What is the symbolism of the mirror in Chapter Seven?

The mirror is one of the few clear, almost overt, symbols in the book. The essential value of that symbolism is summed up in Tilly's commenting that after the mirror arrived, she was able to see herself for the first time. This comment on her physical reflection can be seen as having a metaphoric echo in Mama's reference to how Delphine's presence is triggering change in both her and Tilly - Delphine herself, like the mirror, is helping the two women see more of themselves than they have ever seen before. The same point could also be made of Cass, although Delphine's effect on her is somewhat more indirect.