What thematic idea is embodied in Hawthrone's use of a "magic circle" or "circle" surrounding Hester and Pearl throughout the novel?This reference...

In Chapter II of "The Scarlet Letter," Hawthorne writes,

Those who had before known her [Hester], and had expected to behold her dimmed and obscured by a disastrous cloud, were astonished, and even startled, to perceive how her beauty shone out, and made a halo of the misfortune and ignominy in which she was enveloped.  It may be true, that, to a sensitive observer, there was something exquisitely painful in it.

One of Hawthorne's themes is the Individual vs. Society.  By the use of this "halo" or "circle" around Hester and her child, the conflict of the individual against a restrictive society becomes apparent.  As a child born of Nature--"worthy to have been brought forth in Eden"(Ch. VI)--and not within the restrictive Puritan society, Pearl does not get along with the other children who are cruel to her:

Pearl was a born outcast of the infantile world....the destiny that had drawn an inviolable circle round about her; the whole peculiarity, in short, of her position in respect to other children....the little Puritans, being of the most intolerant brood that ever lived...

And, as she plays, Pearl sometimes makes a circle herself; that is, she surrounds the scarlet A with weeds, a reminder to her mother of her terrible isolation.

Later, in Chapter XXII, "The Procession," as Dimmesdale in his ethereal state passes Hester without recognition, Hester senses within herself

that her whole orb of life, both before and after, was connected with this spot [the scaffold] as with the one point that gave it unity.

So the circle is represented here as the circle of the round scaffold which isolates and defines Hester.  Isolated from the community by her sin, Hester has come to be defined by the letter that she wears upon her bosom.  True, the meanings of the A change, but the end result is Hester's aloneness and conflict with society.  Yet, although the readers' sympathies are attached to the individual's freeing him/herself of the circular constraints of society --Dimmesdale's exposure of his letter and his confession, Hester's going to England after Dimmesdale dies--Hester returns to the humble cottage and picks up the scarlet A and willingly replaces it upon her clothing. Evidently, Hester cannot break free of her misfortune and ignominy imposed upon her by the restrictive Puritan society.