Coincidence rangefinder

A coincidence rangefinder (stereoscopic, parallax, or split-image rangefinder) is a type of rangefinder that uses mechanical and optical principles to allow an operator to determine the distance to a visible object.

The device consists of a long tube, with two lenses facing forwards at either end, and an operator eyepiece in the center. Two prism wedges which, when aligned result in no deviation of the light, are inserted into the light path of one of the two lenses. By rotating the prisms in opposite directions using a differential gear, a degree of horizontal displacement of the image can be achieved.