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"'Each station should be like a beacon on the road towards better things, a centre for trade of course, but also for humanizing, improving, instructing.' Conceive you - that ass!" (2.2)
Sure, a "beacon": a beacon of immorality and evil. The manager is a hypocrite. Nothing he does helps the stations improve. In fact, under his authority, everything has decayed.
[The manager's uncle]: "'Ah! my boy, trust to this—I say, trust to this.' I saw him extend his short flipper of an arm for a gesture that took in the forest, the creek the mud, the river—seemed to beckon with a dishonouring flourish before the sunlit face of the land a treacherous appeal to the lurking death, to the hidden evil, to the profound darkness of its heart. It was so startling that I leaped to my feet and looked back at the edge of the forest, as though I had expected an answer of some sort to that black display of confidence." (2.2)
We're used to thinking of nature as a relatively friendly, nurturing place—Mother Nature, and all that. But the manager's uncle assumes that Nature is evil, and Marlow obviously agrees. Awesome! Another binary smashed.
There was no joy in the brilliance of sunshine. (2.5)
Sunlight is supposed to be symbol of truth or a blessing from God. Here, it's the exact opposite: there's "no joy" in its brilliance or heat. It's oppressive, just like everything else.