Symbolism, Imagery, Allegory
The novel stresses Esther's personal crisis by repeatedly showing how she doesn't recognize herself in the mirror and in photographs of herself: she's lost all sense of who she is. When Esther finally sees herself in the mirror after her attempted suicide, all she sees is a spooky grin, and she drops her mirror in horror. The flip side of this is that it's easy for Esther to make up stories about herself and take on aliases, as when she takes on her Elly Higginbottom persona. This splitting up of Esther's personality into multiple personalities also means that she tends to see herself in other characters, as when she recognizes Joan as a double for herself.