You walk on them all the time, probably without noticing them. They are small round glass lenses, many quite handsome, set into a cast iron plate in the sidewalk or in a loading platform at the edge of a building so as to let light into the space beneath them, and they are found all over Tribeca. Known as vault lights, they are an important part of our architectural heritage.

They were first used in the 1840s, a time when Tribeca and similar neighborhoods were changing from residential to commercial and when cast iron construction was entering its heyday. Properties being converted into—or rebuilt as—warehouses would generally include so-called vault spaces extending under the sidewalk, and some owners took to installing skylights in the sidewalk to bring daylight into the spaces and thus make them more usable. But the skylights and their support frames typically blocked the sidewalk or loading dock and even obstructed a building’s entrance.

Enter one Thaddeus Hyatt, who in 1845 patented a system of setting round glass lenses into cast iron sidewalk panels to accomplish the same thing far more efficiently. Soon his lenses, often referred to as Hyatt Patent Lights, were all the rage.

They were called lenses because they had an added advantage: a prism attached to the underside of the glass  bent the light in various ways so as to focus it on a given area underground, further enhancing the space’s value.

Not only were vault lights used in Tribeca, but they were also made and sold here. One company selling them was Jacob Mark & Sons at 7 Worth Street; another was Leo Popper & Sons at 143-47 Franklin Street. Popper sold not only vault lenses but just about everything made of glass, including lampshades, lighthouse lenses, artificial moose and fish eyes and typewriter keys. The business lasted until 1971.

When electric lighting arrived, the value of vault lights declined, although they had a latter-day use for lighting subway platforms. As the years passed, the expense of maintaining them caused many property owners to remove them entirely, often replacing them simply with steel diamondplate. But some stellar examples remain, for example at 119 Hudson Street (in front of the Issey Miyake store) and at 155 Franklin Street (mounted in a vertical loading-dock surface).

But the champ may be 165 Duane Street, which boasts vault lights installed not only horizontally and vertically but also on a sloping surface near the southeast corner of the building—a trifecta.