Deck Prisms

You can see them in Museum Shops and Nautical Stores, in Gift Boutiques and Catalogs. They’ve been made in miniature as earrings and as desk curios. Or, in full size, they sit atop illuminated bases or swing in specially made chain mounts. They’re Deck Prisms, large pieces of cast glass in the shape of hexagonal pyramids. The purest ones are in slightly bluish “clear” glass, but there are versions in blue cobalt glass and in other colors – impractical for their original use, but more pleasing to the eye, especially when illuminated.

etsy image

A hexagonal Deck Prism. A hexagonal hole would be cut in the deck and

the prism would be placed into it, pointed side down.


As the documentation accompanying these make clear, these are all inspired by the “Deck Lights” or “Deck Prisms” found aboard the Charles W. Morgan, a whaling ship built in 1841 and maintained at Mystic Seaport in Connecticut. They started selling replicas in the gift shop there, and they’ve been spreading ever since.

The idea is simple and elegant – these prisms were inserted into hexagonal holes chiseled into the deck, and provided light to the deck below. The somewhat rippled sides of the hexagonal prism served to refract and diffuse the light, instead of letting it fall into a mere circular patch, as a disc of thick glass set into the deck might do. It was watertight, but let in light, and didn’t require the use of lamps or candles, and so avoided the risk of fire onboard. Most of the newly-purchased ones go on display by themselves, but some people are installing them on actual ships, where they add a touch of nostalgia (and, one suspects, a chance of painful bumps on the head).

Where and when did this idea originate? A survey of websites reveals the curious consistency of the internet – the result of people cribbing from the same websites or from each other, without checking for independent corroboration1the concept dates back to the 1840s, the sites say, and possibly centuries back before that. But the claims are suspicious and vague. None of them cites a source and they smack of assumption and wishful thinking. That one certain decade – the 1840s – looks as if it is suggested by the date of the launching of the Charles W. Morgan. But there is no evidence that the Morgan was launched with deck prisms in place. 

Searching through books, I find a couple of references to deck prisms in historical romances. Kerry Lynne’s 2013 novel The Pirate Captain: Chronicles of a Legend makes frequent mention of the “deck prism” and the greenish-blue light it casts. The story is set in the 18th century, so clearly the author (trained as a historian, and a sailor herself) either thought such deck prisms were used then, or stretches the truth for some picturesque scenes. Similarly, Delle Jacobs, in her 2009 novel Sins of the Heart, set in 1813, writes about the “dim light coming in through the deck prism” It makes for a good scene, but is it at all believable? Recent romance novels aren’t a good source for historical fact.

So we return to the original question – where and when did this idea originate?

The Oxford English Dictionary does not even list “deck prism”, but does have “deck light”, giving its first appearance as 1849. Google N-Gram Viewer, which automatically searches publications squirreled away and scanned by Google, doesn’t recognize deck-prism (or any variation of it) at all, and finds deck-light only from the late 1840s onward. It seems unlikely that the romantic pirates or the builders of the Charles W. Morgan would have used them, at least under that name.

At this point, I am indebted to Paul O’Pecko, Director of Collections and Director of the G.W. Blunt White Library at Mystic Seaport, for a critical nugget of information. The original name for a thick glass piece set into the deck or the wall of a ship, it turns out, is not deck light or deck prism, but bullseye. And with that revelation comes a vast outpouring of information, and a possible early history of the idea. I hope the reader will pardon some etymology and some speculation on my part, and a side excursion into the world of glass technology.

            For most of us, the first association of “bullseye” is the target, used in archery, pistol shooting, darts, and other recreations. But this use seems to be relatively late, dating from 1833, according to the Oxford English Dictionary. A search using the Google resources turns up earlier usages, including one from 1813 in this sense2

But an even earlier use of bullseye is to mean a glass lens, or a thick glass window. “Bullseye” lantern, meaning a glass lantern with a thick concentrating window is cited by at least 18003. One finds reference to “bullseye” spectacles and “bullseye” meaning “lenses” from the early 19th century. Why should “bullseye” be used in this context? To understand why, we must look at the early means of manufacturing window glass.

One popular method was to gather a gob of molten glass on the end of the puntil, the rod used to handle the glass and for glass blowing, and to rapidly spin this to form the glass into a disc. (Another method was to use the pontil as a blow-pipe and blow air into the gob, turning it into a sort of thick glass balloon, which was broken at the end and spun, as before, into a flat disc). The disc was kept in motion until the glass cooled sufficiently to retain the shape. When cool, the large disc of glass, which had nearly parallel front and back faces, was cut into pieces to be used for windows, especially the diamond-shaped panes that were bound together with leaden strips to form the English diamond-paned windows4.

