Microscope 101: Oil Immersion Lens Technique

Oil immersion applies only when the exact objective is marked or specified for an immersion medium. High magnification or a 100x label alone does not establish that oil is required. If the objective model, compatible medium, coverslip requirement, or focusing procedure is unknown, stop and check the manufacturer instructions before adding oil.

A compatible immersion medium reduces refraction at the glass–medium interface and supports the numerical aperture for which an immersion objective was designed. Use only the medium specified for the exact objective; do not substitute cedarwood oil, Type A oil, or another medium without current compatibility evidence.

Locate and center the area at low power, then follow the exact instrument and objective instructions for moving to the immersion objective. Coverslip requirements, medium placement, and quantity depend on the exact objective and preparation. Use only the specified small local droplet or pool at the area to be imaged. Approach slowly because immersion objectives can have very short working distances, and stop before contact or whenever the focusing procedure is uncertain.

After detail is acquired, make only careful fine-focus adjustments within the manufacturer-specified procedure. Clean the immersion medium promptly using the approved method; do not let residue dry. Stop if the slide or objective could make contact, if oil enters unintended areas, or if the setup or controls are uncertain.

Clean immersion oil promptly and safely. Switch off the microscope and nearby equipment as directed. Remove loose dust first, then use only the lens paper, approved optical cleaner, wipe pattern, and inspection method specified by the objective or microscope manufacturer. Ventilate the area and observe the cleaner label, SDS, and flammability precautions. Do not use butane lighter fluid, turpentine, paint thinner, or another improvised household solvent. If cleaner compatibility, residue, or possible damage is uncertain, stop and contact the manufacturer or a qualified service provider.

Bonus Tip: I’ve run out of immersion oil. What type do I buy? And from where?

These days, it’s getting increasingly difficult to find immersion oil in bulk. I’m not 100% sure why, but generally you’ll want at least 1 small bottle so that you have an easy to use dripper or pipette to drop it onto the slide with. Imaging trying to pour just a drop from a large gallon sized container! What a mess that would make!

I did spend a bit searching, and was able to find a decent sized supply bottle, as well as a quality small bottle as well. Here are my suggestions for immersion oil when you’ve run out:

Small Bottle:

Large Bottle:

Clean immersion oil promptly using the exact manufacturer-approved materials and method; stop if residue, damage, or cleaner compatibility is uncertain.

If this helped you, be sure to leave me a comment below and let me know! I’m happy to help answer any questions!