Immersion oil fills the gap between the 100X objective and the cover slip. Only an objective marked for oil takes it, and the cleanup matters as much as the drop.
One drop between the slide and the oil objective.
What an oil immersion objective is
An oil immersion objective is a high-power lens made to look through a drop of oil instead of air. AmScope's 120 Series user guide explains that the oil removes air from the light path to keep the image sharp at high magnification.
The most common one is a 100X objective with a numerical aperture of 1.25 or higher, marked Oil on the barrel. AmScope says to keep oil off lenses not made for it.
Focus on the area you want at 40X first. Finding something new under oil is slow, so pick the spot before the drop.
Check the objective. An oil objective has Oil printed on its barrel, and on most microscopes it is the 100X.
Placing the drop
One small drop is usually enough. Add a fresh one if the view goes soft.
Swing the nosepiece halfway between 40X and 100X, so no objective sits over the slide.
Put one small drop on the cover slip over your spot. Keep the dropper tip off the glass.
Rotate the 100X objective into the drop. The oil spreads across the front lens as it clicks into place.
Focus with the fine focus knob only. At 100X the lens sits very close to the slide, and the coarse knob can drive it into the glass.
Too much oil
Oil running down the side of the objective means the drop was too big. Wipe it off and start again with a smaller one.
Cleaning up
Clean the 100X objective as soon as you finish. Draw a piece of lens paper across the lens with no pressure, then repeat with a clean part of the paper until no more oil comes off.
Oil on the metal around the lens can be wiped away with more paper, keeping clear of the lens itself.
Oil left on a lens dries and hardens, and water-based cleaners do not lift it. A drop of isopropyl alcohol on the paper takes off what dry paper leaves behind.
Choosing an oil
Immersion oil is sold by type, and Type A is a low-viscosity oil. If your microscope's manual names a type, use that one.
The oil is made to bend light the way glass does, with a refractive index of about 1.515. The AmScope oil below lists 1.515 at 23 °C.
Oil bends light the way glass does, so steep light reaches the lens instead of bending away. Gap not to scale. Open the printable diagram
Oil, slides and lens paper on Amazon
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Blank slides
AmScope BS-50P-100S-22 blank slides and cover slips
50 pre-cleaned glass slides, 25.4 x 76.2 mm, with 100 square glass cover slips, 22 x 22 mm and 0.15 mm thick.
