Answer:
The "default" number of lines in each stanza of this poem is four. Four-line stanzas are called quatrains. The only exception in this poem is the third (central) stanza which has five lines and is called quintet.
Explanation:
The third stanza is, in a way, an emotional climax of the poem. That is why it has to stand out formally and visually. The repetition ("The Meadows - mine - || The Mountains - mine -") depicts the speaker's ecstatic feeling towards all the landscapes she would have been able to see had she not "got [her] eye put out".
The fifth line, the one that disrupts the 4-line pattern, reads: "Between my finite eyes," as if the speaker is painfully aware that even though her eyes are finite (limited, conditioned), having two of them would make a world of difference. Then, the following two stanzas decrease the emotional tension and the speaker finally lulls herself into a consolation - maybe it IS better to see with one's soul what other creatures can see with their eyes.
