It is curious that Conrad refers to Decoud as "the lover of Antonia" at the moment when (as we learn later) Decoud is starting to succumb to utter fatal skepticism. I believe Antonia is placed in the sentence so as to be, grammatically, one the "things invisible and existing," which is certainly true of the idealism she represents.

Compare the "things invisible and existing" with the town above that "had no existence," and also with Mrs Gould's line about keeping her wealth unstained "on its only real, on its immaterial side." Throughout, the novel maintains that material objects have less reality than our "invisible" ideas of them.