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Deck Lights / Deck Prisms
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Types: 1 of 4

·Daniel Adamson
·Charles W. Morgan

·Timpson & Lawrence [1855]
·Tiebout [1894]
·Durkee [ca 1915]
·Laughlin [1920]
·Tiebout [1920]
·Davey [1949]
Group of original deck prisms
Group of original deck prisms
See more in the deck prisms gallery

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.

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 shapes which distribute the light.

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.

Deutsche Luxfer Prismen Gesellschaft deck prisms
Deutsche Luxfer deck prisms

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" in the US and "pavement lights" in the UK.

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 (see current manufacturers/suppliers), some in the original forms (or close to it), and some in a newer tank-periscope-like rectangular design.

Preston's Patent Illuminator
In 1818, Grant Preston of Middlesex, Brazier (England) was granted a patent for his "IMPROVEMENT IN THE DECK GLASS RIM AND SAFETY GRATE", No. 4,222. From the patent:
"The outside part consists of a brass or copper strong screw rim, with a broad flat edge to let into the deck as a fixture; the inside part is another rim which screws into the outside or fixed rim, containing a flush glass, but partly convex on the under side. The safety grate is likewise fixed into a screwed rim of the same size and strength as the one that contains the glass, each being made to screw either right or left, so that when the glass is wanted to be taken out for air, it only rests with the person or persons below to take hold of the handles and screw it; it then may be hung upon a hook to prevent rolling about; the safety grate is then ready to screw in in lieu thereof, being quite flush when screwed up; also a ventilating fly fits into the latter rim from below. The handles also answer the end of their being made secure, by padlocking them when the vessel is laid up."
Preston's Patent Illuminator Preston's Patent Illuminator Preston's Patent Illuminator Preston's Patent Illuminator
The Repertory of Arts, Manufactures, and Agriculture — Volume XXXII.—SECOND SERIES. 1818.

Preston's deck light is best known for being original equipment on the ill-fated 1845 Franklin Expedition, when Sir John Franklin sailed in search of the Northwest Passage: the HMS Erebus and the HMS Terror did not have portholes, but used Preston's illuminators instead. The HMS Investigator, sent in 1848 to search for Franklin's lost expedition, also also fitted with illuminators. In 2015, Parks Canada dove the Erebus and recovered an illuminator (and other artifacts):

Preston's Patent Illuminator Preston's Patent Illuminator
Parks Canada underwater archaeologist Ryan Harris
Photo: Parks Canada / Thierry Boyer / 89M0626EF
Photo: Parks Canada / Thierry Boyer / 89M-2015-4533

White and Atkey ad, 1839
White and Atkey, General Ironmongers, Ship Chandlers, Braziers, Copper-Smiths,
Zinc, Iron, and Tin Plate Workers, Smiths, Locksmiths, and Bell Hangers.
Stove Grate, Ship's Hearth, and Water Tank Manufacturers.

By Appointment. 30, High Street, West Cowes.
Agent for Grant Preston's Patent Binnacles, Stoves, and Ventilating Illuminators.
The Vectis directory, or Isle of Wight general guide, 1839

Recovered Shipwreck Prisms
The following pictures of deck prisms recovered from shipwrecks were all provided by and are © 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.

Current Manufacturers/Suppliers
The Charles W. Morgan replicas are made in vast numbers and available many places; I'm not going to list them here.

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