·add. ·noun An optical device for making an indicator diagram for high-speed engines. It consists of a light-tight box or camera having at one end a small convex mirror which reflects a beam of light on to the ground glass or photographic plate at the other end. The mirror is pivoted so that it can be moved in one direction by a small plunger operated by an elastic metal diaphragm which closes a tube connected with the engine cylinder. It is also moved at right angles to this direction by a reducing motion, called a reproducer, so as to copy accurately on a smaller scale the motion of the engine piston. The resultant of these two movements imparts to the reflected beam of light a motion similar to that of the pencil of the ordinary indicator, and this can be traced on the sheet of ground glass, or photographed.