Quatrain 1's imagery based on the seasons makes the speaker sound pretty much decrepit and wasted, "shak[ing] against the cold." Quatrain 2 wasn't quite such a downer at first, but the light of the sunset is distant, the sun has already set, and death is coming pretty relentlessly onward. What makes quatrain 3's fire image different, we think, is that it, for the first time, shows some serious energy at the core of the speaker, still defiantly "glowing" even though most of his fuel is gone. The speaker's attitude here isn't exactly the "rage, rage against the dying of the light" from Dylan Thomas's famous poem, but it's close.
- Lines 9-12: Just like quatrains 1 and 2, quatrain 3 hits us up with some pretty rich sensory imagery. Here, the speaker tells the listener that he can "see" the "glowing" of the fire. And just as with the imagery in the previous two quatrains, quatrain 3's description doubles as a conceit for the last bit of energy that remains in the speaker's life, even though most of the fire (so to speak) of his youth is already past.
- Line 9: Just as line 5 contained an echo of line 1 (see "Symbols: Twilight"), line 9 contains an echo of both of them. In fact, the echo of Line 5 is so strong here that this really counts as a parallelism. Line 5 and line 9 have exactly the same words in the same place, except for two places where they're different. This repetition of the same idea, but with a slight variation, is exactly what the structure of the poem as a whole is all about.
- Lines 10-11: One weird thing that's happening in quatrain 3 is the personification in lines 10-11. The speaker has already made it clear from his wording in line 9 that the "fire" he is talking about is a metaphor for himself, a human being. Since we already know that the fire is a human being, it's kind of bizarre that Shakespeare starts talking about the fire in human terms in these lines, what with the reference to "his youth" (10) and "the death-bed whereon it must expire." It's as if line 9's metaphor has doubled back on itself, like a snake eating its own tail. This kind of craziness is just one of the ways that Shakespeare loves keeping his readers on their toes.