In the first flashback in the novel, the narrative moves from the “present” of Isabel getting to know her extended family in England to a few months earlier, when Mrs. Touchett first meets her in the United States, as seen in the following passage:
She had taken up her niece—there was little doubt of that. One wet afternoon, some four months earlier than the occurrence lately narrated, this young lady had been seated alone with a book. To say she was so occupied is to say that her solitude did not press upon her; for her love of knowledge had a fertilising quality and her imagination was strong. There was at this time, however, a want of fresh taste in her situation which the arrival of an unexpected visitor did much to correct.
This passage both establishes that the narrative has entered into a flashback—“one wet afternoon, some four months earlier”—and also hints at why this flashback is necessary. In showing readers Isabel in her hometown of Albany, James helps them understand an important aspect of Isabel’s character: that she has a “love of knowledge” and that, at this point in time, reading and a “strong” imagination are no longer enough—she wants “a fresh taste” and change of situation, and this is why she ultimately agrees to join her aunt in traveling to England. Ultimately, the flashback—which lasts for two chapters—establishes Isabel as a classic American young person who has a thirst for independence and exploration.
In order to communicate the state of Isabel and Madame Merle’s relationship near the end of the novel, James turns Chapter 49 into a flashback, as seen in the following passage:
Things had passed between [Isabel and Madame Merle] which added no stimulus to sociability, and to appreciate which we must glance a little backward. It has been mentioned that Madame Merle returned from Naples shortly after Lord Warburton had left Rome, and that on her first meeting with Isabel (whom, to do her justice, she came immediately to see) her first utterance had been an enquiry as to the whereabouts of this nobleman, for whom she appeared to hold her dear friend accountable.
As the passage communicates, the flashback (or the “backward glance”) establishes why Isabel and Madame Merle are not on good terms and that their tension is centered on Madame Merle’s “enquiry as to the whereabouts” of Lord Warburton.
The flashback goes on to show how Madame Merle accuses Isabel of sabotaging Pansy’s potential engagement to Lord Warburton and how Isabel comes to realize that Madame Merle is not only attempting to manipulate her with respect to Pansy having a financially advantageous marriage, but she also manipulated Isabel herself into marrying Osmond so that he could have a financially advantageous marriage.
Though this flashback does not reveal the ultimate motive behind all of Madame Merle’s manipulations—that she wants to provide for Pansy because Pansy is hers and Osmond’s daughter from a secret past affair—it sets Isabel up to be ready to receive that information from Countess Gemini in Chapter 51.