What is the significance of the breastplate incident in chapter 7 of The Scarlet Letter?

In chapter 7 of The Scarlet Letter Nathaniel Hawthorne uses the breastplate to create imagery and symbolism that represents the idea that Hester Prynne and her illegitimate daughter Pearl are perceived to be sinful and deserving of their separation from the rest of society.

To set the scene, in chapter 7 Hester and Pearl have gone to Governor Bellingham’s home, ostensibly to deliver a pair of gloves that Hester has embroidered for the governor. However, Hester also has an underlying motive. She has somehow heard that a number of the townspeople feel that she is an unfit mother and are trying to have Pearl removed from her care. In this chapter Hawthorne describes the beauty of Governor Bellingham’s home, and then focuses on a suit of armor on display in the hallway.

There was a steel headpiece, a cuirass, a gorget, and greaves, with a pair of guantlets and a sword hanging beneath; all, and especially the helmet and breastplate, so highly burnished as to glow with white radiance and scatter an illumination everywhere about upon the floor.

The imagery here is meant to communicate the impressive nature of this suit of armor. In a sense, it represents what is considered gallant and courageous by society. Hawthorne characterizes Bellingham by telling the reader that he actually used this armor in battle in the Peqoud War. The fact that the breastplate is able to “glow with white radiance” imbues the reader with the feeling that this armor, and by extension Bellingham, is an upstanding member of society.

Then Hawthorne takes it a step farther when little Pearl becomes excited about seeing her mother’s reflection in the breastplate. But Hawthorne doesn’t just draw for the reader an image of a simple, straightforward, proportional reflection. Instead, he takes this opportunity to present Hester symbolically.

. . . owing to the peculiar effect of this convex mirror [the breastplate], the scarlet letter was represented in exaggerated and gigantic proportions, so as to be greatly the most prominent feature of her appearance. In truth, she seemed absolutely hidden behind it.

With this passage, Hawthorne has created for the reader an image that symbolically represents society’s perception of Hester. She is defined by her earlier deed, adultery. For now at least, this deed, like the overblown reflection of the “A” on her breast in the breastplate’s reflection, is all anybody sees when they see Hester. It completely defines her role in the community. The fact that it is represented this way in the governor's armor hints that Hester is certainly in danger of losing her daughter due to his disapproval. Hester and Pearl are saved separation by Reverend Dimmesdale, who is at the governor's home at the time and makes an effective appeal on her behalf.

At this point, the reader would probably be surprised to find out that the community’s view of Hester is going to change in a positive way over the coming years. Hester’s quiet kindness and sympathetic nature is eventually going to reform her image in the eyes of the townspeople.

What happens in the incident with the breastplate in The Scarlet Letter?

Chapter 7 of Nathaniel Hawthorne's The Scarlet Letter brings the reader into the mansion of Governor Bellingham. Hester, under the guise of returning some mended gloves to the governor, comes with the intention of fighting the magistrates to keep her child Pearl under her guardianship. Pearl is clearly intrigued by the opulence of the Governor's home, which stands in stark contrast to her hut with her mother.

Pearl's function in the novel is to serve as a reminder of her mother's sin and this moment underscores that. Pearl pauses to admire her reflection in the breastplate of the armor and uses the polished surface to again bring light to her mother's sin. 

Calling her mother to the armor, she tells Hester, "I see you there! Look! Look!'

Hester humors her child and peers into the mirrored surface and is brought face to face with an enlarged image of her ignominy. The convex nature of the breastplate magnifies the scarlet A and diminishes all surrounding it. The symbolism is clear here, as Hester will again face the magistrates as she did on the pillory and will have to acknowledge her sin, a sin magnified in the face of the men who are eager to remove her child from her.

Explain the incident with the breastplate in The Scarlet Letter.

The breastplate, which belongs to the Governor, is a symbol of his and the state's authority:

"This bright panoply was not meant for mere idle show, but had been worn by the Governor on many a solemn muster and training field, and had glittered, moreover, at the head of a regiment in the Pequod war. For, though bred a lawyer, and accustomed to speak of Bacon, Coke, Noye, and Finch, as his professional associates, the exigencies of this new country had transformed Governor Bellingham into a soldier, as well as a statesman and ruler."

When Hester sees herself reflected in it, she sees herself and her "A" as the state sees her: "Hester looked, by way of humoring the child; and she saw that, owing to the peculiar effect of this convex mirror, the scarlet letter was represented in exaggerated and gigantic proportions, so as to be greatly the most prominent feature of her appearance."

This short incident reminds us of how officialdom sees Hester; it is not how the population sees Hester as she begins the transformation from adulteress to "Able" or "Angel."