To what does the narrator compare the windows of the house?"The Fall of the House of Usher"

As he approaches the mansion of his old school friend,
Roderick Usher, the narrator describes the "melancholy House of Usher" as desolate and
terrible.  The house has bleak walls and "vacant eyelike windows." So haunting is this
house and its environs that the narrator suffers "an utter depression of soul."  This
"sorrowful impression" repeats itself by the end of the first paragraph as Poe again
makes mention of the "vacant and eyelike windows."

This
technique of repeating and louping bizarre traits is what Poe termed "arabesque."  With
the House of Usher, there is not only a repetition of the phrase "eyelike windows," but
there also is a double entendre upon the title which the narrator
later explains:

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...so identified the two [brother and sister] as
to merge the original title of the estate in the quaint and equivocal appellation of the
"House of Usher"--an appellation which seemed to include, in the minds of the peasantry
who used it, both the family and the family
mansion.

Thus, the house with
vacant eyelike windows assumes a personality, a personality that reflects the changes to
Roderick and his sister Madeline.  And, as the narrator stays with Roderick Usher, he
notices in the man "the luminousness of his eye had utterly gone out."  As Roderick
becomes madder, the narrator describes his eyes as having a "wide and rigid opening." 
Later, Roderick's eyes "were bent fixedly before him, and throughout his whole
countenance there reigned a stony rigidity." And, as Roderick descends into madness, so
too does the stony House of Usher suffer fissures that parallel and reflect the
deterioration of Madeline and Roderick Usher, whose eyes become vacant like those of the
mansion so that both houses of Usher are destroyed.