAGE CLIPS to hold the slide in place. The slide is now ready for viewing.
PROCEDURE D:
a)Using the coarse adjustment, turn the low power objective down or raise the stage as far as it will go. Watch what you are doing from one side.
b)Look through the OCULAR. Slowly lower the stage by turning the coarse adjustment. Keep doing this until the letter comes into focus.
c)Bring the image into the sharpest possible focus by using the FINE ADJUSTMENT KNOB. Try changing the diaphragm setting as well. Proper illumination increases the clearness of the image.
d)Move the slide slightly in various directions. Note carefully the corresponding movements of the image.
e)Now, using a pencil, draw what you see on the sheet given.
f)Proceed as follows to view the letter under medium power. Return the letter to the center position on the stage. Make sure the letter is still in sharp focus. Turn the nosepiece carefully so that the medium power objective is above the slide. Make sure the slide or coverslip NEVER touches the lens. A slight turning of the fine adjustment is sometimes required.
NOTE: Never use the coarse adjustment when you are on medium power. If you have problems, ASK.
g)Now, using a pencil, draw what you see.
h)Proceed as follows to view the letter under high power. Make sure that the letter is still in sharp focus. Then while looking directly at the slide and coverslip from the side, turn the nosepiece carefully to bring the high power objective above the slide.
i)Now, using a pencil, draw what you see.
PROCEDURE E:
Magnifications are only expressed in DIAMETERS. The following example will illustrate this term. If an object is magnified ten diameter (10x), the image will be 10 times longer and 10 times wider than it would appear if the object were viewed from a distance of 25 cm with the unaided eye. The magnification is printed on each ocular and objective lens.
Examine your microscope and note these magnifications. THE OVERALL MAGNIFICATION OF A MICROSCOPE is the PRODUCT OF THE MAGNIFICATION OF THE OBJECTIVE AND OCULAR BEING USED. Thus, if a 10x ocular is combined with a 40x objective, the overall magnification is 10 x 40, or 400 diameters (400x). Calculate the magnification that your microscope can produce and place them next to the pencil drawings you have completed.
PROCEDURE F:
Objects that you examine under a microscope are so small that you cannot measure them with the smallest unit on your ruler, the millimetre (mm). Therefore you need a smaller unit. The one commonly used is the MICROMETRE (μm). 1 μm = 0.001 mm, or 1000 μm = 1 mm.
LOW POWER
Proceed as follows to estimate the size of a microscope object under the low power objective of the microscope.
a)Place a clear plastic ruler on the stage. Focus on it using the low power objective and a 10x ocular.
b)Move the ruler around until you have it in a position that measures the diameter of the field of view. Remember that one division is 1 mm. Note that the marks on the ruler appear very wide.
c)Record the diameter of the field of view in mm and μm.
4)The size of the object can now be estimated by comparing it to the size of the field of view. Put your slide back on the stage. Estimate the height of the letter in mm and μm by labeling each drawing with their respective sizes.
MEDIUM POWER
The same procedure cannot be used for medium power. The marks appear too far apart to be accurate. Proceed as follows to estimate the size of an object under medium power.
a)Divide the magnification of the medium power objective by the magnification of the low power objective.
b)Now divide the power of the low power field by the number you obtained in step (a). The result is the diameter of the medium power field of view. Suppose the medium power objective is 10x and the low power objective is 5x.
10x = 2 Suppose the diameter of the low power field is 1000 μm. 1000 μm = 500 μm is the diameter
5x 2 of medium power
field
HIGH POWER
The diameter of the field of view is calculated in the same way as medium power. Simple substitute "high" instead of "medium" in steps (a) and (b) above.
DISCUSSION QUESTIONS
1.a)Compare the orientation (to place with all important parts facing in a certain direction) of the image of the letter as seen through the ocular with the orientation of the letter when you looked at it without the microscope.
b)Describe what happened when you moved the letter in various directions.
2.a)What happened to the size of the letter when you switched to medium power?
b)What happened to the field of view when you switched to medium power?
Low Power
Medium Power
High Power
MINI ACTIVITY
DETERMINING THE FIELD OF VIEW FOR YOUR MICROSCOPE
1.What is the power of your ocular?
What is the power of the low power objective?
What is the total power of your microscope at low power?
Call this MAG1.MAG1 =
2.With a ruler, measure the field of view at low power = μm
Call this fd1.fd1 =
3.Calculate the total power of your microscope at medium power:
Call this MAG2.MAG2 =
4.Use the formula:MAG1fd2
─────── = ──────
MAG2fd1
and calculate the field of view for your microscope at medium power.
field of view for medium power = μm
5.Calculate the total power of your microscope at high power:
Call this MAG3.MAG3 =
6.Use the formula:MAG1fd3
─────── = ──────
MAG3fd1
and calculate the field of view for your microscope at high power.
Field of view for high power = μm