Every once in a while I'd see one of these at a flea market or in an antique shop and have always gravitated towards them. I didn't know what they were until recently - deck prisms. Cast glass prisms of varying shapes and size were countersunk into the decks of wooden ships. Because the risk of fire was too great to use candles or lamps below, the prisms were used to light the rooms below deck by refracting the daylight. Still in very small production today.