 Four Stages of making Crown Glass

Making Crown Glass from The Cabinet Cyclopedia 1832

The four top illustrations show in cutaway view how a bubble of glass would blown on the

Pontil, pierced, and spun out into a flat disc. The bottom illustration shows the glassblower

spinning the pontil with the molten disc on it until it hardens.

The only portion that could not be used for this purpose was the very center, about which the glass disc was spun, and to which the pontil was attached. This section was necessarily thicker than the rest, a somewhat conical piece of glass without parallel front and back surfaces, and with an opaque center where the pontil was attached. It was unsuitable for window glass. Because of its shape, however, it was perfect to use as raw material to make lenses from. One name for this portion was the crown, and from it one could make Crown Glass Lenses. We still call low-index soda-lime glass a “crown” glass (in distinction to the higher index “flint” glasses that were made of lead-based starting materials. “Crown” glass was also used to refer to windows made from this section of the glass. Not useful for windows that must be optically clear for viewing, it was nevertheless useful for places where viewing was not important, but the admission of light was. Hence, crown glass was often used in places like transoms. “Crown” glass was also called “Bullseye” glass, though. Such circular pieces were called “bullseyes”, as were the lenses sometimes made from them. Lanterns using such lenses as light concentrators were “bullseye” lanterns.

Very Clean Glass Bullseye photo

Photo of decorative glass bullseye


A Bullseye from the crown of a glass disc shown alone, and as inserted into a leaded window

as a decoration.

Why? It’s significant that another name for these lenses and pieces of glass was bullion. “Bullion” is a word with an interesting and confused history, but “bullion” glass seems to be related to “bullion” referring to bars of precious metal, like gold or silver. Both can ultimately be traced back to a French word meaning “molten”, and related to “boil” (although there are competing etymologies). It surely cannot be coincidence that the similar-sounding “bullion” and “bullseye” mean the same thing. There is a characteristic tendency in British English to take foreign words and convert them into something more familiar and English. Thus,  for example, the French “Chartreuse” became “Charterhouse”5 It would not be at all surprising if the English glassmakers (who were noted for this type of glass) transformed the French term “Bullion” into the English term “Bullseye”6.

The use of the term “bullseye” then extended to items made from it, including lenses. The concentric circular patterns in the glass then provided the impetus to name the target made of concentric circles also a “bullseye” – which would thus have no direct bovine roots at all. (I have found examples of “Bull’s Eye” used with respect to a target going back as far as 1818, but it seems to have been used for glass even earlier)

But we are getting afield from our main point, which is the use of the term “bullseye” for thick pieces of glass set into the deck of a ship or into the sides to admit light without letting in water.

At this point we enter into likely surmise. Just as the despised, unwanted, and therefore inexpensive bullseyes were used for transom lights, some inventive and enterprising ship’s officer must have seen that the thick pieces of glass could be put to use for the same purpose by cutting holes in the deck and inserting the thick pieces of glass to provide below-deck illumination, or putting them in ship sides. Set firmly in place with oakum and sealed with putty, they would not leak. Being thick, they would not break under the pressure of seamen’s feet. On warships, such thick pieces of glass could provide an extra barrier between illuminating flames and the gunpowder in the powder magazine.

No documentation supports this series of conjectures, but they do provide a plausible explanation for the use of thick glass as deck lights, and explains why they have the same name as the central portions of crown glass. The word “Bullseye” in any sense is conspicuously missing from Samuel Johnson’s 1755 Dictionary of the English Language and from Noah Webster’s  1806 A Compendious Dictionary of the English Language. But the 1817 edition of William Falconer’s New Universal Dictionary of the Marine does contain “Bullseye” as “a term given to patent reflectors let into the ports or deck”. Significantly, the earlier edition from 1769 lacks the word. This is the earliest use we have of “bullseye” for such a ship’s light. A slightly later one is in the Index to Nautical Terms and Phrases in Henry Barnet Gascoigne’s The Path to Naval Fame from 18258 . I have found one from 1829, with many uses in the early 1830s. A use of “Bullseye” for the light in a powder magazine comes from 1833.

Falconer's Dictionary 1830

 Bullseye I

Bullseyes defined in 19th century dictionaries

 In 1818 Grant Preston of Burr Street, Aldgate in Wapping, England was granted patent #4222 for “Improvement in the Deck Glass Rim and Safety Grate” . From the patent and attached testimonies, it appears that he made a more secure framework for a lens-shaped piece of glass set into the deck, and added an adjustable ventilator grate.

Preston Patent 1818

Illustration from Preston’s 1818 Patent

From the material available in The Repertory of Arts, Manufactures, and Agriculture  Vol. 32 for February 3, 1818, it is clear that these were used in the decks above passenger cabins in East and West Indiamen. One customer says they are “a great improvement over the bulls’ eyes formerly used.”

