Periscope

A periscope was an instrument for observation from a concealed position.

On Dulkis, the Jamie McCrimmon, Cully, Kando, and Zoe Heriot all used a periscope in a bunker among some ruins on the Island of Death. Through it, they spied on the mining operation conducted by the Dominators and Quarks. (DW: The Dominators)

Commander Ridgeway used a periscope in his submarine while searching for Sea Devils. (DW: The Sea Devils)