KNOWN to Ruskin as the father of British Landscape painting, Richard Wilson's is scarcely now a household name. He was one of the founders of the Royal Academy in London, but, compared with the likes of Constable and Turner, whose landscape work owes so much to his pioneering imitation of Italian art, he is largely overlooked.

Indeed, it was Turner's proudest claim that he walked "in the footsteps of Wilson", and, as Robin Simon and Martin Postle show in this exhibition, he often sought out the exact location where Wilson had set foot half a century before.

It is more than 30 years now since the last great retrospective that David Solkin showed at the Tate, as well as in Cardiff and at Yale; so a re-evaluation of Wilson's work offers a significant tercentenary celebration that has already been acclaimed on the other side of the Atlantic.

By 1907, the Welsh National collection owned 40 or so works thought to be by Wilson, some of which have since lost that attribution and now stand testimony to the quality of many of his students. Yet the National Gallery in London owned none of his works until it bought a pair of landscapes of the River Dee from Agnew (Colnaghi Fund) in 1953, one of which is included in this exhibition.

That painting, Holt Bridge on the River Dee (1761-62), to give the game away, is a textbook piece for helping us to understand why Wilson is crucial in transforming European art. It was painted four years after Wilson returned from seven years abroad, but the composition must have seemed foreign to those who saw it then.

At first glance, as intended, the effect of the light suggests that this is a view by one of the great 17th-century classical landscape artists, Gaspar Dughet or Claude Lorrain, of the Roman Campagna. Rain approaches from the west, threatening the group of three figures who laze beneath a tree in the foreground, a compositional device allowing the artist to make us look at the hazy landscape beyond.

But the river is not the Tiber. Rather, it is the Dee in Wales, near Mold, where Wilson had briefly lived near more affluent cousins of his mother's family after the death of his father in 1728 before he moved, aged 16, to London to train as an artist, at first predominantly as a portraitist, but also as something of a landscape painter.

A son of the rectory, Wilson (1714-82) had grown up at St Cadfarch's, Penegoes, in Montgomeryshire. The medieval church he knew from his childhood was torn down in the Victorian period to be replaced in 1877 by a fine church by John Prichard (1818-86), the diocesan architect for Llandaff; but Turner had made a point of visiting the rectory in 1798 on his first jaunt into Wales.

Mold clearly impressed the teenager, and the view of Holt Bridge seamlessly marries the open light style of the Italians with a half-remembered, half-visionary recollection of his Welsh homelands.

The sweeping space afforded by such open skies would be in stark contrast to the filth and squalor Hogarth had immortalised in Gin Lane in the London to which Wilson had returned. Whereas the Mediterranean sun always makes street scenes of Rome or Naples look bawdy or scruffy at worst, there is something stiflingly dirty about London in the same period.

Family connections rather than the jobbing London artist to whom he was apprenticed in Covent Garden first brought him success at the Court of Frederick, Prince of Wales, a far cry from his own childhood in the Principality. Before leaving London, he ended up portraying the future George III and his brother Prince Edward Augustus in 1748-49, with their tutor, Dr Ayscough, and also the Jacobite heroine Flora Macdonald, after her release from prison.

What he made of the Georgian London he first encountered as a teenager is not, however, the subject of this show. None of his society portraits is included, and nor are there any of his topographical paintings, such as his view of the Inner Temple after the disastrous fire of 4 January 1737, or his painting a decade later of the Foundling Hospital, for Thomas Coram's Foundation.

Rather the exhibition begins in medias res as we find Wilson, aged 38, already installed in Rome in 1751, in the Eternal City of the benign Pope Benedict XIV (1740-58), at the high point of the Grand Tour. He had left England the year before, on 12 March 1750, determined to reach Rome, but writing to his sister, "God only knows if ever we shall meet again."

Entering the spaciously staged exhibition, we are at once confronted with the artist himself, portrayed by the younger and better-known German painter whom he befriended in his first year in the city, Anton Raphael Mengs (1728-79). Wilson is in day dress, his hair caught up in a silk turban. In front of him on his easel we glimpse a corner of a landscape painting that he has begun. The year was 1752, the year in which his home country (and all British colonies) finally accepted the Gregorian calendar that had been introduced in Rome in 1582.

Joshua Reynolds, the Adams brothers, and Gavin Hamilton were among the illustrious British artists there at the time. Stubbs joined them in 1754. But Wilson chose not to live in their artistic commune, finding instead, at the outset, a palace on the Piazza di Spagna which was large enough to accommodate a studio. There, he taught European pupils, many of whom are represented in this show, and from which a friend, Thomas Jenkins, operated as an agent for Grand Tourists.

Wilson at least socialised with the British artistic colony, but he was rather more influenced by the French at the Académie de France, especially the celebrated marine-landscape painter Claude-Joseph Vernet.

It was at Vernet's persuasion that Wilson abandoned portraiture to concentrate on landscape painting. Cardiff visitors have an advantage over those at Yale, as they can judge for themselves, in Gallery 9 of the permanent collection, the wisdom of this advice.

The three early portraits currently displayed there are all somewhat hapless examples of English portraiture of the mid-Georgian period. In Boy with Apples, anonymity alone conceals the embarrassment of the pouting youth of post-pubescent privilegem who idles with an apple as if he cannot recall the cause of Adam's fall.

