Abstract: A gear driven and spring biased elongated generally diametrically extending wiper assembly is provided for at least the central area of a circular sight glass. The gear drive enables high torque transfer to the wiper assembly and insures precise movement thereof and the spring biasing of the wiper assembly insures the desired frictional contact of the wiper assembly with the opposing sight glass surface. In addition, the wiper assembly is mounted wholly within a central opening formed in base structure for the associated sight glass assembly to be secured to the exterior of a vessel having a viewing port formed therein.