He addresses his comrades who are on board saying: .
When you have to attend to things of that sort, to the mere incidents of the surface, the reality--the reality I tell you---fades. The inner truth is hidden luckily, luckily. But I felt it all the same; I felt often its mysterious stillness watching over me at my monkey tricks, just as it watches you fellows performing on your respective tight ropes for---what is it? Half a crown a tumble. (36).
What Marlow is saying is that while he is in the Congo, though he has to concentrate on the petty little everyday things, he is still aware of what is going on around him and of the horrible reality in which he is in the midst of. Marlow's crew, however, are not only oblivious to the reality that Marlow is exposed to, but also their naiveté is so great, they can't even comprehend a place where this, so called, reality would even be horrifying. .
Quite surprisingly, this mentality does not pertain exclusively to the Englishmen in Europe. At one point during Marlow's voyage down the Congo, his boat hits an enormous patch of fog. At that very instant, a "very loud cry" is let out (Conrad). After Marlow looks around and makes sure everything is all right, he observes the contrasts of the whites and the blacks expressions: .
It was very curious to see the contrast of expression of the white men and of the black fellows of our crew, who were as much strangers to this part of the river as we, though their homes were only eight hundred miles away. The whites, of course greatly discomposed, had besides a curious look of being painfully shocked by such an outrageous row. The others had an alert, naturally interested expression; but their faces were essentially quiet (41-42).
Once again, we see the ingenuousness of the Europeans, even if they were exposed to the reality. Conrad uses this fog to surround Marlow's experience and express this blindness found in his fellow Europeans.
