Periscope
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A periscope is an instrument for observation from a concealed position. In its simplest form it is a tube in each end of which are mirrors set parallel to each other and at an angle of 45° with respect to the line between them. It may be used as a toy or for seeing over people's heads in a crowd. This form of periscope, with the addition of a simple lens, was used for observation purposes in the trenches during World War I. More complex periscopes, using prisms instead of mirrors, and providing magnification, are used on submarines.
Early examples
Johann Gutenberg, better known for his contribution to printing technology, marketed a periscope in the 1430s to enable pilgrims to see over the heads of the crowd at the vigintennial religious festival at Aix-la-Chapelle.
Periscopes, in some cases fixed to rifles, were used in World War I to enable soldiers to see out of the trenches.
Naval use
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A simple, fixed naval periscope using plane mirrors was built by the Frenchman Marie Davey in 1854. Thomas H. Doughty of the US Navy later invented a prismatic version for use in the American Civil War (circa 1891).
The invention of the collapsible periscope for use in submarine warfare is usually credited to Simon Lake in 1902, who called his device the omniscope or skalomniscope. There is also a report that an Italian, Triulzi, demonstrated such a device in 1901 calling it a cleptoscope.
A modern submarine periscope incorporates lenses for magnification and functions as a telescope. It typically employs prisms and total internal reflection instead of mirrors. It may have additional optical capabilities such as range finding and targeting. The mechanical systems of submarine periscopes are typically hydraulically powered and need to be quite sturdy to withstand the drag through water. The periscope chassis may also be used as to support a radio or radar antenna.
However, the most modern submarines no longer use periscopes. The United States Navy's Virginia-class submarines instead use photonics masts, which lift an electronic imaging sensor set above the water. Signals from the sensor set are transmitted electronically to workstations in the submarine's control center. While the cables carrying the signal must penetrate the submarine's hull, that hull opening is much smaller and more easily sealed — therefore less expensive and safer — than those required by periscopes.
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