While they speak, this relative walks through the room, apparently unaware of the narrator. what was the narrator's reaction to seeing this person?

It happened in "The Fall of the House of Usher" (by Edgar Allan Poe) that while Usher and the narrator speaks, Usher's sister walks through the room, apparently unaware of the narrator. The narrator's reaction to seeing Usher's sister when he sees her pass through the room was with an utter astonishment not unmingled with dread.