"A prism inserted into the deck which provides light down below”  

Marine Terms Online 

 

“… [e]quivalent to a skylight, a thick glass pane set in the deck in order to let sunlight illuminate the deck below… [t]he top of the pane is flat set flush so it can be walked on, the bottom may be angular and pyramidal shaped to better disperse the light (in which case it may be called a ‘deck prism’” 

The Nautical Cyclopedia, Gentile, Gary 1995, p. 43..

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