This glass prism with a slightly green tint is 4½ inches high and very heavy. It was used like a window on vintage sailing ships to get light below the main deck.

Known as vault lights, sidewalk prisms, or pavement lights, these bits of glass that are flush with the sidewalk are there to allow sunlight into the spaces below . The Invention of Vault Lights ...

During a recent house sale, a 4½-inch-high heavy glass pyramid was for sale. The item was inexpensive, but no one knew what it was used for. It was almost too heavy to lift, so it wouldn't be a pr… ...