I want to talk about deck prisms because they’re clever and pretty looking.
old ships, the only sources of light below deck were often candles and
oil burning lamps, both of which could easily be dangerous on a wooden
ship. Another (safer) solution to the lighting problem was the use of
deck prisms. These were made of glass and set into the upper deck, flush
with the boards, and they drew light down into the lower deck that was
the equivalent of a 30 watt bulb.
This example is from the Charles W. Morgan, a wooden New Bedford whaleship built in 1841 that is still afloat, and the last of its kind.