Plath wrote “Mirror” in free verse, which means the poem doesn’t have a regular meter. Whereas some free-verse poems feature lines of radically different lengths, the lines in “Mirror” are all very close in length. The similar line lengths therefore create a visual effect of regularity. Furthermore, on a sonic level, the equal line lengths also help establish a general sense of sustained continuity. Thus, even though “Mirror” doesn’t have a repeating rhythmic structure, Plath still manages to establish a kind of regularity that contributes to the poem’s serene and placid tone. Yet despite this overall regularity, Plath also modulates the poem’s rhythm through the use of punctuation both within and at the end of individual lines. As an example, consider the three lines that open the poem:

     I am silver and exact. I have no preconceptions.
     Whatever I see I swallow immediately
     Just as it is, unmisted by love or dislike.

Whereas the opening line contains two full sentences, the second line flows freely, even spilling over to the next line. The comma in the third line marks a brief pause that interrupts the flow ever so slightly. The punctuation in these lines subtly modulates the rhythm in a way that Plath sustains throughout the rest of the poem.