In the days before electricity, light below a vessel's deck was limited to candles, oil and kerosene lamps. A clever solution for the light problem was the deck prism. Laid flush into the deck, the prism point drew light down below decks. This prism is a smaller reproduction of Mystic Seaport's last remaining original Charles W. Morgan deck prism. Size: H 2.5in x W2.5in x L 2.5 For gifts, awards and decoration. Pyramid shape, 6 sided solid glass. Also note that only clear and light green deck prisms were used on the ships so sunlight could be dispersed to below deck.