The testimonials come from 1817. Since they refer to multiple voyages, and since this is clearly an improvement over previous bullseyes, the use of such glass bullseyes has been going on for some years prior to this.  The silence of dictionaries from 1806, 1769, and 1755 suggests that its use was probably not that much earlier than 1817.  

(I’m indebted to the website The Glassian for the reference to Preston’s patent. )

A very similar idea seems to have been independently developed on land, for quite a different purpose. In the growing cities, paved sidewalks were coming to be used, but they often covered basement rooms that also needed illumination. Thick pieces of glass, specially cast and carefully shaped (unlike the irregular Crown Glass bullseyes) were set into cast iron frames that were set inthe sidewalk. These admitted light to the spaces beneath, but were thick enough to be walked on without damage. Such structureswere called Vault Lights. Some authors credit Thomas Hyatt withinventing the concept in 1845, but Hyatt himself did not claim to have invented the idea9. The earliest case I can find is a March 8, 1834 patent issued to Edward Rockwell of New he patent is so old that the illustrations accompany a patent document online that is hand-written. The drawing shows an annular metal frame (probably cast iron) set with Victorian filigree, surrounding a circular glass disc that is “stepped” in bullseye fashion. A cross section shows that the disc is flat on the upper surface, but is a sort of stepped cone on the lower surface, allowing many surfaces for the light to scatter into the space below. The ring reads Rockwell’s Patent Vault Light — New York. Magazine records show that he was awarded a silver medal for the invention10.

Rockwell Vault Light Patent Rotated

Rockwell’s thick glass Vault Light from his 1834 patent, the earliest Vault Light patent I have found.

Rockwell's Vault Lights seen from above

The way Rockwell’s Vault Lights looked when seen from the sidewalk. They

appear to be dark circles set in the sidewalk.


Thomas Hyatt’s “Illuminating Vault Cover” was patented on November 12, 1845, U.S. Patent 4266, , and consists of many circular thick pieces of glass set in a cast iron framework. The advantage of his construction, he notes in his patent, is that the many smaller pieces of glass are less liable to fracture than a single plano-convex piece. Hyatt doesn’t name him, but seems to have Rockwell in mind. Hyatt’s illustration shows that his glass pieces were simply thick cylinders, so there was no refracting or scattering structure to help conduct light into the space illuminated. Nevertheless, Hyatt, too, won a silver medal in 1855 from the American Institute in New York11.

Hyatt Vault Light Patent

From Thomas Hyatt’s first Vault Light patent.

Park Service B

Views of some of Hyatt’s glass prisms. He was looking for efficient ways to spread the light below.

Over the next seventy years, many others proposed designs for such Vault Lights. One of the more interesting from our point of view is patent 17096, granted on April 21, 1857 to George R. Jackson of Rye, New York. Jackson’s Vault Lights are of several possible shapes, set into a cast iron frame, conspicuous among which are hexagonal pyramids, just like the Mystic Seaport deck prisms.

Jackson Vault Light Patent

Jackson’s patent for Vault Lights is the first to use hexagonal prisms.

Not only did the pyramidal prism help disperse the light underneath,

it also allowed him to tessellate the sidewalk with close-packed prisms.


Jackson states in his patent that


My improvement in illuminating vault-covers consists in the peculiar shape of the glasses employed by me for closing the apertures in the metallic portions of said covers – viz, glasses in the form of an inverted pyramid, or other equivalent polygonal form which will, by reflection and refraction, laterally diffuse the descending rays of light uniformly throughout an apartment, and especially the upper portion thereof – substantially as represented in Figure 1, of the accompanying drawings

Jackson Hexagonal Vault Light Patent Rotated

Another illustration from Jackson’s patent shows how his hexagonal pyramids (left) did a better job of dispersing light underground than the simple glass discs of his competitors (right)

In Jackson’s patent we have, so far as I have been able to find, the first expression of the Charles W. Morgan “deck prism” shape, and for exactly the usually stated reasons. Jackson recognizes the earlier work by Hyatt and Rockwell, calling them by name.

Jackson, too, won a silver medal at the 28th annual Fair of the American Institute in October 1856. It seems to have been some sort of rite of passage for vault light inventors12.

Who was Jackson?13

He was born in New York City on June 4, 1811, and as a boy was apprenticed to a “whitesmith” – an iron worker who does fine finishing work. He founded his own firm on Centre Street in New York City in 1839, then entered into a partnership with a Mr. Cornell. Cornell and Jackson lasted until 1846 or 1848, when Cornell died. He partnered for a time with L. Taylor, who also died shortly afterward. He then joined James J. Burnet to form the Excelsior Iron Works at 340-352 East 14th Street. The building was destroyed in a fire in 1869.

