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A periscope is an instrument for observation over, around or through an object, obstacle or condition that prevents direct line-of-sight observation from an observer's current position. Periscopes are used in tanks and submarines where it is necessary to see from a protected position. The periscope is the eye of the submarine. It was invented and developed solely for the purpose of providing a means to view the surface without fear of detection by surface craft. While it is primarily simple in principle, actually it is a complicated piece of apparatus. It is probable that all the navies of the world have similar instruments with only minor variations. Periscopes have also been used for viewing the interiors of nuclear reactors. Simple periscopes are sometimes used by onlookers at parades, for seeing over the heads of the crowd.