Parallelism
Parallelism is the repetition of the same grammatical or syntactical structure in a poem. The repeated structures are often used to contribute to the rhythm of the lines and to emphasize their meaning. Parallelism tends to contribute a sense of insistence to an expression.

The repetition of the form of the clauses "while he slaked his thirst. . ." and "as he drank. . ." reinforce the temporal dimension of Narcissus's growing passion. The clauses that follow each of these, "another thirst grew. . ." and "he saw before his eyes a form. . ." describe the beginnings of his self-obsession.