Lens Cleaning Technique

1. Moisten one end of a high-quality cotton swab with one drop of Kodak lens cleaner. Keep the other end dry.

2. Clean the optical surface with the wet end. Dry it with the other end, using a circular motion.

3. Use a hand aspirator to remove lingering dust particles.

4. Start with the scanning objective and work upward in magnification, using a new cotton swab for each objective.

5. When cleaning the eyepiece, do not open the lens unless it is absolutely necessary.

6. Use alcohol for difficult cleaning, and only as a last resort use xylene. Regular use of xylene will destroy lens coatings.

f. If your microscope is not functioning properly, report the problem to your laboratory instructor immediately.

4. Turn on the substage illuminator and look through the eyepiece. You will see a lighted circular area called the field of view.

You can measure the diameter of this field of view by focusing the lenses on the millimeter scale of a transparent plastic ruler. To do this, follow these steps:

a. Place the ruler on the microscope stage in the spring clamp of a slide holder finger on a mechanical stage or under the stage (slide) clips. (Note: If your microscope is equipped with a mechanical stage, it may be necessary to use a short section cut from a transparent plastic ruler. The section should be several millimeters long and can be mounted on a microscope slide for viewing.)

b. Center the millimeter scale in the beam of light coming up through the condenser and rotate the scanning objective into position.

c. While you watch from the side to prevent the lens from touching anything, lower the objective until it is as close to the ruler as possible, using the coarse adjustment knob and then using the fine adjustment knob (fig. 33). (Note: The adjustment knobs on some microscopes move the stage upward and downward for focusing.)

d. Look into the eyepiece, and use the coarse adjustment knob to raise the objective lens until the lines of the millimeter scale come into sharp focus.

e. Adjust the light intensity by moving the iris diaphragm lever so that the field of view is brightly illuminated but comfortable to your eye. At the same time, take care not to overilluminate the field, because transparent objects tend to disappear in very bright light.

f. Position the millimeter ruler so that its scale crosses the greatest diameter of the field of view. Also, move the ruler so that one of the millimeter marks is against the edge of the field of view.

g. Measure the distance across the field of view in millimeters.

5. Complete Part B of the laboratory report.

6. Most microscopes are designed to beparfocal. This means that when a specimen is in focus with a lower-power objective, it will be in focus (or nearly so) when a higher-power objective is rotated into position. Always center the specimen in the field of view before changing to higher objectives.

Rotate the low-power objective into position, and then look at the millimeter scale of the transparent plastic ruler. If you need to move the low-power objective to sharpen the focus, use the fine adjustment knob.

Iris Diaphragm Watch

Adjust the iris diaphragm so that the field of view is properly illuminated. Once again, adjust the millimeter ruler so that the scale crosses the field of view through its greater diameter, and position the ruler so that a millimeter mark is against one edge of the field.

Try to measure the distance across the field of view in millimeters.

7. Rotate the high-power objective into position, while you watch from the side, and then observe the millimeter scale on the plastic ruler. All focusing using high-power magnification should be done only with the fine adjustment knob. If you use the coarse adjustment knob with the high-power objective, you can accidently force the objective into the coverslip. This is because the working distance (the distance from the objective lens to the slide on the stage) is much shorter when using higher magnifications.

Adjust the iris diaphragm for proper illumination. Usually more illumination when using higher magnifications will help to view the objects more clearly. Try to measure the distance across the field of view in millimeters.

8. Locate the numeral 4 (or 9) on the plastic ruler and focus on it using the scanning objective.

Note how the number appears in the field of view. Move the plastic ruler to the right, and note which way the image moves. Slide the ruler away from you and again note how the image moves.

9. Examine the slide of the three colored threads using the low-power objective and then the highpower objective. Focus on the location where the three threads cross. By using the fine adjustment knob, determine the order from top to bottom by noting which color is in focus at different depths. The other colored threads will still be visible, but they will be blurred. Be sure to notice whether the stage or the body tube moves up and down with the adjustment knobs of the microscope that is being used for this depth determination. The vertical depth of the specimen that is clearly in focus is called the depth of field (focus). Whenever specimens are examined, continue to use the fine adjustment focusing knob to determine relative depths of structures that are clearly in focus within cells, giving a three-dimensional perspective. It should be noted that the depth of field is less at higher magnifications.

Critical Thinking Application

What was the sequence of the three colored threads from top to bottom?

Critical Thinking Application

What was the sequence of the three colored threads from top to bottom?

10. Complete Parts C and D of the laboratory report.

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  • gustava
    What was the sequence of the three colored threads from top to bottom?
    6 years ago

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