Answer:

periscope

Answer	Crossword Clue
PERISCOPE	Sub apparatus
PERISCOPE	Submarine's viewing instrument
PERISCOPE	Sherman tank device

PERISCOPE	Tubular submarine device
PERISCOPE	Sub viewer
PERISCOPE	Optical instrument used in trenches.
PERISCOPE	Nautilus adjunct.
PERISCOPE	Optical instrument with mirrors.
PERISCOPE	Optical instrument at sea.
PERISCOPE	Submarine instrument.
PERISCOPE	Seagoing eye

PERISCOPE	View-finder for Rickover
PERISCOPE	Viewer on the Nautilus
PERISCOPE	Means of looking above and beyond
PERISCOPE	View enhancer