Pictures of nothing

In his monumental defence of his hero, Turner, the great 19th-century critic John Ruskin argued that the painter's claim to greatness lay in the realism of his art. As many of the 35 Turner watercolours in the National Gallery of Ireland's collection make clear, he was indeed, for part of the time anyway, an exemplary realist. Just consider the meticulous precision of his topographical studies made at home and abroad. These are pleasing, solidly crafted works, but they are not quite what Ruskin had in mind.

Rather he was thinking of the more familiar Turner, the painter of dazzling spectacle, of vertiginous Alpine passes, of immense storms at sea, of the splendours of Venice transfigured by the radiance of the southern sun. Here, the devout Ruskin felt, Turner captured the greatness of God's handiwork. The real was the beauty of nature as a direct reflection of divinity. It is one of the ironies of art history that painter and critic were very likely at cross purposes on this point. Turner gave little or no sign of attaching any importance to religious faith. The closest he came to voicing any personal thoughts on the matter was his famous deathbed observation: "The sun is God."

He may well have thought so, to judge by his lifelong obsession with light. Tracing the pattern of his work in the National Gallery watercolours, we see him periodically dispense with the solid matter of topography. Ships and buildings, mountains and forests, land and sea all dissolve in iridescent veils of mist, withering heat hazes or dazzling bursts of pure sunlight. One can sympathise with the baffled critic who accused him of "painting pictures of nothing".

The heady romanticism of his Alpine views, self-consciously invested with a sense of the sublime and awesome power of nature, can seem now a shade contrived, even theatrical. But when he sketches A Shower over Lake Lucerne with just a few swathes of pale, toned-down colour, there are no rhetorical flourishes. This is the real thing, the density of experience contained in a fleeting moment, delivered intact to us in, as the man said, a picture of nothing.

Watercolour is at the heart of Turner's art, not just because he worked so hard at it and was so good at it, but because he managed to translate certain characteristics of the medium, including its incomparable speed, translucence and immediacy, to the altogether slower, more solid world of oil painting.

In this he, and the other giant of English landscape painting, Constable, are routinely identified as precursors of the French Impressionists. Routinely, and unfortunately, for to categorise them as prototypical Impressionists is to miss most of the point of what they were about and deny the substance of their achievements in their own time.

The Turner watercolours' January outing in the print room of the National Gallery is an unfailingly popular annual fixture. This year's show also marks the centenary of the Vaughan bequest which brought 31 of the watercolours to the gallery. Henry Vaughan, a 19th-century collector of independent means, was "a great Turner man". On his death in 1899, galleries throughout the British Isles benefited from his generosity. It was he who stipulated the limited, winter display of the watercolours. He was presciently aware that over-exposure to light would damage them. As it happens, it was also an excellent way of keeping public interest in them alive. Knowing that the works are there just four weeks a year makes us appreciate them all the more.

The Turner Watercolours can be seen at the National Gallery of Ireland until January 31st