Oil Immersion Techniques

The oil immersion lens derives its name from the fact that a special mineral oil is between the lens and the microscope slide.

Oil is used as a medium primarily because its refractive index is the same as glass's refractive index. This similarity prevents the loss of light due to diffraction (the bending of light rays as they pass through the air). Utilizing oil in this way enhances the resolving power of the microscope.

Parfocalized objectives allow students to go to oil immersion from low power or high-dry. When the microscope has been brought into focus at a specific magnification, rotation of the oil immersion lens is feasible without fear of striking the slide.

Prior to rotating the oil immersion lens into position, a drop of oil is placed on the slide. Caution should be taken to avoid the oil from becoming cloudy; if this happens, the slide should be discarded. Other precautions that should be taken include opening the diaphragm as much as possible to increase the resolving power of optics and keeping the condenser at its highest point. If multiple colored filters are available for the lamp housing, it is best to use blue or greenish filters to improve resolving power. At the end of the laboratory period remove all immersion oil from the lens tip with lens tissue.

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