Alliteration
Alliteration is the repetition of intial consonant sounds for a particular poetic effect. The repetition of a consonant sound might contribute to the musical quality of the poem and might also enhance the meaning of an expression.

Here, the repetition of the initial "f" of "form" and "face" is alliterative and suggests the connection between the two ideas--the "form" Narcissus sees in the water resolves into a reflected "face." Alliteration can also be found in the "l" of "loved" and "leaping"; the connection between these words might be seen to emphasize the emotional upheaval of Narcissus.