pair of opera glasses. In side is a periscopic arrangement so that you are looking through the ends as shown in fig 2. Focussing the object you see it as depicted below
using the thumbscrew marked x in fig 1 you twist this until the object is perpendicular with its inversion as shown, and the same distance from the thin black line in the middle. The rangefinder is now set and on raising the dial cover marked v in fig 2 the range is shown.