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[Marlow]: "It was the farthest point of navigation and the culminating point of my experience. It seemed somehow to throw a kind of light on everything about me—and into my thoughts. It was somber enough too—and pitiful—not extraordinary in any way—not very clear either. No, not very clear. And yet it seemed to throw a kind of light." (1.15)
Marlow describes meeting Kurtz as an experience that "throw[s] a kind of light […] into my thoughts." Great! Light probably helps him clear up some of those vague ideas he has about Kurtz, right? Not so much. It's still "not very clear"—and the separation between light and dark is getting even fuzzier.
"It [Africa] had ceased to be a blank space of delightful mystery—a white patch for a boy to dream gloriously over. It had become a place of darkness." (1.18)
When he's a kid, Marlow sees the map's blank spaces as full of mystery and wonder. (Notice that "mystery" and "wonder" are exactly the opposite of what we expect to associate with whiteness.) But when Western explorers fill in that map, it becomes dark—another reversal of traditional imagery.
"But there was in it one river especially, a mighty big river, that you could see on the map, resembling an immense snake uncoiled, with its head in the sea, its body at rest curving afar over a vast country, and its tail lost in the depths of the land. And as I looked at the map of it in a shop-window, it fascinated me as a snake would a bird - a silly little bird." (1.18)
Ooh, ooh, we've got this one: the Congo is like a snake, one of the oldest symbols of evil and deception. But Marlow is fascinated by it, hypnotized like a "silly little bird." We guess he just can't look away.