Why do some objectives require immersion oil or water?
The resolving power of an objective lens depends largely on its numerical aperture (its NA), which in turn depends on the refractive index of the medium between the specimen and the lens. A higher refractive index means the lens can gather more light and deliver images of better resolution and intensity. When air, with a relatively low refractive index, is the medium between specimen and lens, lower NA objectives perform to their capabilities. Higher NA objectives, however, need a higher refractive index to fulfill their capabilities, and oil provides that higher index. For optimum performance at an NA above 1.0, you should also oil the top lens of the condenser to the bottom of the slide. Immersion objectives carry the inscription oil or oel. Some objectives, particularly those used in the observation of living biological specimens, require water as the immersion contact medium. These objectives carry the inscription “WI.” • In order to preserve the darkness of the background during d
