Exercise 1.2 - Use of Oil Immersion (100x)

LEVEL I

Materials

• Microscope equipped with 100X, oil immersion lens
• Immersion oil
• Prepared slide of bacteria or suitably small objects

Procedure

1. Place the prepared slide on the microscope stage. Using the 10X objective, focus the microscope on an appropriate field containing bacteria. Center the bacteria in the field of view, by manipulating the lateral movement knobs.

2. Rotate the nosepiece to the 40X objective and refocus with the fine focus. Again, center the object which you wish to examine.

3. Rotate the nosepiece so that an intermediate position between the 40X and 100X objectives is obtained.

4. Place a small drop of immersion oil 10 on the center of the viewing area of the slide.

5. Continue to rotate the nosepiece so that the 100X objective is rotated into the oil.

   Do not under any circumstances place the 40x objective in the oil

   Use only the fine focus and refocus your specimen.

6. Determine the shapes of the bacteria and draw them in the place provided.

7. Immediately after using the oil, remove any residual oil from the slide and from the front of the 100X objective by gently rubbing with lens paper dipped in xylol, toluene, or better, a xylol substitute designed for this purpose.

   Nearly all organic solvents, and especially xylol and toluene, are potentially hazardous. They are flammable and readily enter the boddy by direct absorption through the skin or lungs. Consistent high exposure to thses solvents has been linked with liver damage and potential carcinogenesis. Use all solvents sparingly when needed and always in a well- ventilated area.

   Once oil is placed on the slide, the 10X and 40X objectives will no longer be useful until the oil is removed. The oil will blur any image attempted with the lower magnifications. Use oil only when necessary and after completing all work at the lower magnifications. In order to return to work at the lower magnifications, the slide must be completely cleaned of any residual oil and dried. For this reason, oil immersion is employed only when necessary, and only after thorough observations at the high dry magnification (40X). 11

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Drawing of bacterial cell types

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