How does Peter Hoeg use imagery in Journey into a Dark Heart?

While on the train, David challenges Joseph K., saying that the only African pictures that make their way to Europe are dark images of forests, which are accompanied by legends of murder by the natives. Joseph counters this claim by saying that Africa is dark, and he opens the shades of the train car's window to prove it. It is pitch black outside. David then blows out the oil lamps, and, after a moment, the formerly black landscape appears in the moonlight. David uses this as proof that Africa may not be as dark as outsiders think.