Immersion oil is used with high power objectives such as light microscopes.
Light microscopes have an upper limit to their resolving power of marginally over 1,000 x. At this level of magnification, the microscope needs to direct every available amount of light in order to achieve a clear image. The immersion oil helps to reduce the refraction since it has a refractive index equal to glass. As a result, it forms a continuum between the objective lens and the slide, thereby successfully ensuring that more light is directed towards the specimen and ultimately, a clearer image.
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Immersion oils are commonly available in two viscosities; low viscosity (A), and high viscosity ( B). The low viscosity oil is applied to the airspace between slide and objective, the high viscosity oil is applied between the condenser and the slide.
How to Use it:
Type A: Locate a specimen on the slide and center it in the image field. Rotate the nosepiece until the 100x objective lens is just to one side of the slide. Place a single drop of immersion oil on the slide cover slip and place a drop directly on the objective lens. Combined, both drops ensure no air is trapped in between. Rotate the 100x objective into place and adjust the fine focus to fully resolve the image.
It is very important to carefully clean the oil off your objective lens before it dries. Carefully wipe oil from all glass surfaces with a folded piece of clean lens paper. Moisten a piece of lens cleaning paper with lens cleaning fluid and wipe away any residual streaks of oil.