What is one way in which Edgar Allen Poe creates a unity of effect of longing in the raven?

Well, Edgar Allan Poe believes that all short stories should have a "Single Effect" and that all activities, events and details of the story should contribute to this effect. The summary of the poem "Raven" goes like this "The unnamed narrator is alone in his house on a cold December evening, trying to read. As he is about to fall asleep, he hears a quiet knock at his door, but decides to ignore it. He says that he has been reading in the hopes of relieving his sorrow over Lenore, his beloved, who has passed away."