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Deck Lights / Deck Prisms
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Vessels
·Daniel Adamson
·Charles W. Morgan

Catalogs
·Timpson & Lawrence [1855]
·Tiebout [1894]
·Durkee [ca 1915]
·Laughlin [1920]
·Tiebout [1920]
·Davey [1949]

A deck light or 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.

Group of original deck prisms

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 "dead light" or "deadlight" are sometimes used, but more often refer to non-opening, plain-glass lights.

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 (vaults); in that application, they are called "vault 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.


Recovered Shipwreck Prisms
The following pictures of deck prisms recovered from shipwrecks were all provided by (and ©) Captain Daniel Berg, expert wreck diver (see aquaexplorers.com), author of many books on the subject, host and producer of the Dive Wreck Valley TV series, holder of 6 current US diving equipment patents, and much more.
Deck prism recovered from the wreck of the Cornelia Soule Deck prism recovered from the wreck of the Frank Buck Deck prism recovered from the wreck of the Iberia Deck prism recovered from the wreck of the Republic
Capt. Dan holding a reeded deck light from the wreck of the Cornelia Soule, a three-master schooner which ran aground in 1902 with a load of cut granite jetty stones. "Spike" prism from the wreck of the Frank Buck, a copper-clad wooden schooner whose history is unknown. Captain Ed Slater holding an unusual truncated rectangular prism from the wreck of the steel-hulled freighter Iberia, built 1881, which sank in 1888 following a collision with the Cunard luxury liner Umbria. Haywards vault light/deck prism from the wreck of the Republic, a White Star Line steamship built 1903, which sank in 1909 following a collision with the Italian steamship Florida.

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Notes
¹ Jane Stevens, on the Maine PBS Quest program "Shipwrecks!" (#202)