As a wee lad growing up in and around NYC, I often marveled at the tiny glass windows set in the sidewalk in front of older stores. I thought them to be coke bottles buried upside down in the concrete. Then I saw one broken out and was able to peek into a wonderful and mysterious underground cavern (AKA basement). When I got older and was gainfully employed making deliveries of heavy boxes to different stores in Manhattan, I had the opportunity to venture into some of these vaults. They were fascinating areas (as far as storage basements went) animated by people walking over the frosted glass prisms up on the sidewalk overhead. Casting temporary shadows like fast moving clouds on a sunny day.
Basically basements that extended out under the sidewalk, they were designed with glass prismed roofs so as to utilize as much natural light as possible. Once a common design element, they have long since fallen out of favor. And though some remaining examples are embraced and restored, most are replaced with regular concrete or plate steel. Yet another bit of history slowly but surely being lost to time. :-(
A nice pic of an backlit bank of lenses can be seen here.
Here’s a link to some excellent photos and documentation of examples in Victoria, British Columbia.