Prismatic Lens

From International Dictionary of Marine Aids to Navigation
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An assembly consisting of a bullseye together with concentric dioptric and (sometimes) catadioptric rings, the foci of which are coincident with, or lie within the boundaries of, the source, when correctly focused (in sense 2 of 2-2-155).

Ch2 Fig10.jpg

Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)

International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM