Periscope

n.	1.	A general or comprehensive view.
	2.	an optical instrument of tubular shape containing an arrangement of lenses and mirrors (or prisms), allowing a person to observe a field of view otherwise obstructed, as beyond an obstructing object or (as in submarines) above the surface of the water.

Noun

1.

periscope - an optical instrument that provides a view of an otherwise obstructed field

Aqua-Lung, air cylinder, aquascope, bathyscaphe, bathysphere, benthoscope, diving bell, diving boat, diving chamber, diving goggles, diving helmet, diving hood, diving mask, diving suit, scuba, snorkel, submarine, swim fins, wet suit