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MICROSCOPY
Objectives
After completing this exercise, you will be able to
- Define magnification, resolving power, contrast, field of view, parfocal, parcentral, depth of field, working distance;
- Describe how to care for a compound light microscope;
- Recognise and give the function of the parts of a compound light microscope;
- Accurately align a compound light microscope;
- Correctly use a compound light microscope;
- Make a wet mount;
- Correctly use a dissecting microscope;
- Describe the usefulness of the phase-contrast, transmission electron, and scanning electron microscopes;
- Use your skills to enjoy a fascinating world unavailable to the unaided eye.
Introduction
A microscope contains lenses (for example, transparent glass), which focus radiation (such as light rays) emanating from a specimen to produce an image of that specimen on a surface sensitive to the radiation (like the retina, the light-sensitive layer of the eye). Table 1 below presents the three most important properties of lenses and their images.
TABLE 1 Important Lens and Image Properties
Property Definition
Magnification The amount that the image of an object is enlarged – for example, 100x.
Resolving Power
The extent to which object detail in an image is preserved during the magnifying process.
Contrast The degree to which image details stand out against their background.
The Compound Light Microscope
The lens of a normal unaided eye can project onto the retina a focused image of an object held no closer than about 10 cm. At this distance, details separated by 0.1 mm are visible. Most cells and related structures are smaller than this, and a light microscope is needed to see them. A microscope placed between the eye and a specimen (usually a section or thin object(s) mounted on a glass slide) acts to bring the specimen very close to the eye so that you can see its details. Ultimately, the greater the
proportion of the retina covered by the final image of the specimen, the greater the magnification. It does this by producing a series of magnified images.
Magnification without enough resolving power is referred to as empty, and with a light microscope, the maximum useful magnification is about 1000 times the diameter of the specimen (1000x). Above this value, additional details are missing. Furthermore, adequate contrast is needed to see the details preserved in an image. Dyes are usually added to sections of biological specimens to increase contrast.
Like automobiles, models of compound light microscopes abound, and these instruments have numerous accessories.
Parts of the Compound Light Microscope
Remove the instrument assigned to you from the cabinet and place it on your lab bench. Use Figure 1- and the chart on the wall of your lab room to identify the various parts of your microscope. Read each step below and manipulate the parts only where indicated. Before you start, make sure the shortest objective is the light path.
- Light source. The compound microscope uses transmitted light to illuminate a transparent specimen usually mounted on a glass slide. Newer microscopes have a built in illuminator (figure 1-1). Locate the illuminator, the off/on switch and perhaps also a rheostat, which is used to vary the intensity of the light. On some models, the switch and the rheostat are combined. Turn on
- Focusing knobs. The coarse focus adjustment knob is for use with the lower-power objectives, whereas the fine focus adjustment knob is for critical focusing, especially with the higher – power objectives. On most modern microscopes, you move the stage of the instrument up and down to focus the specimen. Modern microscopes usually have a preset focus lock, which stops the stage at a particular height. After setting this lock, you can lower the stage with the coarse focus knob, to facilitate changing of the specimen, and then raise it to focusing height without fear of touching the specimen against the objective. There may also be a focus tension adjustment knob, usually located inside if the left-hand coarse focus knob.
- Objectives. The compound light microscopes has a least two magnifying lenses, the objective and the ocular. The objective scans the specimen. Most microscopes have several objectives mounted on a revolving nosepiece. The magnifying power of each objective is labelled on its side. Usually included are these objectives: a 4x low-power or scanning, a 10x medium power (figure 3-4b), an about 40x high dry, and perhaps an about 100x oil-immersion objective. The other number often labelled on the side of nosepiece objectives is the numerical aperture (NA). The larger the numerical aperture, the greater the resolving power and useful magnification.
Objectives are parfocal. That is once an objective has been focused, you can rotate to another one and the image will remain in coarse focus, requiring only slight movement of the fine focus knob. Objectives are also parcentral, meaning that the center of the field of view remains about the same for each objective. The field of view is the circle of light you see when looking into the microscope. Objectives have lengths; the lower-power objectives are shorter than the higher-power ones. That is, the working distance of objectives decreases with magnification. Working distance is the space between the objective lens and the slide. Therefore, the higher the power of the objective in use, the closer the objective is to slide -and the more careful you must be. (a) Record the magnifying power and NA of the objectives on your microscope in Table1-3. If your instrument does not have a particular objective, indicate that it is not present (NP). (b) Describe the objectives in table1-2 and state the function.
