Quote 151
"I own to you that just then I perceived - in a new light, as it were—how unwholesome the pilgrims looked, and I hoped, yes, I positively hoped, that my aspect was not so - what shall I say?—so—unappetizing: a touch of fantastic vanity which fitted well with the dream-sensation that pervaded all my days at that time." (2.14)
Marlow hopes he looks prettier than the pilgrims, even though his stunning good looks might get him eaten by his own cannibal aides. In this situation, caring about your appearance goes beyond vanity right toward madness. (Seriously, wouldn't the rational response be to uglify yourself as much as possible?)
Quote 152
"Were we to let go our hold of the bottom, we would be absolutely in the air - in space. We wouldn't be able to tell where we were going to - whether up or down stream, or across - till we fetched against one bank or the other - and then we wouldn't know at first which it was." (2.15)
Marlow knows that if he follows the manager's orders and begins sailing again in the fog, left will become right, upstream will become downstream, and they will probably die. It's good to know that he's clinging to some shred of sanity.
Quote 153
"You can't understand. How could you? - with solid pavement under your feet, surrounded by kind neighbours ready to cheer you or to fall on you, stepping delicately between the butcher and the policeman, in the holy terror of scandal and gallows and lunatic asylums - how can you imagine what particular region of the first ages a man's untrammelled feet may take him into by the way of solitude - utter solitude without a policeman - by the way of silence - utter silence, where no warning voice of a kind neighbour can be heard whispering of public opinion? These little things make all the great difference. When they are gone you must fall back upon your own innate strength, upon your own capacity for faithfulness…The earth for us is a place to live in, where we must put up with sights, with sounds, with smells, too, by Jove! - breathe dead hippo, so to speak, and not be contaminated." (2.29)
Marlow claims that his audience cannot understand his feeling of utter loneliness and the ensuing madness without being there. He describes how isolation from one's fellow man can mess with one's sense of reality, that without public opinion, one cannot judge the morality of one's actions.