The sixteenth chapter follows Lucy as she attempts to grapple with the previous chapter's momentous incident: her second kiss with George. She confronts George and he delivers a gripping speech on why she can't stay with Cecil. Lucy refuses to listen to him in the moment, but at the end of the chapter the narrator uses an idiom to capture the moment in which she grasps George's point:
The scales fell from Lucy’s eyes. How had she stood Cecil for a moment? He was absolutely intolerable, and the same evening she broke her engagement off.
Originating from the Bible, the idiom in the first sentence is commonly used to describe moments of realization. The image of scales falling from someone's eyes to indicate a healed blindness was used about the Apostle Paul, when he embraces Christianity in the book of Acts. The narrator suggests that Lucy similarly heals from her blindness to Cecil's intolerability. This pivotal realization is sparked by an event that seems relatively mundane: it is when Cecil refuses to join their game of tennis that Lucy reaches her decision to break up with him.
Lucy, George, Freddy, and Freddy's friend had been playing tennis in the previous chapter. In this chapter, Lucy sends George away because he kissed her again. Freddy tries to get another tennis game going, but given that George is gone, they need Cecil to step in for the game to work. Freddy kindly asks him to join them "just this once," adding that it's his friend's last day. Cecil refuses, claiming that he is good for nothing but books. This is the last straw for Lucy and it is what makes her realize that he is "intolerable."
Needless to say, Lucy does not break off the engagement because Cecil doesn't want to play tennis. Rather, she breaks off the engagement because George's point about Cecil finally sinks in when she witnesses his haughty unwillingness to let loose and do the people around him—her brother in particular—a simple favor. Selfish and unaware, Cecil doesn't realize that joining the game would in fact be helpful and kind. One could say that George's speech is what begins to destabilize the scales on Lucy's eyes and that the impact of Cecil's refusal is what finally made them fall down.
The idiom captures the juxtaposition between the triviality of Cecil not wanting to play tennis and the momentousness of Lucy's first real instance of self-honesty—not to mention agency—at the end of the chapter. While it is a serious idiom with religious connotations, it is also a rather commonplace phrase. The idiom makes the mood simultaneously somber and light, as the reader both feels shocked and amused that this is what would make Lucy see what everyone around her has long since recognized.
Another element that contributes to the mixed mood at the end of the chapter is the pace. After long portions of action and dialogue, the time speeds up immensely with these final sentences. Forster both upends and fulfills the readers expectations when the scales fall from Lucy's eyes, as this feels simultaneously very sudden and also like something that could have (perhaps even should have) happened a long time ago. Her termination of the engagement seems very abrupt, but it has also been a long time coming.