Periscope

A periscope is an optical instrument, typically a tube fitted with mirrors or prisms at each end, that lets an observer see over, around or through an obstacle that blocks a direct line of sight. Johannes Hevelius described an early lens-based version he called a polemoscope in his 1647 work Selenographia, but the first practical naval periscope is credited to the Frenchman Hippolyte Marie-Davy in 1854, a vertical tube with two small mirrors fixed at 45 degrees at each end. The instrument became essential naval and military equipment over the following century: Simon Lake fitted periscopes to submarines from 1902, Howard Grubb refined the design during the First World War for use in trenches and gun turrets, and Rudolf Gundlach patented a rotary periscope in 1936 that was first used in Polish tanks before British and American forces adopted the design.