A fibre optic ring illuminator normally used as a reflected light source with a low-powered microscope is here used as a substage illuminator with a stereo zoom microscope, to give uniform dark field (dark ground) illumination. This development was primarily for use in colour video recordings giving good contrast pictures but can also be used successfully in cine and photomicrography, at magnifications ranging from X 1.5 to X 124. Its application is particularly suitable for the observation of biological specimens.