The captain's words, like the chilly night breeze, passed through the increasingly dim staircase. Alice subconsciously hugged her arms, following closer behind Duncan, and as they descended further, she finally understood what the captain meant by "the light is black".
There was indeed light in the lower deck cabins—at least structurally and layout-wise, the chambers she saw had the same support pillars, and on these pillars hung ever-burning oil lamps. These lamps were burning, but the flames made the vicinity of the lamps even more dim than the distance.
Yes, the closer to the oil lamp, the dimmer the light, with the lamp itself almost completely shrouded in shadow. Only its vague outline could be faintly discerned, while a little further from the lamp, the light gradually increased—the corners of the cabin were nearly as bright as the deck above.