Symbolism, Imagery, Allegory
The girls are sometimes described as seeming to be "possessed" (1.2), and that otherworldly quality often comes with a light that illuminates them. The image first shows up in a snapshot that the narrators find of the Lisbon house before the suicides begin:
The upper-right second-story window contains a blur that Mrs. Lisbon identified as Mary Lisbon. [. . .] In the photograph Mary is caught in the act of blow-drying her hair. Her head appears to be on fire but that is only a trick of the light. (1.4)
Here the fire is a mirage, a trick, not to be trusted, but it seems to have a foreboding quality.
Fire shows up again in a seemingly harmless scene. After Mr. Lisbon rakes his leaves, he lights the leaf pile:
... like the rest of the fathers, but his anxiety over the fire's getting out of control would diminish his pleasure. He patrolled his pile, tossing leaves into the center, tidying the conflagration, and when Mr. Wadsworth offered him a sip from his monogrammed flask, as he did every father on his rounds, Mr. Lisbon would say, 'Thanks no, thanks no." (3.67)
Mr. Lisbon is doing his fatherly duty, keeping his yard looking nice, but he feels the threat of everything in his nice suburban home going out of control. He can keep the fire in the leaf pile contained, but there's another fire smoldering that he's not aware of.
Light also represents the narrators' understanding of the girls. Since the boys' relationship with the Lisbon sisters consists in large part of spying on them, the imagery of light pervades the novel: the light in the girls' bedrooms that lets the boys watch them going about their lives. As long as the lights are on, there's life in the house and the boys can watch and wonder. In fact, when they're first allowed in the house, for Cecilia's fateful party, the light almost introduces them to the girls:
We were directed to the downstairs rec room. […] and as we descended, the light grew brighter and brighter, as though we were approaching the molten core of the earth. […] for the first few seconds, the Lisbon girls were only a patch of glare like a congregation of angels. Then, however, our eyes got used to the light and informed us of something we had never realized: the Lisbon girls were all different people. (1.42)
Later the light becomes the last signal from the girls to the boys, a flashlight flashed on and off. And seeing an eerie light fro the bedroom, the boys realize that the sisters have created a shrine to Cecilia with candles and glowing incense.
From the windowsill, from cups suspended on clothes hangers, from old flowerpots, from cut-out milk cartons, the candles burned. At night we saw Bonnie tending the flames. Occasionally, finding candles drowning in their own wax, she dug runoff trenches with a pair of scissors; but most often she watched the candles as if their outcome held her own, the flames almost extinguishing themselves, but, by some greed of oxygen, persisting. (4.133)
The makeshift shrine, with its homemade candles and candleholders, is connected to the girls' own fate. The boys believe that by keeping the candles alive the girls are keeping themselves alive. However, they're also calling them to death:
The candles were a two-way mirror between the worlds: they called Cecilia back, but also called her sisters to join her. (4.155)
The night of the mass suicides, the narrators return to the rec room, drawn by a light down there in an otherwise darkened house. They see Bonnie hanging there. Again, the light reveals important knowledge about the girls:
We had never known her. They brought us here to find that out. (4.206)
Even after the girls all die and their parents move away, the flames continue to be a sign of the tragedy. The boys can't shake their obsession with the Lisbon sisters: "a haunted quality persisted about the Lisbon house, making us see, whenever we looked, a flame shape arcing from the roof, or swinging in an upstairs window" (5.30). Talk about holding a torch for someone; the Lisbon girls will never be forgotten.
BTW, "Lux" means "light" in Latin. In modern use, it's a unit of illumination. We'll let you ponder that one. Great essay topic.