Objective Objective Magnifying Power (ObMP)
Total Magnifying Power (ObMPxOcMP=______x)
Numerical Aperture (NA) Low-Power Medium-Power High-dry Oil-immersion
- Ocular. The magnify lens you look into is called an ocular (figure 1-4a). Oculars are generally 10x (a) Since each objective has a different magnifying power, the total magnification is calculated by multiplying the magnifying power of the ocular by that of the objective in use. What is the ocular magnification power (OcMP) of the ocular (s) on your microscope? __________x Calculate the total magnification for each of your microscope’s ocular/objective combinations, and then record them in Table 1-3. (b) Describe the ocular in table 1-2, and then state its function. (c) Your microscope will have one or two oculars mounted on a monocular or binocular head, respectively. There may be a pointer mounted in an ocular so that you can easily show a specimen detail to your instructor or another student. (d) If your microscope is monocular, determine your dominant eye: Look for a small object on the far wall of your room with both eyes open. Form the thumb, and index finger of one hand into a circle and place this circle in your line of sight, at arm’s length, so that it surrounds the object. Close your right eye. If the object shifts out of the circle to your left, your right eye is probably dominant. If the object remains in the circle, your left eye is probably dominant. This time close your left eye and go through the process again. If the object shifts to the right, left eye is dominant. The more pronounced the shift, the greater the dominance. If there is no shift, neither eye is dominant.
Aligning a Compound Light Microscope with In-base Illumination and a
Condenser with an Iris Diaphragm
Aligning your Microscope properly will not only help you see specimen detail clearly but will also protect your eyes from strain.
- Rotate the nosepiece until the medium-power objective is in the light path. Open the iris diaphragm.
- If it is not already there, place the prepared slide of stained diatoms on the stage; center and carefully focus on it. Skip steps 3 and 4 if your microscope is monocular. Skip step 5 if your microscope does not have a control to adjust the height of the condenser.
- If your microscope is binocular, adjust the interpupillary distance. Hold a different ocular tube with each hand and, while looking at the specimen, pull the tubes apart or push them together
slide with the specimen (the letter e) right side up on the stage. Center the e in the field of view, and carefully bring it into focus.
- In Figure 1-7, draw the image of the e as you see it through the ocular. Record the total magnification used in the line at the end of the legend. Is the image right side up or upside down compared to the specimen?
Compared to the specimen, is the image backward as well as upside down?
(yes or no)______
In summary, the image is inverted with respect to the specimen.
- Move the specimen to the right side while watching it through the microscope. In which direction does the image move?
- Move the specimen away from you. In which direction does the image move?
- Remove the slide and put it away.
Depth of field
The depth of field is the distance through which you can move the specimen and still have it remain in focus. Remember, the working distance – the space between the objective lens and the coverslip- decreases with increasing magnifying power. Therefore, the higher the power of the objective in use, the closer the objective is to the slide- and the more careful you must be.
- Obtain a prepared slide of three crossed colored threads. This exercise requires care, since you are probably not yet at focusing on a specimen. Once you have the threads in focus (using first the low-power objective and then medium-power objective), you need only use the fine focus knob with the high-dry objective. After switching to the high – dry objective try rotating the fine focus knob to focus with the high dry objective. After switching to the high – dry objective, try rotating to fine focus knob to focus ½ turns away from you and then a full turn toward you. If you have not found the plane of focus,next try 1 ½ turns away from you and 2 full turns toward you, and so on. If you work deliberately, you will find the plane of focus and won’t crack the coverslip. (a) How many threads are in focus using the low-power objective?______ medium power objective?__________ high-dry objective? (b) With which objective is it easiest to focus a specimen?_________________
(c) At which objective is it easiest to focus a specimen?___________________
- Specimens have depth. Continue using prepared slide of three crosss colored threads. (a) Use the high-dry objective to determine the order of three threads mounted on the slide and record the results in Table1-4. Each slide label has a code on it. When you believe that you have discovered the correct order, check with your instructor to find out if you are correct. (b) Focusing carefully with the fine focusing knob, move from the bottom to the upper thread. Did you move the knob away from or toward you?_________________
- Remove and put away the slide.
- Viewing sections of three-dimensional structures makes interpretation of the original shape quite difficult? Location Color Middle Closest to slide Closest to coverslip
a. Examine a slide of the cortex of the mammalian kidney with your compound microscope. Complete the hypotheses as to the three – dimensional shape of the structures labelled renal corpuscles and nephron tubules. The shape of renal corpuscles is ____________________ The shape of nephron tubules is ________________________
Using the Iris Diaphragm to improve Contrast
- Place a specimen of unstained fibers on the stage. Locate and focus on these fibers using the medium-power objective. Make sure the condenser and iris diaphragm and correctly set.
- Close the iris diaphragm
Does this produce increase or decrease contrast?___________
Although this procedure is useful when viewing specimens with low contrast, it should be used only as a last resort because resolving power is also decreased.
- Remove and put away the slide.
Units of Measurement
The basic metric unit of length at the light – microscopic level is the micrometer (μm). An even smaller unit, the nanometer (nm) is often used at the electron-microscopic level.
1000μm=1mm
- If your microscope is equipped with an ocular micrometer, your instructor may provide directions on how to accurately measure specimen details.
