The writer spoke of acute bodily illness--of a mental disorder which oppressed him. (3)
Notice that Usher is afflicted by a mental illness while his sister is afflicted with a physical illness.
I reined my horse to the precipitous brink of a black and lurid tarn that lay in unruffled lustre by the dwelling, and gazed down--but with a shudder even more thrilling than before--upon the remodelled and inverted images of the grey sedge, and the ghastly tree-stems, and the vacant and eye-like windows. (1)
This reflection is the first instance of doubling we see in the text. The motif is repeated in the inverted relationship between the Usher twins.
Perhaps the eye of a scrutinizing observer might have discovered a barely perceptible fissure, which, extending from the roof of the building in front, made its way down the wall in a zigzag direction, until it became lost in the sullen waters of the tarn. (5)
This crack reveals that something is wrong in the Usher family, and of course foreshadows the collapse at the story’s ending.
A cadaverousness of complexion; an eye large, liquid, and luminous beyond comparison… (8)
The special quality of Usher’s eyes suggests that he has special abilities of perception. And, indeed, he can recognize elements of the supernatural that take the narrator longer to recognize.
We painted and read together; or I listened, as if in a dream, to the wild improvisations of his speaking guitar. (15)
This is another case of doubling; reality is reflected, though distorted, in the worlds of fiction and music.