The use of reflecting mirrors for the purpose of observing from cover is no novelty, and during the trench warfare of the Crimean War 1854-5 a device was patented which scarcely differs from the simple mirror periscope of the World War.
Krupp for use for the outer tube of the German navy periscope used before the war, and a similar steel was developed and used in the British service, but it is costly and more difficult to machine to the required accuracy than is the case with bronze.
The modern submarine periscope consists essentially of a long tube, the top of which is just above the water when diving, while the lower end passes through a stuffing box on the shell of the boat into the control-room.
The use of aircraft for anti-submarine work led to the demand for a periscope which could be used for looking overhead.
A neat rainguard made of sheet metal, to the same curve as the body of the periscope and almost 8 inches long, is attached to the upper prism box by two spring straps.
Quite recently, the camera obscura has come into use with submarine vessels, the periscope being simply a camera obscura under a new name.
For purposes of torpedo attack the periscope is used as a range-finder to determine the distance the target is away, and also in connexion with tables to determine the correct time to fire the torpedo, allowing for the speed of the enemy, course, etc. Officers of submarines have devised various mechanical devices to avoid calculations, and these have been added to the periscope.
The periscope when installed in the submarine is used for two purposes: (a) general observation for submerged navigation; (b) for correctly aligning the submarine when firing a torpedo at a target.
When in use, it is held at right angles to the periscope above the upper window by a bayonet catch; when not in use, it is lowered and sprung round the body of the periscope just below the upper prism box.
Continuous use of a periscope is very trying for the observer's eyes, and for use in bright weather light-filter screens are provided to reduce the glare.