The Greeks saw the rainbow and asked what it was, and their answer was a person. Iris was the daughter of Thaumas, whose name means Wonder, a sea god born of Pontus and Gaia, and of Electra, whose name means amber or the shining one, a daughter of Oceanus and the great stream that girdles the world. The parentage is a riddle solved: a rainbow is the child of sea-spray and sunlight, of the deep and the burning sky. Her sisters were the Harpies, the storm-winds that snatch, and the Greeks did not miss the contrast. The Harpies tear things away from the world; Iris connects things to it. Where her sisters were the violence of the storm, she was the sign that the storm is passing, the coloured arc that joins the rain-dark earth to the brightening heaven. The word iris simply means rainbow, and in Homer the rainbow itself is already a portent: Zeus stretches the purple bow across the sky as a sign to mortals, of war or of bitter storm, and men look up and understand. The goddess is only the sign given feet, wings and a voice.

In the Iliad, Iris is the messenger of Olympus, and the poem gives her more errands than any other divine herald. When Paris and Menelaus prepare to duel for Helen, it is Iris who comes to Helen's tower in the likeness of Laodice, the loveliest of Priam's daughters, and draws her out to watch the fight for her own fate. When Zeus needs to warn Hector or to halt a god, Iris runs. When Hera and Athena yoke their chariot to descend and aid the Greeks, Zeus sends Iris to bar their way with threats of wrecked wheels and wounded horses, and the goddesses turn back. When Poseidon refuses to leave the battlefield, it is Iris who must carry Zeus's command to the Earth-Shaker himself, and she delivers it with a diplomat's care, adding a softener of her own, that even the proudest god might heed a brother's warning without shame. Sent by Hera, she rouses Achilles to save the body of Patroclus. She flies to the house of the winds to summon Boreas and Zephyrus to kindle the hero's pyre. And in the last book, when Thetis must be fetched to Olympus and Priam must be sent into the enemy camp to ransom his son, Iris carries both messages, descending to the old king in the dark and speaking courage into his heart. By the Odyssey, Hermes has taken over the herald's road and Iris has stepped aside, a quiet shift in the divine household that the poets never explain.

The most solemn of her offices is recorded not by Homer but by Hesiod. Far beneath the earth runs the river Styx, the cold, loathsome water by which the gods must swear their greatest oaths. When a quarrel arises on Olympus and an immortal is suspected of lying, Zeus sends Iris on swift wings to bring back the water in a famous golden pitcher, a long journey to the edge of the world where the Styx falls shrieking down a cliff of rock. The god who swears falsely by that water does not die, for gods cannot, but falls into a breathless coma for a full great year, untended, unfed with nectar and ambrosia, and for nine years more is barred from the feasts and councils of heaven. This is why the artists painted Iris with a pitcher in her hand: not a wine-jug but the oath-vessel, the most dreaded cup in the universe. The rainbow goddess who seems all brightness carries the coldest water in creation, and with it the guarantee that even gods must tell the truth.

Her deepest loyalty belonged to Hera. Callimachus gives us the most intimate portrait of any Olympian attendant: Iris sits at the foot of Hera's throne and never leaves it, never looses her girdle or her swift sandals even in sleep, dozing with her ears pricked like a hound at its master's heel, in case the queen should speak and need a runner. The loyalty could carry her into dark work. In Euripides' tragedy of Heracles, it is Iris who escorts Lyssa, the demon of Madness, down to Thebes on Hera's orders, to drive the greatest of heroes into the frenzy in which he will kill his own wife and children. Iris does not invent the cruelty; she only delivers it, protesting gently that she serves necessity, the perfect handmaid doing her mistress's will. Poets gave her a marriage to match her nature: her husband was Zephyrus, the mild West Wind, and their child was Pothos, Longing, one of the darts of love. The Greeks could hear the meaning in it. Wed the rainbow to the west wind, and what is born is yearning, the ache you feel when the sky clears and the arch of colour fades before you can reach it.

Later ages kept her busy. Ovid gives her one of the most beautiful scenes in the Metamorphoses: when Juno needs a dream sent to Alcyone, Iris dons her robe of a thousand colours and trails her rainbow across the sky to the cave of Sleep, a hollow mountain near the land of the Cimmerians where no light ever enters, and the goddess of the rainbow stands glowing in that darkness until even Sleep can hardly lift his heavy eyes. Virgil made her Juno's instrument in the Aeneid, sent down in disguise to fire the Trojan women to burn their own ships, and sent again to speed Turnus to war, and sent a final time to Dido's pyre, where she cuts the lock of hair that releases the queen's struggling soul, dew on her golden wings staining the light with a thousand colours as she flies. And everywhere she left her name. The Greeks gave it to the iris flower for its many colours; physicians gave it to the coloured ring of the eye; physics took iridescence for the play of rainbow on a soap-bubble or a beetle's wing; and when a new metal was found in platinum ore in 1803, its salts dissolved into such a variety of brilliant colours that its discoverer named it iridium, after her. Every time rain and sunlight meet and the arc appears, the oldest explanation is still the loveliest: a golden-winged goddess is running an errand, and the rainbow is the print of her passing.