Abstract: An instrument assembly is mounted to view the inside of a furnace or boiler, its probe extending into a port in the furnace wall. To remove slag built up within the port, a deslagger unit is mounted upon the assembly and includes a shiftable blade surrounding the probe lens. A displacing mechanism mounted upon the assembly is periodically actuated to rectilinearly shift the blade within the confines of the port to dislodge built up slag therein.