The Under Secretary for War and later (1779) baronet Edward Lloyd of Pengwern is painted honestly, which is not much of a commendation, and the High Sherriff of Merioneth, Richard Owen of Ynysmaengwyn (1684-1760), painted around 1748, frankly appears dull.

I kept having to remind myself that, although Wilson knew the Twickenham where Alexander Pope held court from 1719 to 1744, the social critic and playwright Richard Sheridan was not born until the year Wilson reached Rome, via Venice, Ancona, and Terni; little wonder that his British sitters look so painfully one-dimensional.

Wilson had tried landscape rather than just cityscapes before he set off; his ruinous view of Caernarvon Castle of the mid 1740s (it is well worth searching out in Gallery 4) remains the earliest Welsh scene painting, offering a manipulated panorama and a social observation of a divided society, as we see both a gentleman relaxing and some peasant figures working.

In Rome, the light and the examples of landscape around him were plentiful, and included the frescoes in the Palazzo Colonna, painted by an earlier visitor to Rome in the previous century, Gaspar Dughet (1615-75). Here, too, he could see at first hand the paintings of Claude Lorrain, and the savage landscapes of the Neapolitan Salvator Rosa. A brilliant recapitulation of Rosa's work comes in the 1752 canvas Landscape with Banditti: The Murder.

The central conundrum with which the exhibition plays masterfully is that a Welsh Anglican visitor to papal Rome reprises the work (imitation, in the sense of Samuel Johnson's indebtedness to Juvenal in 1738 for his poem London) of predominantly French Catholic artists from an earlier century to explore his own native country.

The rectory boy remained nervous of the Catholicism he found himself up against. A much repeated composition shows a bend in a river beneath a steep gorge with a chapel on the headland. Since it was engraved in 1825, it has always been called The White Monk, after the title of the engraving.

The 1760s version that we see here is from the Cardiff collection, whereas our American cousins saw the primary version once owned by the Earl of Bridgewater and now in Toledo, Ohio. In both pictures, and in the studio copy on show, Wilson depicts a turn in the river which the curators have located on the Aniene, which flows through Lazio. At Subiaco, it is closely associated with St Benedict, and there are several monastic communities along its length.

In the foreground, a young couple hide playfully under a parasol from a passer-by who trundles off on a donkey, obliviously. They seem equally unaware of the artist working on the bank near by. Wilson painted, as well as often drew, en plein air, an unusual practice at the time, which he continued in England on his return.

The lovers distract us from seeing the much smaller figures on the opposite bank, where one of the Greyfriars flagellates a Benedictine,

bowed over as he submits to discipline. The monkish figures themselves are diminutive, but their action is telling, and serves to pass judgement on the casual carryings-on of youths.

In another painting, Solitude (1762), two Franciscans, or possibly rustic hermits, one of whom is reading, deliberately draw our attention from a distant scene in a forest clearing beyond where eight or nine Whitefriars complete the Stations of the Cross, standing around a large outdoor cross in front of a small chapel. A monkey on a column on the opposite side of the picture gives away the joke.

Wilson's accomplishments with a pencil are as notable as his pain_tings. In particular, the presentation drawings commissioned by the 2nd Earl of Dartmouth in 1754 are out_standing. The Dartmouths' album, found at Patshull House near Wolverhampton, was exhibited in 1948 and sold in 1954. Only the whereabouts of 26 drawings from a reputed 68 is now known.

Wilson's fellow-countryman Thomas Jenkins framed the drawings with rich lilac borders, some of which have now decayed to a softer lavender that sets off the views of Rome with a suitable grandeur. The Temple of Minerva Medica and the Via Nomentana offer 18th-century views of the ruined imperial city.

But this exhibition offers more than just a chance to see 18th-century views through the lens of a respectable traveller. The work of many of his pupils and disciples is included, a rich testament to their ability, and offering an introduction, mine at least, to the German Adolf Friedrich Harper, who died in the same year as his master.

The enigmatic, half-glimpsed details in the Neapolitan oil sketches of Thomas Jones (1742-1803) are increasingly known, but it will be the work of J. M. W. Turner and of John Constable which most gallery-goers will more immediately recognise.

The last exhibit, Turner's Lake Avernus: Aeneas and the Cumaean Sybil, proves to be a copy of a work of Wilson which his 35-year-old follower painted even before ever crossing the Alps himself (in 1814 or 1815). Furthermore, it was intended for the Hoare family at Stourhead as a pendant to one of Wilson's pictures commissioned a generation earlier.

Following Wilson's example, Constable set Malvern Hall, Warwickshire, off centre in the composition of a painting of that title (c.1820) which is more about morning summer light than about architectural detail. "Poor Wilson. Think of his magnificence, think of his fate!" wrote Constable in after years.

As I walked across Cathays Park away from the exhibition, I, too, worried that an artist who had descended into chronic alcoholism and increasing debt in the 1770s, and who was taken advantage of by his publishers, has had to wait so long to be restored to a rightful place in the pantheon. 

"Richard Wilson (1714-1782) and the Transformation of European Landscape Painting" is at the National Museum of Wales, Cathays Park, Cardiff, until 26 October. Phone 029 2057 3000.

www.museumwales.ac.uk