An optical instrument that permits an observer to view his surroundings
along a displaced axis, and hence from a concealed, protected, or submerged
position. The simplest periscope, used in tanks, has two parallel reflecting
surfaces (prisms and mirrors).
An auxiliary telescopic gunsight may be added. Submarine telescopes have
a series of lenses within the tube to widen
the field of view, cross-wires, and
a range-finder, and can rotate and retract.
AND OPTICAL PHENOMENA