A transformation that takes place throughout "Mirror" is the personification of the mirror; through personification, the poem transforms a mirror into a speaking, feeling narrator. Then, in the jump between the first and second stanzas, the mirror transforms into a lake. Lastly, the woman in the poem sees herself transforming from a young girl to an old woman.
The mirror's transformation into a lake makes the woman's transformation from a young girl into an old woman more vivid.
The transformation, through personification, of an inanimate mirror into a talking narrator, makes the poem's discussion of appearances more interesting.