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Don't Grope About in the Dark
Don't Grope About in the Dark

Vault lights being constructed in New York City, 1907
Vault lights construction
New York City, 1907

Luxfer Prismen

Prism glass: group of vault lights, deck prisms, coal covers, etc
Display of vault lights,
deck prism, coal plates, etc.

Luxfer Elektroglas Feuersicherefenster

Vault lights in Etna, California
Illuminated sidewalk lift
Etna, California, 2010

Luxfer Prism Company of Toronto

Cut of sidewalk vault from Badger's Illustrated Catalogue of Cast-Iron Architecture
Sidewalk Vault (Plate LXXXIII),
Badger's Illustrated Catalogue
of Cast-Iron Architecture

Lith. of Sarony, Major & Knapp
449 Broadway, NYC
Vault lights at Penn Station, 1938
Penn Station, 1938
Prism glass is architectural glass used to redirect daylight (sunlight and skylight) into interior spaces through refraction and reflection— "daylighting". Before electric lighting became common around 1900, light was provided free of charge by the sun, but at night by candle, lamp, or other flame. Penn Station (right) was a glorious example: the glass roof let in sunlight which fell through the glass-embedded floor to illuminate the tunnels below. Since sunlight is the superior light source, while flames are dim, smelly, smoky, expensive and dangerous, anything which could extend the reach of the sun's free and safe light to interior spaces would make that space more useful and valuable.

Group of original deck lights
Group of original deck lights,
bottoms shown (tops are flat)
Deck lights were the first form of prism glass— the earliest known patent is Wyndus' of 1684: GREAT AND DURABLE INCREASE OF LIGHT BY EXTRAORDINARY GLASSES AND LAMPS; sadly, the details are not specified. Fire at sea is disaster, more so on a wooden ship or with a flammable cargo. Safely lighting a ship's interior with daylight and prisms instead of flames was a practice widely adopted. Colliers and lime cargos were especially dangerous (slaking lime becomes very hot). The glasses work both directions: they daylight the hold, but also show on deck any light from a cargo fire.

Shop window before Luxfer prisms
Before: open grate
Eventually the idea was adapted for land use as vault lights (UK pavement lights). Light used to be provided to vaults and basements by open grates, which were difficult to walk on (especially with women's shoes) and obviously let in water. By placing glass lenses in an iron (or later concrete) frame, daylight could be introduced below while excluding the weather. Iron nubs protruding above the glass surface protect the glass and aid traction. Shop window after Luxfer prisms
After: vault lights

Rockwell's 1834 vault light (private collection) Rockwell's 1834 vault light patent drawing Rockwell's 1834 vault light (in situ)
Private collection (Amityville) Patent drawing X8,058 In situ (Brooklyn, New York)
In 1834, Edward Rockwell patented a round iron plate with a single large lens, but Hyatt later complained in his own patent application, "These glasses are extremely liable to fracture, and when broken leave large and dangerous openings within their rims...". Rockwell's plates are rare today; I know of only three examples of the iron covers and none of the lenses.

Thaddeus Hyatt Thaddeus Hyatt's Patent Basement Extension Hyatt Light (Brown Brothers #4)
Thaddeus Hyatt
(Library of Congress)
Thaddeus Hyatt's
Patent Basement Extension
Brown Brothers #4
21" Hyatt Light

As an improvement on Rockwell's design, Thaddeus Hyatt in 1845 patented his Vault Cover, an illuminated iron plate set with numerous small, tougher bulls-eye lenses, protected with protruding iron knobs. His Hyatt Lights worked well and were a hit; they made him a rich man. From the 1850s on, Hyatt spent much of his time and fortune fighting for abolition of slavery.
Diagram showing light passage through semi-prism vault lights
The next big advance was Hayward Brothers' 1871 Improvements in Pavement Lights. Pendant prisms had been tried before but did not work well. Edward Hayward's idea was to use a semi-prism shape instead: "...the object of my Invention is so to construct them [prisms] that they may not simply allow the light to pass through, but that they may also direct the light in an inclined direction into the rooms it is desired to light." Using a combination of internal reflection and refraction, the prisms could re-direct and disperse light from the vertical to the horizontal, throwing light from the one bright area deep into the backs of otherwise useless spaces.

How light is bent by a prism tile
Prism tile bending light
The most familiar type of prism glass are prism tiles, which take the reflection/refraction idea of semi-prism vault lights and applies it to vertical windows.
American 3-Way Prism Company tile
American 3-Way Prism Co
Trademark tile
They were based on James G. Pennycuick's 1885 patent for window glass with horizontal prisms on the inner surface, and introduced commercially in 1897 by the Luxfer Prism Company. They were an instant success and the tiles were very widely used. Prism tile installations can still be seen in many small towns where they were especially popular as transom lights above storefront windows. Like vault lights, prism tiles take concentrated light from the sky which falls at the front of a room and redirects and diffuses it to the darker parts of a space. Sometimes called generically "Luxfer tiles", they were also made by the American 3-Way Prism Co (with whom they later merged), Solar Prism Co, Condie-Neale Glass Co, Berger Manufacturing Co and others.

Fresnel lens diagram
How a Fresnel lighthouse lens bends light
The Fresnel lens was developed by Augustin-Jean Fresnel in 1822 for magnifying lighthouse lanterns. It is essentially the outer surface of a double-convex lens, but without the core glass. It works the same as a solid lens would, and makes it possible to build compound lenses much larger than a single lens could ever be. There is much about Fresnel elsewhere, so I won't duplicate here. Fresnel lens at the San Francisco Maritime National Historical Park
1st Order Fresnel, SF Maritime
Photo: NPS

Steam boiler gage glasses look like skinny desk prisms. The plain style are just solid glass, but the "reflex" type (right) have prisms on the steam side: "Type B, Reflex, is made for extra ease in locating liquid levels. This gage glass has vertical prisms extending the full viewing length of the glass. The prism side faces the liquid. The sections of the prisms covered with liquid do not reflect light. The sections of the prisms not covered with liquid reflect light brilliantly. Thus the liquid level stands out extra clearly from a great distance away."

Of the MACBETH brand (made by Corning), the smallest is #1 at 4½" long, and the largest the #9 at 13 3/8" long. Pictured is #8, which is 12 58" long, 1 516" wide, and 1116" thick. Marked on one side (by dulling the glass, either sandblasting or acid etch) "TYPE B-8 / REFLEX" where / indicates a line break. The other side reads "MACBETH ® BRAND / ALUMINOSILICATE / MADE IN U.S.A." See the MacBETH gage glass instruction sheet.

MACBETH brand steam boiler GAGE GLASS, made by CORNING