The word " periscope" was first applied to this instrument.
The modern practice is to take rapid observations rather than to keep the periscope above the water all the time.
Zeiss made a periscope 7 metres long, main tune 150 mm.
Of these the most remarkable is the German Giant Periscope, two specimens of which exhibited in the collection of trophies in the Imperial War Museum, Crystal Palace, have excited considerable popular interest.
In an ingenious periscope designed by Messrs.
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 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 the periscope is not in use, the prism is lowered and protects the upper lens in the body.
It is therefore necessary to train the periscope round when taking observations on different bearings.
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.
And in the World War, while optical instruments of this kind were elaborated and improved, the periscope as such came into use for the infantry garrisoning trenches.
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.
This periscope is considerably larger than any others, and was designed for observing over obstacles of between 9 and 26 metres in height.
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.
To facilitate this mechanical lifting, gear is provided which is readily controlled, and can raise or lower the periscope at a speed approaching 25 ft.
The use of aircraft for anti-submarine work led to the demand for a periscope which could be used for looking overhead.
In the skysearching periscope the upper prism can be rotated by mechanism inside the periscope, so that aerial observations can be readily made before the submarine " breaks surface."
Quite recently, the camera obscura has come into use with submarine vessels, the periscope being simply a camera obscura under a new name.
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.
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.
The simplest form of periscope, and that most generally used by troops, consisted of a tube, rectangular in section, provided with two mirrors, the upper of which, inclined at an angle of 45° to the axis of the tube, reflected the image of the foreground vertically downwards to a second mirror, also inclined to the axis at 45° into which the observer looked.