Emily is talking about how “The Poets” even after they have died continue to illuminate the world through the “Suns” (and there’s a son’s pun in here too) of their art. However, the word “Wicks” as defined as “a corner of the mouth or eye” (OED) implies the phenomena of averted vision which is when you have to slightly look away from a star to actually see it (“The Poet’s influence) better.
In “We grow accustomed to the Dark” Emily gives us the image of the neighbor who, after saying good bye to us, extinguishes their lamp and thus we are left to stand out on the street in darkness where we will need to find our own way and travel by our own light, we must “fit our vision for the Dark”. This is what Emily is telling us her profession is, she is like the neighbor who holds the lamp and greets us in the night and who inspires us to continue on in the darkness to the next poet’s lodge in some vast wilderness.
The first line of the poem is both humble and speaks to the mortality of the poet. She describes “The Poets” as mere lamps whose flame will eventually “go out”. “The Poets” are instruments whose function is to cast “vital Light” and thus it is this “vital Light” which is important, not “The Poets” themselves, it was what “The Poets” illuminate which inspires and lights the way. If life were a vast wilderness, then “The Poets” are the residents of that landscape who guide us on our way towards dawn.
Yet though “The Poets” are only “but Lamps” who cast light into the darkness, they also amplify this “Light”, they are a “Lens” that disseminates the “circumference” of the smallest wick of light which allows those of us who are unable to see the light otherwise. “The Poets”, like the great scientists, such as Newton, capture some essence of truth and beauty and amplify it so that the rest of us can see it too. It’s not that “The Poets” invented beauty or the scientist invented gravity for they were there all along, but “The Poets” and the scientist give voice and language to these things, they cast light on what was already there but had been hidden in the wilderness. And once the light has been cast, “Each Age” of sons (and daughters) afterwards will be able to carry this light to each successive age, like a pilgrim carrying a horn of light through the darkest of winter storms so that they will be able to make camp at the next lodge.
Thus the poet sees the universe with inverted vision, they can see the light that escapes us because we only look directly at it, but “The Poets” are able to look slant, to look a little to the side to see the truth hiding in the halo of a distant star. “The Poets” then report back to us and give us the tools we need to see as they have.