Abstract: A furnace peep sight utilizes a pair of glass plates removeably mounted within passageways of a truncated conical body journaled for rotation and partly mounted within the wall of a furnace. A passageway, useful for sighting purposes, passes through both of the glass plates extending transverse to the longitudinal axis of the body so as to position, in one angular location of the body, one of the glasses inwardly towards the interior of the furnace and the other glass plate located outwardly from the exterior surface of the wall. Further rotation of the body permits the glass plates to be aligned transverse to a sight hole otherwise coaxially aligned the openings in the body.