ORIGINAL: AW1Steve
It sounds like a "deck prism". (I have an inverted one on my side board on a light stand). They are a triangular peice of glass, installed flat into the top of the deck, with the lower part extending below deck , and using the prisim effect to intesify and cast light below decks on sailing ships.
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Looks like the same idea by different means. When did they tart using this? Mid to late 1800s I'd guess.