He was enchained by certain superstitious impressions in regard to the dwelling which he tenanted, and whence, for many years, he had never ventured forth…
This quotation comes from the narrator’s description of Roderick’s mental illness, as he delves deeper into Roderick’s specific fears. A major aspect of Roderick’s anxiety is his belief that the Usher manor itself has caused his illness. Nevertheless, Roderick refuses to leave. Although it’s debatable whether the house itself has any power of its own, Roderick is correct in that he is trapped inescapably in his own madness. He is isolated in his own mind and frightening interpretation of events. Not even the narrator, a guest from beyond the insular world of the Usher family, can coax Roderick from his gloom.
…the more bitterly did I perceive the futility of all attempt at cheering a mind from which darkness, as if an inherent positive quality, poured forth upon all objects of the moral and physical universe, in one unceasing radiation of gloom.
The narrator makes this comment after describing his first few days at the Usher manor, during which he and Roderick paint and read together. This vivid description of Roderick’s depressive mood echoes Roderick’s belief about the house, that is, it seemingly has the power to create its own darkness and fear. The narrator assigning this power to Roderick's mind highlights the way madness in the story is an internal force, created and nurtured by a person’s own mind. The narrator’s efforts at cheering Roderick cannot possibly succeed.