Don't Grope About in the Dark
Vault lights construction
New York City, 1907
Display of vault lights,
deck prism, coal plates, etc.
Illuminated sidewalk lift
Etna, California, 2010
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.
Penn Station, 1938
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.
Group of original deck lights,
bottoms shown (tops are flat)
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.
After: vault lights
In 1834, E. Rockwell patented a round iron plate with a single mammoth
bulls-eye 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 giant glass jewels.
As an improvement on Cornell, 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.
(Library of Congress)
patent basement extension
|Brown Brothers #4
21" Hyatt Light
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
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.
They were based on Pennycuick's 1885 patent
for window glass with horizontal prisms on the inner surface, and introduced
commercially in 1897 by the Luxfer Prism Company.
Prism tile bending light
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 "Luxfer tiles", they were also made by the
American 3-Way Prism Company
(with whom they later merged), Solar Prism Company,
Condie-Neale, Jupiter Prism Company and others.
Am. 3-Way Prism Co
How a Fresnel lighthouse lens bends light
|The Fresnel lens was developed by
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.
1st Order Fresnel,
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
"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 5/8" long, 1 5/16" wide, and 11/16" 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.