The current exhibition "Turner's Watercolour Explorations 1810-1842", tucked away in two dimly lit rooms on the Clore Gallery's third floor, is a delightful exception, constantly surprising and frequently fascinating. It is a triumph of good museum keeping. The exhibition stems from Eric Shanes' researches into the group of 390 watercolours in the Bequest known collectively as "Colour Beginnings": a collection of sketches, some of which have been identified as studies for finished works and others which seem like abstract fields of washed colour. There's not a pencil mark in sight.
"Pictures of nothing and very like" was William Hazlitt's description of Turner's work but, as these pictures show, it was a very deliberate sort of nothing: the nothing of mist and air. He painted light like no one else and within these studies are contained the basic elements of nature. They are depictions of atmosphere and pure weather: suggestions of sun, wind and rain; the sun rising and setting on blurred horizons and occasionally the barest hint of a solid form, a boat, perhaps, or the side of a cliff.
Studies such as these were not, of course, intended as finished works, yet Turner kept them in his studio and included them in his bequest to the nation, stipulating that they must all be kept together. They were his starting points: under-paintings to which he might have added 19th- century details, but which he chose to leave in a formless state. He prided himself on "never losing an accident" - acknowledging the unpredictable behaviour of watercolour on wet paper, and yet even the most experimental flicks of his brush look deliberate.
It was recognition of this assured experimentation that led the French Impressionists and later the American Abstract Expressionists to hail Turner as the father of modern painting. Rothko, in particular, was smitten by his fields of washed colour and it was because of the Turner Bequest that he gave his own work (the Four Seasons Murals) to the Tate in 1969. Rothko isn't the only later painter pre-dated in these sketches: there's Whistler, time and again; and Bonnard; and even Paul Klee in a picture dating from the 1820s and described in the catalogue as ?A Paper Test? - the double question-marks giving the title a modern, almost surreal feel that isn't entirely inappropriate.
This, according to Shanes, was Turner exploring the absorbency of paper, but the blobs of colour have vaguely suggested forms: a couple of trees, some square blocks (buildings, perhaps), a patch of grass, some hills; and so the paper test became a little picture. Its mood is so like Klee that one imagines he must have seen it, yet this is the first time that these wonderful works have been on public view. On this page last week I described the current Alfred Wallis exhibition as one of the most refreshing and inspiring shows of the year: this little show at the Tate is even better.
Tate Gallery, London SW1 (0171-887 8000). To 8 June