You've got an old school stereoscopic rangefinder. It works the same way your eye's depth perception works. When you view an object through the stereoscopic rangefinder, unless the focus dial is correct, it'll look blurry and/or like two images. Then you turn the dial until the dual image focuses into one clear image. What you're doing when you turn the dial is moving the lenses so that the line of sight converges on on point. Then, through a simple mechanical computer and trigonometry the rangefinder, by knowing the exact distance between the lenses and the convergence angle, displays the range to the target. This type of ranging system has been used on naval warships for line-of-sight naval gun laying for over a century (of course, it's now obsolete).

Mike