The poem has an alternate ABAB rhyme scheme. This gives the poem a flow and helps to emphasize the beauty of the persona being described in the poem.
Alliteration can be seen in the second stanza line 5, "..serenely sweet.." and in fact there is a repetition of 's' sounds throughout this stanza which is the use of sibilance. This again serves to give the poem a sense of flow and rhythm conveying a strong sense of meaning throughout the poem.
Finally throughout this poem the idea of the contrast between light, dark and brightness is an image that is repeated.
"And all that's best of dark and bright,"
"Thus mellowed to that tender light,"
"Or softly lightens over her face,"
"The smiles that win, the tints that glow."
All of these references serve to reiterate the beauty and innocence of the woman being described.
The speaker uses a simile in the first stanza of the poem. He writes:
She walks in beauty, like the night
This simile begins a comparison of a woman's beauty and a starry night. It evokes both calm and awe.
The dark-light imagery continues in the second stanza in lines 7 and 9-10.:
One shade the more, the one ray the less
Which raves in every raven tress/ or softly lightens o'er her face:
In addition, the speaker uses alliteration in line two, repeating the "C" or "K" sound and the "S" sound:
Of cloudless climes and starry nights.
The main idea of the poem is that a woman's outward appearance reflects a virtuous soul inside.