Abstract: The invention provides novel apparatus for detecting and marking defects in flat glass advancing on a conveyor, for example a ribbon of float glass. There are two viewing stations on opposite sides of the center of the conveyor and spaced longitudinally so that there is staggered viewing of the two halves of the width of the advancing glass. Light reflected obliquely through each half is received on a viewing screen mounted above the respective side of the conveyor. Images of defects are recognized by examiners or cameras viewing the screens and the defects are marked by operation of appropriate marker guns positioned above the conveyor.