How to make a Wet Mount
In the mid-seventeenth century, Robert Hooke used a microscope to discover tiny, empty compartments in thin shavings of cork. Repeating this historic observation is a good way to learn how to prepare a wet mount.
Materials
Per student:
Compound microscope, lens paper, a bottle of lens- cleaning solution (optional ), a lint-free cloth (optional) Cork
Razor blade Glass microscope slide Glass coverslip Dissecting needle
Per student group (4):
Dropper bottle of distilled water (dH 2 O)
PROCEDURE
- Carefully use a razor blade to cut a number of very thin shavings from a cork stopper. Place them on a glass microscope slide.
- Gently add a drop of distilled water.
- Place one end of a glass coverslip to the right or left of the specimen so that the rest of the slip is held at a 45^0 angle over the specimen.
- Slowly lower the coverslip with a dissecting needle so as not to trap air bubbles.
- Observe the wet mount, first at low magnification and then with higher power. Air may be trapped, either in the cork or as free bubbles. Trapped air will appear dark and refractive around
its edges. This effect is due to sharply bending rays of light. Draw what you see in figure 1-11. Note the magnification used to make the drawing.
- Clean and replace the slide and coverslip as indicated by your instructor.
Microscopic Observations
Examining the microscope world is both challenging and fun. Most of the macroscopic world has been explored, but the microscopic world is barely touched. Yet the microbes in it are essential to our very existence. So be an explorer and see what you discover!
MATERIALS
Per Student
Compound microscope, lens paper, a bottle of lens-cleaning solution (optional), a lint-free cloth Glass microscopic slide Glass coverslip
Per group
Pond water or some other mixed culture in a dropper bottle Dropper bottle of protoslo
PROCEDURE:
- Obtain a drop of pond water or other mixed culture from the bottom of the bottle.
- Add a drop of Protoslo this methyl cellulose solution slows down any swimming microorganisms.
- Make a wet mount. (figure 1-9)
- Observe the wet mount with your compound microscope. Start at the upper – left corner of the coverslip and scan the wet mount with the low-power objective. When you find something interesting, focus on it and switch to the medium – power objective, and then, if necessary, the high-dry objective.
- Draw what you find on Figure 1-12 and note the total magnification.
- Attempt to identify it using the resource books provided you by your instructor. If successful, write its name under your drawing.
- Clean and replace the slide and coverslip as indicated by your instructor.
- Put away your compound microscope.
- Is the image of the specimen inverted as in the compound light microscope? (yes or no)
- Describe the type of illumination used by your dissecting microscope. Is there a choice?
Other Microscopes
In future exercises you will examine pictures taken with other types of microscopes. Some will be of living cells taken with a phase-contrast microscope or similar instrument, including those using the Normaski process. Others will be very thin – sectioned, heavy metal-stained specimens taken with a transmission electron microscope or TEM. Still others will be of precious metal-coated surfaces produced by signals from the scanning electron microscope or SEM. Table 1- summarizes the technology and use of these microscopes.
Microscope Technology Use Phase-contrast and Normarski process
Converts phase differences in light to differences in contrast.
Observations of low contrast specimens (often living)
Transmission electron Increases resolving power by using electrons in a vacuum and magnetic lenses instead of light and glass lenses, respectively.
Preservation of greater specimen detail allows for magnifications up to 1,000,000x or more (usually dead materials)
Scanning electron Forms TV-like picture from a secondary electron signal, which is emitted from surface points excited by a thin beam
Investigation of the fine structure of surfaces (usually dead specimens)
of electrons drawn across the surface in a raster pattern
MATERIALS
GROUP
Photographs of TEM micrographs (negatives) or digital images. Photographs of SEM negatives or digital images
PROCEDURE
- Examine some photographs of TEM micrographs (negatives)or digital images. The darker areas are more electron-dense in the specimen than the lighter areas.
- Now look at some photographs of SEM negatives or digital images. The lighter area corresponds to the emission of greater numbers of secondary electrons from that part of the specimen; the darker areas emit less.
- What type of microscope (compound light, dissecting, phase-contrast, TEM or SEM) would you use to examine the specimen listed below
Specimen Microscope Living surface of the finger Dye-stained slide of a section of the finger Gold-coated bacteria on a single cell of the finger Unstained section of a biopsy from the finger Heavy metal-stained, very thin section of the finger
POST-LAB QUESTIONS
- What is the function of the following parts of a compound light microscope? a. Condenser lens
b. Resolving power
c. Contrast
- Which photomicrograph of unstained cotton fibers was taken with the iris diaphragm close?___________
- Describe how you would care for and put away your compound light microscope at the end of the lab.
- Describe how to make a wet mount.
- A camera mounted on a __________________microscope took this photo of a cut piece of cork.
- Why were humans unaware of microorganisms for most of their history?
