In light microscopy, oil immersion is a technique used to increase the resolving power of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the numerical aperture of the objective lens.
Two Leica oil immersion objective lenses. Oil immersion objective lenses look superficially identical to non-oil immersion lenses.
Oil-immersion objective in use
The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microscope and were possibly invented in their present compound form in the 17th century. Basic optical microscopes can be very simple, although many complex designs aim to improve resolution and sample contrast.
Scientist using an optical microscope in a laboratory
A miniature USB microscope
The oldest published image known to have been made with a microscope: bees by Francesco Stelluti, 1630
Basic optical transmission microscope elements (1990s)