A pivoting, compensating parallelogram linkage system converts rotary motion into axial linear motion of a portion thereof suited for critical uses such as carriage of a moving mirror in a Michelson interferometer. The compensating includes a coupling provision that in response to angle change in one part of the linkage imposes an equal but opposite angle change in another part of the linkage. The coupling may include a motor to reciprocate the portion usable as a carriage.
