A periscope is a vertical tube which people inside submarines can look through to see above the surface of the water. a long tube with mirrors fitted in it, used to look over the top of something, especially to see out of a submarine (peri- (from ; PERICARDIUM) + skopos ). Optical instrument (see optics) used in land and sea warfare, submarine navigation, and elsewhere to enable an observer to see the surroundings while remaining under cover, behind armour, or submerged. A periscope includes two mirrors or reflecting prisms to change the direction of the light coming from the scene observed: the first deflects it down through a vertical tube, the second diverts it horizontally so that the scene can be viewed conveniently
Optical arrangement which feeds light (usually from the sky background) into the slit of a spectrograph These can be used when the object being observed would otherwise fill the slit and so no sky signal would be recorded