a procedure in which a black circular shield is used to block most of the vertically directed rays of light (e.g., the field is dark), and a circumferential, suitably angled, mirrored surface is used to direct the peripheral rays horizontally against the object, thereby reflecting the light vertically through the objective lens and along the optic axis; thus, the object is well illuminated in a contrasting dark background. SYN: dark-ground illumination.