Two of his sons joined the firm, along with nephews, and some plaques, like the one still in place in the historic Smith, Gray, and Co. building in New York, observe that the iron frontwork was done by “George R. Jackson and Sons Foundry”. Perhaps the foundry was considered distinct from the company, which was named “George R. Jackson, Burnet, and Co.” by this time. Jackson was a member of the East River Association, an industrialist group promoting the interests of manufacturers. He died in September 1870 after a long illness. He lived at 85 East 10th Street. His partner James J. Burnet survived him by twenty years.

An advertisement in the New York Times for the Excelsior Works in 1859 says that it was founded in 1839, which is perhaps literary license for Jackson’s being in the cast iron business since that year.

The Vault Light was exactly the sort of thing that Jackson would be interested in, since it provided a market for his cast iron. Jackson filed patents for many iron devices. His patent for the Vault Light came when he was 47. James J. Burnet is listed as a witness on the patent. Jackson’s patent 1851 also used pyramidal prisms. An ad in the New York Times in 1859 says that the Excelsior Iron Works manufactures “patent vault lights”

Jackson Newspaper ad

Newspaper advertisement for Jackson’s Iron Works. Note that he sells “Patent Prismatic Lights for vaults, areas, skylights, &c. (etc.)”


Intriguingly, Thaddeus W. Hyatt filed another patent, #21050, granted on July 27, 1858, for a Vault Light using “an inverted pyramidal, polygonal, or conical form….for the purpose of producing a wide-spread and perfect diffusion of the rays of light which might pass through said cover into the apartment beneath…” The description is so similar to Jackson’s, right down to the wording, that you have to wonder why a separate and later patent was granted. Even more intriguing, the patent, although said to be invented by Hyatt, was assigned to George R. Jackson. Perhaps we see here the outcome of a patent battle14.

Hyatt Hexagonal Vault Light Patent Rotated

From Hyatt’s later hexagonal Vault Light patent.

Vault Light Phosforus

Illustration showing how the Vault Lights directed sunlight to areas under city sidewalks.

Some websites, speculating on the origins of deck prisms and vault lights, suggest that the sidewalk devices were inspired by ship’s deck lights. But it appears that deck lights and vault lights may have been independent inventions. The Vault light patents make no mention of prior use in ships. There are references to Deck Lights in advertisements dating from 1851. The Official Description and Illustrated Catalog of the Great Exhibition of the Works of Industry of All Nations lists the invention of Vault Lights for ships in 1851 (Vol. II, p. 6489) by James Barlow of 14 King William Street, Mansion House, City of London.

Barlow was a Furnishing Ironmonger, born in 1797, who was also a prolific inventor. Another of his inventions listed in the Official Description for 1851 is a siphon-tap for kegs. An illustration of this appears in George Meason’s Official Illustrated Guide to the London and South-West Railway from 1858. Barlow died in 1862 in Leatherhead.

I haven’t been able to find an illustration of Barlow’s Vault Light, but it probably wasn’t in the form of an inverted pyramid. It seems unlikely that the ship’s prism was in that pyramidal form. If it were, the earliest patents for Vault Lights would have copied that design, and at least one of them would have mentioned the inspiration. More importantly, I suspect that an ship’s designer coming up with a prism for use below decks would not have come up with that potentially dangerous pointed prism. The below-deck space was cramped, with a low ceiling. The glass “bullseyes” would be low-profile and without dangerous points. Jackson’s pyramids make more sense under a New York sidewalk with a ceiling high enough to avoid hitting one’s head on. I suspect that the deck prisms of the Charles W. Morgan type may have been repurposed sidewalk prisms, rather than ones specifically intended for ships. Indeed, modern catalogs of deck lights show several low-profile examples that would be less likely to cause injury if accidentally struck.



Notes and References


Five Black Arts: A Popular Account of the History, Processes of Manufacture …edited by William Turner Coggeshall Online here:

The Landmarks of New York, Fifth Edition: An Illustrated Record of the City … By Barbaralee Diamonstein-Spielvogel p. 230

He was a member of the East River Association, an industrialist group. See

The New York City Draft Riots: Their Significance for American Society and …

 By Iver Bernstein

(no page given)


Landmarks Preservation Committee (NYC) Report



More information on him here:


  • Reference: Hyatt Assigns Vault Light patent to George R. Jackson

Congressional Series of United States Public Documents Executive Documents Printed by Order of the House of Representatives during the Second Session of the 35th Congress, 1858-59, Volume 986; Volume 1010 p. 176, Patent #21,050)


There was an article in the New York Times on May 19, 2002 that drew a connection between sidewalk Vault Lights and Deck Prisms, but it didn’t suggest that the hexagonal deck lights derived from Vault Lights, didn’t delve into the history of Deck Prisms, and didn’t exhume any of the Vault Light patents. Here it is: New York Times Article on Vault Lights