Answer: periscope

AnswerCrossword Clue
PERISCOPEOptical instrument at sea.
periscopea device (eg a tube with mirrors) by which an observer can look at something which is not in the direct line of sight
periscopeAn apparatus consisting of a tube attached to a set of mirrors or prisms, by which an observer (typically in a submerged submarine or behind a high obstacle) can see things that are otherwise out of sight
PERISCOPEView enhancer
PERISCOPEMeans of looking above and beyond
PERISCOPEViewer on the Nautilus
PERISCOPEView-finder for Rickover
PERISCOPESeagoing eye
PERISCOPESubmarine instrument.
PERISCOPESub apparatus
PERISCOPEOptical instrument with mirrors.
PERISCOPENautilus adjunct.
PERISCOPEOptical instrument used in trenches.
PERISCOPESub viewer
PERISCOPETubular submarine device
PERISCOPESherman tank device
PERISCOPESubmarine's viewing instrument