Great Works: Diana and Callisto, 1556-59 (188cm x 206cm), Titian

National Galleries of Scotland, Edinburgh National Gallery, London

The moving finger writes, and having writ moves on...' Yes, I knew you would recognise that familiar gobbet from the agitated pen of Edward Fitzgerald, whose thrillingly inaccurate translation of various poems by Omar Khayyam was first published in pamphlet form in 1859. Fingers are very important in art too – an excellent book was written about them in 2010. It's called The Finger: a Handbook, and it was written by Angus Trumble, who is Senior Curator of Paintings and Sculpture at the Yale Center for British Art in New Haven, Connecticut. Fingers, being so wheedlingly curious, get everywhere in art, the long and the short varieties, the crookedly gnarly or the pearly-straight-and-oh-so-evenly-tapering. Think of the extraordinarily extended fingers of Parmigianino's figures; or of the bolt of electricity that zaps across from the extended first finger of a very gruff God to the First Man on the ceiling of the Sistine Chapel; and, most pertinently today, the crucial, nay pivotal, importance of the finger in this mid-career Titian, which has just been rescued for the nation, sparing us the horrible humiliation of being obliged to see it elsewhere in the world. It is currently on display – go and look at it for yourself – in an altarwise, owlish light in Gallery 1 of the National Gallery.

Reflecting on Fitzgerald's words for a moment or two leads to some surprising conclusions. Literature, we discover as we read these words, is a time-based medium. The way the line unspools causes us to mimic its meaning in the mind's eye: we see that laborious movement of the finger even as we read those words, and especially so if we read them out loud to ourselves. Try it. There is no frozen moment to separate us from the freedom to do so. Painting represents such a frozen moment, and that is part of its eternal, and wholly irrational, poignancy. The moment is always arrested – how could it not be? Tarquin's murderous dagger is always poised to strike the helpless Lucretia over at the Fitzwilliam Museum in Cambridge – another Titian, incidentally. The innocents will forever be in the throes of being butchered by Rubens at the National Gallery. How could it possibly be otherwise? There will never been any resolution, any redemption. A painting is a fixative, a terrible trap.

Here the pointing figure is accusatory. The painting pivots about this finger, this gesture of the pagan goddess Diana, the supreme huntress (chaste and fair in the words of Ben Jonson, a murderer himself, with a branded thumb), and the story is fresh plucked from Ovid, who was such a favourite among Renaissance poets and painters. So much that was unsayable and undepictable was voyeured through the cheeky lens of Ovid and other classical authors, so much shape-shifting – men, women changed to stags, rivers, stones – so much happy gender-bending. Here it is Diana, full of a magnificently posed and icy wrathfulness: that near-uprightness seems to suggests probity in her own eyes, might and right on the side of the powerful, as opposed to the pitifully crumpled figure of the nymph Callisto, who lies pregnant just feet away, belly slopped over like a pregnant sack. Diana knows that she has been betrayed. That is the crux of it. She loved this nymph, and the nymph allowed another to tumble in the hay with her. And not just any other. Jupiter has impregnated the girl – once again, he was a Jupiter in disguise. This time he had come in the guise of Diana herself. A small stream separates them from each other, and a gorgeously exhausted hunting dog measures it out for us in idle spans. There is such a clustering of flesh here. The groups of naked figures seem to divide into two partial camps: Diana's on one side of the stream, Callisto's on the other. Callisto herself lies helpless as a thrown-down thing.

The lighting is odd in this painting. It seems inconsistently, fitfully lit. There is no uniformity of light, no careful gradation. It lacks the brilliant clarity of execution that we find in abundance in Diana and Actaeon – oh, there are so many Titians in the National Gallery, and now there is half of another one (soon we may feel that we have a complete set and the game will be over) because it was bought in conjunction with the National Galleries of Scotland, and it is likely to spend its luckless life commuting. Or perhaps it was merely painted in fits and starts. That would explain the sense of discontinuity of treatment. Such light as the figures possess who crowd towards the front of the picture plane seems to come from the way he has painted the flesh itself. That fleshly glow, that aura of fleshiness, lights the scene like a taper. Towards the back, on the other hand, among the turbulence of the threshing trees, and what, we sense, is an oncoming storm – perhaps this is Nature conveniently finding itself in accord with the angry mood of Diana – there seems to be an encroaching gloom, an uncertain, brooding light which seems to be edging towards a darkening of the sky.

In fact, there is much about this painting which is not entirely satisfactory. That is not to say it is not a great painting. There are multiple degrees of greatness within the work of any great artist, and especially within the work of an artist whose working span was so long: Titian painted into his eighties, and according to a recent estimate (which may be inaccurate because many attributions are often vexed and uncertain), he left approximately 1,800 paintings to his name. Vermeer left fewer than 40. There is an overall lack of clarity about the painting, a certain tonal muddiness, and what we might be tempted to call an insecurity about the painting of some of the figures. Does that belly really convince us? Is it not a little crudely painted? How much of this painting was the achievement of Titian's workshop and how much was painted by the master's hand alone? We shall never know. Nevertheless it is a scene of tight, high drama. Diana will never forgive this nymph. Callisto will never stop being in half minds about that assignation with Jupiter. Had she only known who she was dealing with! Life is so messy when there are so many intermediate beings.

ABOUT THE ARTIST

Often ranked beside Michelangelo, the long-lived Tiziano Vecelli (c. 1487-1576) was among the greatest of all Venetian painters. Much influenced by Giorgione, he was in demand by many of the courts of Europe. He is celebrated most of all for his brilliant use of colour.