"A prism inserted into the deck which provides light down below.” Marine Terms Online. “The deck prism consists of a cast glass prism in a metal farm, mounted flush with the deck… [t]he prism reflects light coming below, admitting much more light than you would expect from the size of the hold cut in the deck.” Commission and Decommissioning: Preparing a new boat, seasonal launching and layup, Keith, Lawrence, 1988, p. 14.
“… [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..
On coal ships deck prisms were used to monitor the cargo below. If the coal caught fire, light from the flames would be collected by the prism and be visible on deck.
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