Abstract: A sight tube for ultraviolet monitoring of liquid and capable of handling industrial scale flow rates for example 40 gallons (180 liters) per minute in which the shape is especially designed to allow oppositely disposed windows, through which the ultraviolet light passes, to be relatively close together. The sight tube has an interior the central section of which has a flat sided portion adjacent and parallel to the windows, the flat sided portion being flanked by two side portions which are thicker than said flat sided portion, the tube having cylindrical end sections joined to the central section by intermediate sections which provide gradual transitions between the section shapes.