Check the Objective Lens Style
High power objective lenses on optical microscopes come in dry or oil immersion types. Before carrying out any work, the user must first understand what type of lens the microscope has. Oil immersion lenses work by the user placing a drop of special oil on the slide and carefully positioning the lens in the oil to view the specimen. Using oil with a high power dry lens, however, can foul the lens by damaging the lens coating. A user must always take the precaution of checking which high power lens style a microscope uses before commencing work.
Understand the Focus Knobs
Many microscopes have two focus knobs, one for coarse adjustments and one for fine. Clicking a high power objective lens into position brings it close to the slide. Microscope users should manipulate the coarse focus adjustment knob when watching the lens from the side of the microscope to reduce the risk of driving the lens into the specimen. When looking through the microscope eyepiece, the user should only manipulate the fine focus knob. Understanding which focus knob is which enables the use of the correct knob during operation and reduces the risk of damaging the lens or slide.
Focus Away From the Specimen
When bringing the specimen clearly into view using a high power lens, the user should preferably do this by adjusting the focus so that the lens moves away from the slide. The user should first view the lens from the side of the microscope, using the coarse focus knob to move the lens as close as possible to the slide and then view the specimen through the eyepiece and use the fine focus knob to move the lens away from the specimen to bring it into view. This precaution prevents the user from driving the lens through the slide, which may happen if the user focuses by moving the lens towards the slide.
Cleaning the Lens
A simple precaution that can help extend the useful life of a high power oil immersion lens involves the user cleaning it after use. Immersion oil left on a lens may harden and removing the oil after it hardens increases the chances of damaging the lens. Lens paper, manufactured especially for cleaning optical lenses, provides the best option to remove the oil after use while it is still wet.