The critic I.A. Richards has provided a useful set of terms to describe the parts of a metaphor. The tenor is the "gist" or "point" of the metaphor, the phenomenon the metaphorical turn of phrase is trying to convey. The vehicle is the concept conveyed by the word or group of words that is used to make the explicit comparison.

The final metaphor of this passage is the most elaborate. Narcissus's passion is compared to fire (passion is the tenor; the concept of fire and burning is the vehicle). In itself, this is a highly conventional, even clichéd metaphor, but it's handled here so that it points out the irony of Narcissus's situation. Narcissus "kindles" the fire of his passion by looking into pool and causing his face to be reflected back to him and he "burns" with the very passion his own reflected face arouses in him.