How has Montag changed from the beginning of Part One to its end? (Please provide details from the book, with references to light and darkness in this answer.)
In "The Hearth and the Salamander," Montag moves from a robotic follower in his society to a man who thinks and acts as an individual.
The opening sentence of Bradbury's Fahrenheit 451--"It was a pleasure to burn"--indicates the initial attitude of Montag in this narrative. This mentality is soon challenged when Montag encounters a pedestrian named Clarisse McLellan on his walk home from work. "[W]ith eyes so dark and shining and alive," the ebullient Clarisse speaks of nature and real feelings; and, as Montag envisions her dress as "white and...whisper[ing],"she tells him that she likes to smell the grass; further, she loves to let the rain hit her tongue. In fact, sometimes she stays out all night, "walking, and watch[ing] the sun rise." Before she parts from Montag, she challenges, "Do you ever read any of the books you burn?" Further, she questions him, "Why are you laughing?....You never stop to think what I've asked you," implying that Montag does not analyze anything that he does.
In contrast to this lively encounter with the girl whose face his feels "refracted your own light to you," Montag enters his house and opens the door to a bedroom that seems like "the cold marbled room of a mausoleum after the moon has set." He finds his wife near death from an overdose on the "dark bed," so he calls the emergency number. Not wanting any outside light to display her nearly dead body, Montag lights his igniter:
Two moonstones looked up at him in the light of his small hand-held fire; two pale moonstones buried in a creek of clear water over which the life of the world ran, not touching them.
This metaphoric passage expresses the feelings of Montag, who perceives himself distanced from the spiritual lights of real living.
After his ordeal with the desensitized Mildred, who expresses no knowledge of her having overdosed the night before, Montag recalls his conversations with his lively neighbor who has challenged his complacency. Try as he may, Montag cannot forget this girl "with eyes so dark and shining and alive" and realizes that he is, in fact, not happy: "He wore his happiness like a mask and the girl had run off across the lawn with the mask." Later on, when he joins the other firemen on a call to burn, he is impressed with the passion of a woman who sets herself on fire with her books rather than part from them. Almost impulsively, Montag catches a few of the woman's books as they soar toward him. Once home, he hides these books behind a grill in a wall. At home his wife speaks of the wonders of the "talking walls" of the virtual reality that she enjoys. However, Montag finds them "senseless," and, pulling the ear bud from her ear, he tries to talk to her about the woman and the books and how behind each book, for which she was willing to die, there was a real person who wrote it. But all he sees in her eyes is "a kind of cataract unseen but suspect far behind the pupils." When he asks her to turn off the parlor with its talking walls, she argues, "That's my family."
As he contrasts the passion of Clarisse with his wife, an insipid and shallow woman, and her friends who live only vicariously with the parlor walls and the stories they watch, Montag begins to understand how technology has dehumanized his society and works toward conformity and desensitization. At his visit to Montag, Beatty has explained how a "safe" society has been built, concluding, "We must all be alike." In this end, Beatty contends that books cause conflicts and must be burned because "Fire is bright and fire is clean."
After Beatty departs, Montag has reflective thoughts that reveal to him the necessity to keep books as they spur independent ideas and beliefs in the minds of people. Therefore, he urges his wife to help him.
We've got to start somewhere here, figuring out why we're in such a mess, you and the medicine nights, and the [speeding] car, and me and my work. We're heading right for the cliff, Millie. God, I don't want to go over.
He tells Mildred that they will now "start over." Montag now realizes that life is a construction-destruction cycle and books teach the history of this human experience, thus providing people with the ideas they need to continue and rebuild.