Dark ground microscopy is used for ?

Darkfield / Dark ground microscope in which reflected light is used instead of transmitted light used in the ordinary microscope. The essential pa of the dark field microscope is the dark field condenser with central circular stop, which illuminates the object with a cone of light without letting any ray of light fall directly on the objective lens. Light rays falling on the object are reflected or scattered onto the objective lens with the result that the object appears self-luminous against a dark background. The contrast gives an illusion of increased resolution, so that very slender organism such as spirochetes, not visible under ordinary illumination, can be clearly seen under a dark field microscope. Treponema pallidum immobilization (TPI) - pg no 375 The first in the group is the test introduced in 1949. the test serum is incubated with complement and T.pallidum maintained in a complex medium anaerobically. if antibodies are present, the treponemes are immobilized that is rendered non-motile when examined under a dark ground microscope Ref: Textbook of Microbiology; Ananthanarayan and Paniker; 9th edition; Pg no: 11