A ghost climbing a ruined staircase with a lamp is not the image most people expect from a poem about moonlight, but Longfellow opens with exactly that and makes it work. The moon "glides" through "the damp / Mysterious chambers of the air," so the whole sky becomes a derelict house. Its clouds are "crumbling walls," and the gaps between them are windows. When the moon slips behind a cloud and reappears, the phantom is "concealed" and then "at the windows seen again." Calling the apparition "full of pain" gives the light a sorrowful, haunted mood.
The third stanza reverses that mood. The same architecture that held a ghost now holds a monarch, as the moon "walks the terraces of cloud, / Supreme as Empress of the Night." The movement is much the same, gliding and then walking, but the manner changes from furtive to regal. In three stanzas one object has been given two opposite personalities, and that sets up the poem's argument.
Stanzas four to six bring the light down to the speaker's own street. He "recognize[s] no more / Objects familiar," and the path to his door becomes "an enchanted avenue." The building imagery follows the moon to earth. The elm-trees "drop their curtains down" like a stage set, and he walks "by palace, park, and colonnade" as in "a foreign town." Judging by the imagery, the setting is quite ordinary, a path, some trees and a quiet street, and moonlight turns it into marble that "glimmers in the empty square." The words "silent" and "empty" matter here. The transformed town has no people in it, which makes it feel like a dream or a theatre before the actors arrive.
The form supports this gentle effect. The poem is eight quatrains in iambic tetrameter, rhymed ABAB, and the rhythm is smooth enough to mimic the gliding it describes: "As a PALE PHANtom WITH a LAMP." The rhymes are simple and full (lamp/damp, proud/cloud, feet/street), so nothing distracts from the images. That evenness makes the seventh stanza land harder. It opens with a single word, "Illusion!", which interrupts the flow, and the lines that follow are deliberately plain: "the common life of every day," "the sober gray." After the marble and the palaces, "sober gray" is a deflating phrase, and it is meant to be.
The last stanza states the idea outright. "In vain we look, in vain uplift / Our eyes to heaven, if we are blind." The repeated "in vain" sounds almost like a reproach. Then the poem closes on a balanced, nearly proverbial line: "We see but what we have the gift / Of seeing; what we bring we find." The short, plain words at the end make it easy to remember. The pronoun has also shifted from the private "I" of the evening walk to a general "we," so one man's stroll becomes a claim about everyone.
What makes the poem more interesting than a simple moral is that it demonstrates its own thesis. The speaker says the "spirit glorifies / With its own tints the sober gray," and "tints" is a pointed word, since moonlight is close to colourless. But the ghost with the lamp and the Empress on her terraces are also tints. They are the speaker's inventions, laid over a patch of sky and a lit-up street. The poem opens by showing two different moons, one grieving and one triumphant, and only at the end admits that the difference came from the person looking.
Longfellow also stops short of mocking the illusion. "Glorifies" is a warm verb, and "sober" is faintly dull. The poem seems to value the imaginative act even while calling it unreal. That is its lasting appeal for a young reader. It suggests that the wonder of a moonlit street comes partly from the person walking down it, and that this wonder can be carried into daylight too.