Vault lights, also known as sidewalk or pavement lights, are those old glass prisms set into sidewalks to let light into vaults and basements below. They first originated in the 1840s, aboard sailing ships. Prisms were used instead of flat glass to disperse the light over a larger area. Nowadays these bits of glass appear to be purple, but originally the glass was clear. Manganese added to the glass as a decolorizer, eventually turned purple after years of UV exposure from the sun. We saw these vault lights during our tour of underground Seattle last month.
I work in a vault, but sunlight never penetrates it, not even through purple vault lights. Instead like everywhere else, electric lighting has supplanted vault lights. On the rare occasions that electric power is lost, the room becomes as black as a cave. I keep a flashlight on my desk, for just such eventualities, so I can find my way out. I got my fifteen minutes of fame today at work, but it took ten hours to get it. That is too long a time for too little a reward.