From International Dictionary of Marine Aids to Navigation
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The property of those optical elements which alter the direction of a light ray from its original direction by refraction only. (Figs. 10, 11a and 16)

Ch2 Fig10.jpg

Ch2 Fig11.jpg

Ch2 Fig16a.jpg

Ch2 Fig16b.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