This guide is for objectives designed to be used for oil immersion microscopy. Not all objectives would meet this requirement; dry objectives like those used with metallurgical microscopes don't need oil and may be damaged if they come into contact with it.

Procedure

1. Using a lower power objective, center the area of interest on your slide within the field of view.

2. Place a drop of immersion oil onto the cover slip.

3. Carefully turn the nosepiece to bring the oil immersion objective (usually a 100x) into position.

4. Using the coarse focus knob, slowly bring the stage up while observing the lens from the side until the drop of oil on the slide just barely comes into contact with the front of the objective lens.

5. To eliminate air bubbles, it can be helpful to grasp the nosepiece ring and slightly rock the objective back and forth, then return the objective to its stable position with the oil between the lens and the slide.*

6. While looking through the eyepieces, using only the fine focus knob, gradually raise the stage to bring the specimen into focus.**

7. Another approach is to watch the objective lens from the side and slowly raise the stage so the objective lens is as close to the cover slip as possible, and then look through the eyepieces and move the specimen away from the lens until it is in focus.

8. Done! When you're finished working with the oil immersion objective, be sure to clean the objective lens, slide, and any other parts that came in contact with the oil. Not doing so may damage the optics, as the oil itself can potentially dissolve the cement used in the lens assembly if it is left on the objective.***

Cleaning the Objective
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Dip the end of a lint-free cotton swab into a cleaning solution. Shake off any excess fluid. Using the tip of the cotton swab, rub the lens starting from the center and proceeding outwards using a circular motion.

For the cleaning solution, pure distilled water is an effective, safe fluid. Commercial lens cleaners are also suitable. However, avoid cleaning agents that contain ammonia, which may damage anti-reflective coatings.

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Footnotes

* Higher magnification objectives like the 100x often have a concave front lens to correct for flatness of field, and air bubbles can get trapped in the indentation.

** It's important to work carefully here since one can easily crash the objective into the slide, sending pieces of glass and oil into the substage condenser or damaging the front lens.

*** Rottenfusser, Rudi; Wilson, Erin E.; Davidson, Michael W. Microscope Cleaning and Maintenance.

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