It is used to examine live microorganisms
that are either invisible in the ordinary light microscope, cannot be stained by standard methods, or are too distorted by staining that their characteristics cannot be identified.
Instead of a normal condensor, it uses a darkfield condensor that contains an opaque disc. The disk blocks the light that would enter the objective lens directly. Only light that is reflected off the specimen enters the objective lens. Because there is no direct background light, the object appears light against a dark background.