Voigtlander Telemeter (1157)
Info
Rangefinder that can be attached on top of a (old) camera to determine the distance to the subject. The parallax principle is used to, by means of the rotating knob, adjust the image so the subject images from both front lenses are on top of each other. The distance can then be read from the scale on the knob.
The scale runs from 1 to 20m and infinity.
References/Links/More Info
Mercury Camera - Article on rangefinders