Hawthorne uses these chapters move as a bridge between the seven years that have passed, and to develop symbolism and theme. The major action that is taking place is Dimmesdale's developing relationship with Chillingworth, which is directly connected to his failing health, Chilllingworth's conformation that Dimmesdale is guilty of adultery, and Hester's discussion to stop Chillingworth from further harm to Dimmesdale.
These chapters advance several of the central themes in the book, and reinforce the symbolic structure Hawthorne has built into the story. These themes have to do with Chillingworth’s revenge – it’s in Chapter 9 that Chillingworth begins to “treat” Dimmesdale, who is growing ill and constantly clutching his heart – and Dimmesdale’s guilt, which becomes a crisis of faith. In Chapter 10, Dimmesdale and Chillingworth have a conversation about “hidden” sins, and the symbol of the weed that grows from the unrepentant heart is from chapter is introduced. In fact, Chillingworth is himself usurping the role of “spiritual guide” for Dimmesdale; part of his plan is to use Dimmesdale’s own faith and guilt to torment him. In Chapter 11, Dimmesdale’s guilt becomes more acute, and he begins to scourge himself. He comes up with the idea of a nighttime vigil on the scaffold where Hester was humiliated years before, and, in Chapter 12, he goes to the scaffold. He is wracked with pain from his heart, and is joined on the scaffold by Hester and Pearl, but refuses to confess his guilt to the community the following day. A meteor lights up the scene, tracing the letter “A” in the heavens, another symbolic example of how Hester, in wearing her sins on her breast, is free of the darkness of deception that burdens Dimmesdale. In Chapter 13, seven years have passed, Pearl is a little girl, and Hester has rehabilitated herself in the community somewhat. But she is determined to free Dimmesdale of Chillingworth. In Chapter 14, she confronts Chillingworth, saying that she will reveal his identity as her husband, but it also becomes clear that Chillingworth knows that Dimmesdale was Hester’s lover. His malice causes him to physically transform into the embodiment of evil, a change that at first horrifies him but then he embraces. In Chapter 15, Pearl and Hester discuss the meaning of the scarlet letter, and Pearl shows that Hester’s hatred of Chillingworth and her own sin have affected her child, who seems to have an instinctive understanding of Hester and Dimmesdale.
When you look back at these chapters, a few additional themes emerge. These chapters advance the problem of secrecy, and difference between the “real” nature of Dimmesdale as sinner and the “fictive” view of him held by the townspeople, who believe him to be sinless. Similarly, Hester, in openly acknowledging her sin, avoids the guilt that Dimmesdale feels, but her agreement to keep Chillingworth’s true identity secret exacts its own price. Chillingworth, for his part, is a physician, but his real purpose with Dimmesdale is to harm him, not heal him – his thirst for vengeance transforms him into the very “evil” that the townspeople assign to Hester.