A periscope was an instrument for observation from a concealed position.
On Dulkis, 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. (TV: The Dominators)
Commander Ridgeway used a periscope in his submarine while searching for Sea Devils. (TV: The Sea Devils)