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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.

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.