Arts: Boring. Boring. Boring

With some artists, late means late. It means the work of their sixties or seventies or eighties. It means old. But with John Everett Millais, late means anything after about the age of 30. He had a remarkable career. One decade of early, and nearly four of late.

Millais was a child prodigy. In 1848, at 19, he became a founder member of the Pre-Raphaelite Brotherhood, and for about 10 years - from Isabella to The Vale of Rest - he did perhaps the best work of the movement. Then came the great apostasy. For the beady-eyed honesty of Pre-Raph ideals he substituted a series of looser, old-masterly styles. He became successful and very popular: The Boyhood of Raleigh, Bubbles. He was the first painter to be knighted. He said he needed the money.

Now the standard, the established, the absolutely obvious view of all this, is that it was a change for the worse. No doubt Millais did need the money, couldn't afford to go on doing the sort of painting that took a day to fill a square inch; and it's good to recall how irresistible these pressures can be. But excuses go only so far. So far as his art was concerned, Millais might as well have been run over by an early locomotive in about 1860.

Any exhibition of Millais's painting, then, that pays equal attention to his whole career, is unlikely to be just a neutral survey. It's going to have a point to make. It will be venturing a reappraisal. And that's how it is with Millais: Portraits, at the National Portrait Gallery.

Millais painted portraits all his life. In the early work, it was friends and private patrons who sat. Later he became the most demanded portraitist in England, and did a lot of public figures. In this show, the Pre-Raphaelite period is given only its chronological portion. So it's the later work, including Disraeli, Gladstone, Tennyson and Cardinal Newman, plus plenty of smart ladies and cute tots, that dominates the exhibition. And we look for reasons. It can't just be a fame parade, surely?

Reasons are offered, but they're of a peculiar sort. The leaflet which every visitor gets with their ticket says this: "Millais's late career has often been interpreted as a selling-out of the Pre-Raphaelite values of his youth, and his mature works have suffered neglect. Yet, as the exhibition shows, these portraits were greatly admired at the time and, together, form an extraordinary expression of their era."

I don't quite take the force of that "yet" - but it seems to mean that, because the late work was much admired in its time, it's somehow become "unhistorical" of us not to admire it now.

Look to the show's catalogue and you'll find some equally curious lines of persuasion. The leading essay reminds us that people have deprecated late Millais for an awfully long time, implying - I think - that a revaluation must therefore be due. (No: the time for a revaluation is when someone provides grounds for one.) It then proceeds vaguely to impugn the motives of late Millais critics, suggesting that they were, and are, snobby or inverted-snobby aesthetes, resentful of the artist's worldly success and contemptuous of his popularity with a big public. (Suppose this were true; it would again say nothing in favour of the later work.)

The odd thing about this supposed reappraisal is its complete lack of revisionary zeal. It seems to have no particular love for the paintings it would reinstate. It just feels, in an abstract, leisurely, open-minded, even-handed sort of way, that this long-standing preference for one lot of work over another is a rather unfair and unsymmetrical state of affairs; that taste should spread itself out more equitably and more respectfully.

Of course, this isn't explicitly said, but I can't detect any more positive argument for the cause. And, in its absence, the appeal to a sense of balance becomes quite paradoxical. The only way it can get us to feel the "unfairness" of late Millais neglect is by contrast with the enthusiastic attention that's long been given to early Millais. But this enthusiasm it must also play down, lest it prove to be too infectious. In fact, the main problem with this reappraisal is not the feeble case it makes for the late work, but its failure to see how good the early is. And to put this the right way round: the only reason we have for being interested in late Millais at all, interested enough even to be "unfair" to it, is a sense of loss and disappointment. Simply: late Millais is the promise of early Millais, wasted.

The contrast is remarkably sharp. Put it schematically: small pictures against large - a luminous clarity of shape and colour against a glutinous murk - precise delineation against vague evocation. The size point is very striking. You see at once that the early pictures are all tiny, some almost miniatures. This, together with the inexhaustible minuteness of Millais's technique, becomes a very strong psychological device. The image demands extremely close viewing. It can't be seen by more than one person at a time. The viewer is drawn into and held in intimacy with the sitter.

Take the portrait Emily Patmore (1851), one of the wives of the uxorious poet Coventry Patmore. I think it's the masterpiece of the first room, because it fully mobilises this intimacy. It can't be reproduced here, for copyright reasons, but if it were it would fill only a quarter of the page actual size. It's an odd face, made over-oval by its centre-parted hair. It pushes interestedly towards the viewer. Its pinkness is stressed by the plain ground of deep ultramarine against which the figure is set - which makes it a bit icon-like, too. Every bit of it is observed and described.

This is a tendency that can turn quasi-naive at times - as in Eliza Wyatt and her daughter Sarah (1850), where the figures are maybe a little too dollish. Or it can become almost obsessively informative. With the portrait of John Ruskin (1854), standing on a rock in the middle of a Scottish waterfall, you feel that a group of the relevant scientific experts - a botanist, a geologist, a hydrologist - could append a series of explanatory labels to the various natural phenomena depicted. (Of course, Ruskin could have done all this by himself.)

But at the root of these excesses is the wonderful probity of Millais's Pre-Raphaelite style. It takes responsibility for everything it depicts. It makes it its business to know exactly what's happening in the world before it. It never fobs off the viewing eye with "oh, you know, take the hint, et cetera..." It spells out every detail. It recognises the peculiarities of persons and differences of things.

Now make the relevant comparison - say, A Jersey Lily (1878), a portrait of Lillie Langtry, actor and royal mistress. Like many of Millais's later sitters, she was a public celebrity, and you can get a bit of talk out of that. And I don't really want to be rude about this picture. It doesn't seem necessary. Who, passing it in a gallery unlabelled, would slacken their pace? What possible interest does it hold? It's a picture that's designed to deflect all but the most casual attention. Everything is vague and mergy. Face, expression, gesture, texture of flesh, hair or fabric - it's without any sense of the particular.

Boring: this is the most prominent aspect of Millais's late work. You may say "stuffy" or "twee", but the extreme boringness of it is the decisive factor. It's an art that could interest only those who didn't know anything better - who didn't know, for instance, the early Millais. And it's true to the paradoxical nature of this exhibition that it could make its case only if it excised the early work entirely - but then nobody would want to go to see it.

It may seem odd that a show should be organised specifically to promote the cause of dullness. It may seem odd, too, that the arts, whose boast has long been that they are among the most interesting things in the world, should so often be run by people who are clearly among the most boring. But it is true - I don't quite know why. The real surprise is that so much good stuff still gets through.

Millais: Portraits, at the National Portrait Gallery, St Martin's Place, London WC2; every day until 6 June; admission pounds 4, concs pounds 3