GoldFever said:
From Glassian.org-
A deck prism is a prismatic glass set into a ship's deck, to let natural light below. They are especially useful when an open flame would be hazardous, such as with a cargo of coal or gunpowder. On colliers (coal ships), prisms were also used to spy on the cargo hold; light from a fire would be collected by the prism and be make visible on the deck¹ even in daylight.
In normal usage, The prism hangs below the ceiling and disperses the light sideways; the top is flat and installed flush with the deck, becoming part of the deck. A plain flat glass would just form a single bright spot below-- not very useful general illumination-- hence the prismatic shape.
The names "deck light" and "dead light" or "deadlight" are sometimes used, though the latter seems to be uncommon as a reference to prisms, as more often refers to plain-glass panels.
The earliest deck prisms with provenance are from the 1840s. Presumably they were used earlier, but how much earlier is unknown; the origin of the idea is lost, and glass is difficult to date. Very few original specimens remain. The Charles W. Morgan (1841), last surviving American wooden whaler and National Historic Landmark, had a single deck prism left. This ship has now been restored and is residing at Mystic Seaport. The original deck prism has been reproduced and is widely available.
This idea was later borrowed for use in urban sidewalks, to light underground spaces. In that application, they are called "vault lights" or "sidewalk lights". Model Shipways makes a model of the Charles W. Morgan that includes 21 tiny deck prisms!
Deck prisms are alive and well today. They are still being made, some in the original form (or close to it), and some in a newer tank-periscope-like rectangular design.
