At the end of Chapter 10 when Dimmesdale fell asleep, Roger Chillingworth "thrust aside the vestment that, hitherto, had always covered it from the professional eye." What did he see? He saw the letter A which Dimmesdale had been cutting into his chest from the moment Hester was punished for their sin. He was the one who sinned with Hester and being the reverend for the town, could not admit it so he had personally been punishing himself in silence. As for Roger's reaction, he was delighted. He was filled with ecstasy but it was tinged with evil, devilish delight. He finally had his proof as to who his wife and slept with and now, he could put forth even more revenge. He was compared to Satan with the way he was dancing around with happiness because, even though he was happy, it was such a disturbing happiness that no one would truly feel happy.
In chapter 10 of The Scarlet Letter, "The Leech and his Patient", Chillingworth tries everything he can to extract from Reverend Dimmesdale the truth about the ailment that has been consuming him alive. Since Dimmesdale refuses to give way, Chillingworth opts to hint at different things that "could" be wrong with the minister, namely, his spiritual health.
The men keep going back and forth, with Dimmesdale reminding Chillingworth that he (Dimmesale) is the one to deal with his own spiritual matters. Chillingworth lets Dimmesale go along with the certainty that he still will be needed.
At the end of this chapter, Reverend Dimmesdale falls asleep at around noon while reading. This is rare since Dimmesdale's physical state often rendered him sleepless and nervous. Chillingworth is aware of this and enters the room "without any extraordinary precaution". It is here where he moves the Reverend's robe aside and sees "something".
The reader does not get to know in this chapter exactly what it is that Roger actually sees, but we later find out that Arthur has carved a letter "A" on his own flesh. This means that he commiserates with Hester and her own scarlet letter. This is when Chillingworth makes the connection that Dimmesdale is the secret man that Hester refuses to identify as the father of her child born out of wedlock.
Chillingworth's reaction is quite surprising for an elder man. He feels a "ghastly rapture", and he essentially becomes extremely ecstatic. He is compared to Satan himself, and how the devil would behave itself in a situation where it had won someone's soul. However, Hawthorne clarifies:
When Dimmsdale suddenly becomes ill, his congregation becomes very concerned for him. He walks around and suddenly will clutch at his chest in pain. People think he is having heart problems. Since Dimmsdale lives alone, Chillingworth convinces the town leadership to let him move in with the reverend.
Chillingworth moves and starts to take care of Dimmsdale. Chillingworth is Hester's husband. He had sent her to American, but never followed through on his promise to follow her. She spots him while she is on the scaffold. He warns her to keep his identity a secret. Since Dimmsdale doesn't know that he is her husband, he doesn't realize what the man is up to. Chillingworth is trying to find out who the father of Pearl is. He is confused to the illness that Dimmsdale is suffering from and can't find a cure for him. One day when Dimmsdale is asleep, Chillingworth sneaks over to him and pushes his shirt aside. He then sees a mark on the man's chest. He is thrilled to find what he has been looking for all this time.
Now that Chillingworth has the prove that Dimmsdale is the father of Pearl, he can plan his revenge on Dimmsdale. He reacts with joy and dances around when he finds the hidden mark on Dimmsdale chest.