The lensatic compass is (was) the typical military field compass, and is used principally to measure or lay out magnetic bearings. It can also be used to orient a map. The "lensatic" means that the rear sight is provided with a lens that allows you to read the dial while your eye is close to the rear sight, as it should be. The lid of the compass opens up to provide a slot with a crosshair as the front sight. This crosshair is actually a sturdy metallic wire. When you sight a distant object, the magnetic bearing can be read from the dial to an accuracy of a degree or two, with care.