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Hunters for gold or pursuers of fame, they all had gone out on that stream, bearing the sword, and often the torch, messengers of the might within the land, bearers of a spark from the sacred fire. What greatness had not floated on the ebb of that river into the mystery of an unknown earth! (1.6)
The European pioneers head into the darkness of unknown territory bearing little flares of light like torches or glittering swords that represent their vigor and their enlightenment. This is super conventional imagery, which makes us wonder if we're really supposed to take it seriously. Somehow, we don't think Conrad would be quite so obvious.
The sun set; the dusk fell on the stream, and lights began to appear along the shore. The Chapman lighthouse, a three-legged thing erect on a mudflat, shone strongly. Lights of ships moved in the fairway—a great stir of lights going up and going down. And farther west on the upper reaches the place of the monstrous town was still marked ominously on the sky, a brooding gloom in sunshine, a lurid glare under the stars. (1.7)
Conrad isn't giving us some simple light = good/ dark = bad equation. Check out how even the light isn't very cheerful: the sunshine is "brooding" and the town glares "luridly" under the stars.
"And this also," said Marlow suddenly, "has been one of the dark places of the earth." (1.8)
Out of the blue, Marlow declares that London—pretty much the capital of the world in the late nineteenth century—used to be as dark as the interior of Africa. Well, gee, who asked you?