Demonstration

A compound microscope is sometimes equipped with a micrometer scale mounted in the eyepiece. Such a scale is subdivided into fifty to one hundred equal divisions (fig.3.4). These arbitrary divisions can be calibrated against the known divisions of a micrometer slide placed on the microscope stage. Once the values of the divisions are known, the length and width of a microscopic object can be measured by superimposing the scale over the magnified image of the object.

Observe the micrometer scale in the eyepiece of the demonstration microscope. Focus the low-power objective on the millimeter scale of a micrometer slide (or a plastic ruler), and measure the distance between the divisions on the micrometer scale in the eyepiece. What is the distance between the finest divisions of the scale in micrometers? _

11. Prepare several temporary wet mounts using any small, transparent objects of interest, and examine the specimens using the low-power objective and then a high-power objective to observe their details. To prepare a wet mount, follow these steps (fig. 3.5):

a. Carefully clean a microscope slide with soap and water, and dry it with a paper towel.

b. Place a tiny, thin piece of the specimen you want to observe in the center of the slide, and use a medicine dropper to put a drop of water over it. Consult with your instructor if a drop of stain might enhance the image of any cellular structures of your specimen. If the specimen is solid, you might want to tease some of it apart with dissecting needles. In any case, the specimen must be thin enough so that light can pass through it. Why is it necessary for the specimen to be so thin?

c. Cover the specimen with a coverslip. Try to avoid trapping bubbles of air beneath the coverslip by slowly lowering it at an angle into the drop of water.

d. Remove any excess water from the edge of the coverslip with absorbent paper. If your microscope has an inclination joint, do not tilt the microscope while observing wet mounts.

e. Place the slide under the stage (slide) clips or in the slide holder on a mechanical stage, and position the slide so that the specimen is centered in the light beam passing up through the condenser.

f. Focus on the specimen using the scanning objective first. Next focus using the low-power objective, and then examine it with the highpower objective.

12. If an oil immersion objective is available, use it to examine the specimen. To use the oil immersion objective, follow these steps:

a. Center the object you want to study under the high-power field of view.

b. Rotate the high-power objective away from the microscope slide, place a small drop of immersion oil on the coverslip, and swing the oil immersion objective into position. To achieve sharp focus, use the fine adjustment knob only.

c. You will need to open the iris diaphragm more fully for proper illumination. More light is needed because the oil immersion objective covers a very small lighted area of the microscope slide.

d. Because the oil immersion objective must be very close to the coverslip to achieve sharp focus, care must be taken to avoid breaking the coverslip or damaging the objective lens. For this reason, never lower the objective when you are looking into the eyepiece. Instead, always raise the objective to achieve focus, or prevent the objective from touching the coverslip by watching the microscope slide and coverslip from the side if the objective needs to be lowered.

13. When you have finished working with the microscope, remove the microscope slide from the stage and wipe any oil from the objective lens with lens paper or a high-quality cotton swab. Swing the scanning objective or the low-power objective into position. Wrap the electric cord around the base of the microscope and replace the dustcover.

14. Complete Part E of the laboratory report.

Figure 3.4 The divisions of a micrometer scale in an eyepiece can be calibrated against the known divisions of a micrometer slide.

Figure 3.4 The divisions of a micrometer scale in an eyepiece can be calibrated against the known divisions of a micrometer slide.

Diagram Micrometer

Figure 3.5 Steps in the preparation of a wet mount.

2. Place the specimen in the center of the slide and add drop of water to specimen.

1. Clean slide with soap and water and dry it with a paper towel.

1. Clean slide with soap and water and dry it with a paper towel.

Needle probe

Needle probe

Coverslip With Water

2. Place the specimen in the center of the slide and add drop of water to specimen.

3. Slowly lower coverslip to avoid bubbles.

4. Remove excess water with absorbent paper.

3. Slowly lower coverslip to avoid bubbles.

4. Remove excess water with absorbent paper.

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