Answer: alliteration and rhyme
Alliteration is the repetition of identical initial consonant sounds in closely associated syllables or words. In this line, the author uses alliteration when he uses the words "silken sad."
A rhyme is a repetition of similar sounds in the final syllables of two words. Rhymes are most often find in poetic writing. In this case, the author's use of the words "uncertain" and "curtain" are examples of rhymes.
