The captain's words were like the chilly night wind, piercing through the increasingly dim staircase. Alice instinctively hugged her arms and followed Duncan even closer behind. As they descended further, she finally understood what the captain meant by "the light is black."
In the lower deck, there indeed was light—at least structurally and in layout, the compartments she saw had the same support columns, and on those columns hung oil lamps that never went out. The lamps were burning, but the flames seemed to cause the areas around the lamps to be darker than those farther away.
Yes, the closer one got to an oil lamp, the dimmer the light became, with the lamps themselves almost enveloped in shadows, faintly outlining their forms. Conversely, areas further from the oil lamps gradually brightened—the furthest corners of the ship's cabin were almost as bright as those on the upper decks.