A foil is a contrasting literary character who serves two purposes: he is either an opposite to a protagonist, thus demonstrating the latter's qualities by contrast, or is similar except for a vital difference. The foils in Dickens' "Tale of Two Cities" show similarities and differences that illuminate the novel's protagonist, Sydney Carton, and its terrifying antagonist, Mme. DeFarge.
Foil as Reforming Contrast
Charles Darnay is a near-double for Sydney Carton, but an utterly contrasting character. He is dedicated to French idealism while Carton languishes alcoholically in his law offices; they love the same woman, Lucie Manette, but only Darnay has the inside track; Darnay's idealism goads Sydney into sacrificing his own life to save the young lovers from the guillotine. The men are mirror images, but Carton's character undergoes a radical change when, in one sense, he examines his own mirror and sees his defects in comparison. The foil points the way to the protagonist's changes.
Foil to Antagonist
Miss Pross, Lucie's nurse, is Dickens' embodiment of British order, justice and maternal loyalty, a perfect foil to Mme. DeFarge, the horrendous force of misrule who engineers the chaos and injustice of the French revolution. In a melodramatic master stroke, Dickens pits foil against antagonist in the novel's penultimate chapter, where Pross, whose qualities are all mental, physically restrains and kills DeFarge with a pistol. DeFarge had been characterized as "deaf to all entreaties of pity" -- in a fine ironic touch, Pross is rendered physically deaf by the pistol's report. Foil and antagonist swap qualities and infirmities.
Foil to Mirror
The most heartrending foil perhaps in all of Dickens is the unnamed seamstress bound for the guillotine with Carton. Wracked with terror at first, she allows the peaceful and composed protagonist to comfort her, and both face death with equanimity. "I think you were sent to me by heaven," she tells him, and he replies, "Or you to me." They kiss and bless each other, having gone from polar opposites to kindred spirits; the foil has shifted entirely from horror-filled solitude to companionship, and identification, with the protagonist. Dickens implies that Death sends them to eternal togetherness.
Novel Full of Foils
Dickens, who relished the use of foils as exponents of themes, created, in "Tale," a novel that is a foil for itself, a set of contrasts not the least of which is the title. Dickens populates the story with wildly contrasting characters, as if everyone is a foil for someone else, and opens with a classic parallelism that is a mirror and foil for his theme: "It was the best of times, it was the worst of times."