Down below, a deck prism illumination is astonishing. Drilling a round hole in the cabin sole is way easier, and a discarded plastic bottle is far less expensive than a vintage Criitenden & Willcox prism. Separate the bleach if you get real thirsty? Watch your head(pun?).
The main difference here is that deck prisms are flat on top, which limits the amount of light entering the prism. The water in the bottles is filled to the top, creating a dome shaped concentrator. Since water has a refractive index close to glass (~1.3 vs ~1.5) the bottom of the bottle has a similar effect to the deck prism, except with slightly more light.
