Type of light used in dark ground microscopy: September 2005

Ans. B: Reflected light Dark ground microscopy is the most specific and sensitive technique to diagnose syphilis when an active chancre or condyloma lata is present. This technique allows for a presumptive diagnosis of syphilis even before antibodies have developed. Although, unstained living organisms may be observed with an ordinary light microscope, they are seen much more clearly and with a better resolution with a dark ground microscope, which has an optical system that enhances the contrast of unstained bodies. Principle of dark ground microscopy: The dark ground microscope creates a contrast between the object and the surrounding field, such that, the background is dark and the object is bright. The objective and the ocular lenses used in the dark ground microscope are the same as in the ordinary light microscope,however, a special condenser is used, which prevents the transmitted light from directly illuminating the specimen. Only oblique scattered/reflected light reaches the specimen and passes onto the lens system causing the object to appear bright against a dark background Transmitted light is used in the ordinary microscope.