The first line reads like a safety warning: "Dang'rous to hear, is that melodious tongue." Before we see anything of the beloved, we learn that the voice is both sweet and hazardous. The second line escalates, since the eyes are "murd'rous" and "fatal to the sense." The central idea is that beauty works like an attack the speaker welcomes. The speaker knows the wound is coming, keeps gazing anyway, and ends by promising a warm, lasting devotion.
The imagery makes this danger glitter. Love's arrow lies "in a sapphire sheath," so the eye becomes a blue scabbard, and Cupid himself hides "the crystal haunts among," ambushing from inside the gaze. The lips then complete the armoury: "Cupid's bow with rubies strung." The arrow is in the eyes and the bow is at the mouth. Almost everything is described in hard, bright materials (sapphire, crystal, rubies, a "polish'd brow"), so the beloved seems less like a person than a jeweller's display, beautiful and cold enough to cut.
The form is a Petrarchan sonnet, with an octave rhymed abba abba and a sestet rhymed cdcdcd. In the octave, the heavy rhymes on tongue, among, hung and strung keep returning, much as the speaker "o'er that form, enamour'd" hangs and circles without moving on. The sestet loosens into alternating rhymes. The last two rhymes pair "rays" with "decays," so the brightest word in the poem is tied to the darkest. Contractions such as "Dang'rous," "murd'rous," "conceal'd" and "polish'd" squeeze each line into ten syllables, and they also give the voice a formal, slightly antique polish.
The portrait is built from pointing words: "that melodious tongue," "those murd'rous eyes," "that smooth cheek," "that lip," "that polish'd brow." The beloved is assembled piece by piece, as if seen from a respectful distance, and never appears as a whole person. Only in the last line does a name arrive: "Phaon." The name recalls the classical story of Sappho and Phaon, and the poem seems to speak in a lover's voice from that tradition (this is an inference from the name, not something the text states). If so, the usual habit of cataloguing a woman's features is turned around, and it is a man who is admired part by part. The title's "Sonnet X" also suggests one numbered piece in a longer sequence.
The tenses shift quietly. The opening is a general statement, the middle is habitual ("Oft... have I hung"), and at line nine the poem turns with "Still let me gaze," a wish that leads into future vows. Here the central simile appears. The golden hair plays over the pale brow as the "proud Sun revels in resplendent rays" on a "modest lily's leaves of snow." The pairing of proud and modest, and the playful "revels," make the light seem affectionate rather than fierce. Then the speaker borrows the image: "Warm as his beams this sensate heart shall glow." The word "sensate" matters, because this heart can feel. It was wounded by the arrow earlier and is now warmed by the sun.
The ending brings back the poem's darker side. The sonnet that opened with "murd'rous" and "fatal" closes on "decays," and the glow of love lasts only "till life's last hour." The grammar of the final line is slightly open. The heart seems to decay along with Phaon's "self," so the devotion is tied to mortality and not set above it. Love here is warm because it is finite.
The poem is worth studying because it shows how a stock vocabulary of Cupid, jewels, lilies and sunlight can still carry feeling when the form is tight. Each image has a job. The weapons build danger, the sun simile releases it, and the final rhyme links radiance with decay. The poem is a compliment that has been given a sense of risk, which keeps it from reading as a mere list of pretty features.