All crewmembers of a tank have to have some idea of what is going on outside - especially the commander, gunner, and driver. To give the tank crew a view outside, tiny slits in the tank's armor (vision slits) were added with protective guarding plates. These vision slits were too hard to shoot through for an average marksman, and enabled the crewmen to see outside.

The idea of the copula and then of the episcope were further expansions on the concept.

WHERE TO LOOK FOR VISION SLITS

Look for small slits in the tank's armor, usually surrounded by a steel protrusion (see right) which makes it impossible for the vision slit to be hit from the side.