Rosamond begins to get ideas about perhaps attracting other admirers, in order to appease her vanity, and allay her fears about Lydgate's fondness for her growing weaker. It seems like she might try to win Mr. Ladislaw's affections, and seems a little jealous that he likes Dorothea rather than her. She also seems to suspect that maybe her husband has a soft spot for Dorothea, and that might have been part of the reason she was searching for Lydgate.
Eliot contrasts with Rosamond and Dorothea in appearance in order to show a corresponding difference in their character. Whereas Rosamond dresses in a showy, fashionable way that plays to her vanity, Dorothea is attired in a way that suggests her piety, modesty, and humility, which are especially plain to see in contrast with Rosamond. Eliot's allusions to Imogen and Cato's daughter convey the almost theatrical dignity with which Rosamond composes herself; she is all for show, and acts the part, even at home.
Will feels that being seen at Rosamond might convey to Dorothea that he doesn't think exclusively of her; he fears it has made a bad impression on her, and that Casaubon has poisoned her against him. He feels that his dilemma lies in having to make acquaintances in Middlemarch in order to get ahead professionally and socially, versus devoting himself completely to his admiration for Dorothea. Will compares his situation to "the situation in which Diana had descended too unexpectedly upon her worshipper". The allusion to Diana confirms Will's high regard for Dorothea and her virtue, and the parallels between Will's conception of the situation and this example highlight his wishes to please her, and to convey his love to her.
Dorothea is torn by the theme of loyalty vs. desire; she wants to be in Will's presence, but on the other hand, this will be directly against her husband's wishes. Dorothea is not sure at all what she should do in this situation; she knows now of her husband's unreliability and something of his prejudice toward Will as well. If she were to be herself, as she wishes, she would merely go to see Will, and think nothing more of it; but Dorothea is too faithful a creature to be dishonest, and must continue to struggle with her feelings, as Will is with his.
As a short summary of the chapter I have concluded that Dorothea is certain that her husband has gone through a crisis. How much he knows about his illness is not clear to her. She decides to meet Lydgate and find out she drives to Lydgate’s new house in Lowick Gate to meet him away from her husband. There, Rosamond is occupying herself by learning some songs from Will Ladislaw, in her husband’s absence. Rosamond is excited at seeing one of those "county divinities not mixing with Middlemarch mortality" for the first time. Will is dismayed at being seen alone with another woman. Dorothea greets them with her usual warmth and asks for Lydgate. She agrees he should be sent for. Then suddenly changes her mind and hastily departs for the new hospital herself. Will is left feeling deprived and uneasy. He too leaves in a sulky mood. In fact, Dorothea has left on becoming conscious that this is a meeting with Will, she dare not mention to Casaubon. She is also uneasy about Will’s presence, alone with Lydgate’s new bride, both apparently singing together.
When Lydgate returns, Rosamond is curious to know his opinion of Dorothea, especially of her appearance. She also complains about his preoccupation with work in the hospital and at home. Lydgate affectionately tries to convince her of the need to be "something better than a Middlemarch doctor does." Their marriage is still too new for the discussion to be very heated.
Lydgate is happy at Dorothea’s interest in the hospital and feels that she will give a handsome donation towards its work.
