You may have noticed the inscription “oil”, “oel” or “WI” on your microscope objective lens. This refers to an immersion objective, and is typically found on higher magnification objective lenses (50x – 100x). Most microscope objectives are created to allow you to view specimens by using air as the medium between the objective lens and the cover slip.
An immersion objective requires a drop of immersion oil, or sometimes water be placed between the tip of the microscope objective lens and the cover slip. This reduces the refractive index differences between the glass and the imaging medium, allowing the user to achieve a numerical aperature of 1.0 or above, while greatly improving the microscopy image.
When using an immersion microscope objective, the image appears very poor if not using the proper immersion liquid. The letters “WI”, “W”, “Water” and “Wasser” refer to an objective that requires water for the immersion medium. “Oil” and “Oel” objectives require immersion oil. “HI” refers to homogeneous immersion and “Gly” stands for glycerol immersion. Immersion objective lenses are especially useful when observing living biological specimens.
When finished using the immersion oil objective, be sure to clean it properly so it does not attract dust or form a thick foggy film on your lens. If the 100x oil immersion objective is used frequently, it is often a good idea to mount the dry 40x objective on the opposite side of the microscope nosepiece from the oil objective. This reduces the likelihood of accidentally dipping the 40x dry objective into immersion oil as you rotate the objectives.