Julia apologizes to Cuff for her “inconsiderate manner” and Cuff explaining that he spent his whole investigation doing whatever possible not to scare or tip off Rosanna, and therefore cannot be responsible for her death. He also suggests that Rosanna’s suicide may have something to do with the Diamond, and that only Rachel can know. Upon hearing this, Julia puts away her checkbook and tells Cuff that he has “gone too far to go back” and must stay to complete his investigation.
Cuff’s defense allows the reader to reinterpret his decision to hide so much from Betteredge as a means of protecting Rosanna by containing his theory—the same thing Betteredge was trying to do. However, Rosanna’s death is also consistent with his theory, and he still has cause to believe Rachel was involved—and would now feel doubly guilty for hurting Rosanna, all over the man they both love.
However, Julia finds it completely impossible that Rachel has lied to everyone and hidden the diamond; instead, she thinks that “circumstances have fatally misled” Cuff’s suspicions. Cuff, in contrast, insists that “young ladies of rank and position do occasionally have private debts” and Rachel refused to acknowledge or help precisely the people attempting to get the jewel back (himself, Seegrave, and Franklin Blake). This reminds him precisely of the actions of secretly indebted women, as did Rachel’s refusal to have her clothing searched and insistence on leaving when Cuff explained it would impede his investigation. (Betteredge thanks God for making his faith in Julia stronger than his sense of reason.) Cuff explains that he chose Betteredge to assist him because Franklin seemed to see his suspicion of Rachel early on, and Betteredge loudly protests that he “never […] helped this abominable detective business.”
With Julia’s retort, someone—indeed, the only person in a position to do so—finally openly challenges Cuff’s thinking, although it is clear that she is more preoccupied with defending her daughter’s honor than discovering what happened to the Diamond. The demands of the family and the law, or loyalty and truth, diverge; Betteredge knows without a doubt which side he means to stand on (even if he inadvertently played a key role in the investigation). His absurd insistence that he is proud to have a limited sense of reason (despite the esteem in which he holds his own judgment) points to the social distortions of the Victorian class structure (in which “truth” often simply means faith in the word of the wealthy, and Cuff’s scientific method of investigation is therefore of contestable validity). So does Cuff’s postulation of the potential motive behind the crime: secret debts, which suggest that Rachel is hiding much more than is already apparent from her evasiveness during the investigation.
When he first saw Rosanna, Cuff explains, he immediately identified her as Rachel’s accomplice—as well as the person Rachel wanted to frame for the crime. He thinks Rosanna likely helped Rachel sell the Diamond, using her old connections in the London criminal underworld. Cuff declares that he has a certain way to close the case, as well as “a bold experiment” that may or may not work. The first entails monitoring Rachel, bringing in a new servant in Rosanna’s place to spy on her, and “mak[ing] an arrangement” with the money-lender whom she owes. Enraged, Julia rejects this idea. Cuff then proposes the “bold experiment”: tell Rachel about Rosanna’s death and, given that her emotions so often overwhelm her self-control, hope she admits to the crime. Astonished, Julia agrees, on the condition that she gets to tell Rachel herself. Cuff is speechless, and Julia rides away to Frizinghall.
Cuff’s theory of the crime turns Betteredge, Penelope, and Julia’s vision of Rachel’s character on its head, portraying her not only as a secret spendthrift and liar but also as a cold-blooded schemer willing to turn against her accomplice. Instead of buying into further deception to fight this alleged deception, Julia prefers the direct and confrontational tactics of the long-gone Seegrave and takes matters into her own hands. While this may further traumatize Rachel, Julia’s decision to reveal Rosanna’s death also allows her to inform Rachel that the investigation is over.