This passage explicitly equates the treasure with the inner, most subjective aspects of Nostromo's soul (note that it is "himself" who would be lost, and "him" who would be detected). The lighthouse, in contrast, is an artifice of society -- owned by the O.S.N., kept by Linda. Note that its effect would be to socialize the truth of Nostromo's soul: it would shine upon "disgrace, poverty, contempt," which are social consequences. The lighthouse is thus a symbol of communicating the inner world to the outer one -- for better or worse.