Liquid compass; Lens/eyepiece cap?

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Lens/eyepiece cap? found in the compass box. From a navigational instrument?

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ID:	ACO0213.2
Type:	Liquid compass; Lens/eyepiece cap?
Display location:	Not on display
Creator:	Unknown
Date made:	Unknown
Credit:	National Maritime Museum, Greenwich, London, Admiralty Compass Observatory
Measurements:	Diameter: 27 mm;Overall: 2 mm
Parts:	Liquid compass Liquid compass; Box (ACO0213.1) Liquid compass; Lens/eyepiece cap? (ACO0213.2)