THERE HAVE always been deeply rooted prejudices against the idea of painting in watercolours. In the 18th century its detractors argued that it was essentially an unserious medium, fit only for Sunday painters, or for those who were learning to paint. One fact is sufficient proof that it was clearly regarded as illegitimate in those years: it was impossible to gain admission to the Royal Academy as a watercolourist.

Towards the end of the eighteenth century matters came to a head. A new society was formed - the Royal Watercolour Society - in an attempt to give legitimacy to the medium. Its first exhibition was held in 1805, in a gallery off Grosvenor Square, in London. An image of that exhibition is in this show, with works, principally of topographical scenes (as far as one is able to construe it), packed floor to ceiling - on one wall, for example, there are 37 paintings. Each one is gasping for breath elbow- room, and light.

This entertaining show replays those old battles, showing us works by members of that new society, and, on the opposite wall, works by a rival grouping which was formed in its wake. In a small glass case we can examine the Price Book that someone kept of that first exhibition. Each painting is numbered and its painter named. We can read whether it was sold, to whom, and for how much. No 13, an effort by "Glover" selling for a relatively modest pounds 31 10s, was perhaps only part-sold - a certain Mr Ore seems to have left a deposit of pounds 2 2s. Let's hope, for Glover's sake, that he came back to pick it up. A man of that name would have been a collector worth cultivating.

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The exhibition was a resounding success - 11,000 visitors, and 165 works sold out of a total of 275. By the 1830s the battle was won. The new queen herself - Victoria - became an enthusiastic collector of watercolours, thus helping to legitimise the whole enterprise.

The final room of this show demonstrates the range and quality of the painters who were by then working in oils - Samuel Palmer is here, with a characteristic dreamscape, and John Martin with something recognisably turbulent and visionary. By this date it no longer seems to have mattered what exactly the medium was - talent would out, whatever.

But, in the early years so well documented here, the question of subject matter was of paramount importance. Watercolour was deemed fit for topographical works, but not for history painting or portraiture. Where are the portraits in this show? There are none. There are, however, a few allegorical scenes, but even these seem quite different in mood precisely because they have been painted in watercolour and not in oils. Consider Stephen Francis Rigaud's 1807 painting of The Genius of Painting Contemplating a Rainbow, for example. It has an unusually fetching softness and vulnerability which it might have lacked if it had been painted in oils.

The very fact that the subject matter at times seems to be modest, and even humble, once helped to revolutionise our idea of what painting could and should encompass. The qualities of some of these early watercolours of topographical scenes - their fleeting- ness, the fact that they seem to be of the moment and even "on the spot" - caused later painters (the Impressionists, for example) to question the whole nature of painting and what could be its subject matter.

Painting, in later decades, took its cue from ideas about the transience of human experience. The leaden and laboursome monumentality of history painting in oils, once so exalted, looked, to later eyes, like so much dead pigment on canvas.

These watercolourists, these - God blast them! - so-called Sunday painters, were in fact helping to define and to create painting as we now know it.