A collection of Turner watercolours can be seen in Edinburgh, for one month only, at the start of every year. Something to do with topography, suspects Richard Ingleby.

The National Gallery of Scotland's January exhibition of Turner watercolours has long been an annual institution, as much a part of the new year in Edinburgh as is a hangover from Hogmanay. There are 38 of them, bequeathed to the Scottish nation in 1900 by the collector Henry Vaughan with the strict stipulation that they only be shown in the first month of the year "when the light is at its weakest and least destructive".

I'm not sure if Vaughan, a Londoner whose family fortune came from making hats, ever visited Scotland's capital, but I suspect that he didn't. Otherwise he would have known that his precious watercolours would have been perfectly safe for a lot longer than he specified - some days in Edinburgh it doesn't get light at all, save for a brief glimmer of dawn at around lunchtime. In recent times this has all become an irrelevance anyway since the pictures are shown in a dimly lit basement without any hint of natural light. The conditions would be much the same in July, but it is to the gallery's credit, in these days of disrespect for donor's wishes, that they have upheld the terms of Vaughan's bequest for over 90 years. Inevitably these circumstances have given this annual exhibition an air of importance that it wouldn't otherwise have. It's not that the pictures aren't any good - just that the sense of occasion which surrounds their showing has given them a treasured quality that has little to do with the work itself.

The watercolours given to Edinburgh were not the sum of Henry Vaughan's collection. He also owned drawings by Michelangelo and Raphael, as well as Constable's Haywain and numerous other Turners which are now in the National Gallery of Ireland. On the evidence here, however, he had rather conservative taste. Broadly speaking, these 38 watercolours span the whole of Turner's career and there are plenty of fine examples, but Vaughan's leaning was more towards the topographical sketches made for the publishing projects that were the bread and butter of Turner's life than to the late great works on which rest his place as the founding father of modern art.

The Edinburgh exhibition begins with a group of grey-blue views of English towns which look more like the work of Girtin than Turner, and may well be. These belong to the batch of works described as "the Monro school", a reference to Dr Monro's evening academy for promising young painters in watercolour; a class that included Cotman and De Wint alongside Girtin and Turner, and whose members' unsigned offerings all look pretty much the same. They are unremarkable pictures, but set a tentative topographical tone for what follows.

The best of the works on show here are those where Turner allowed himself to step outside the requirements of illustration and into the realms of the sublime, taking landscape to the edge of abstraction. One of the most effective of all is a tiny watercolour of Loch Coruisk painted on a trip to Skye in 1831. Two tiny figures (including Turner himself, perhaps, as one of them appears to be sketching) are perched on a rock above the loch; all around them a swirling vortex of hill and sky seems set to swallow them up. At a glance, it is a little hard to read but, for all his illustrative skills, Turner's genius was never in the detail. It is an image filled with power and presence: the scene seems enormous, but it is painted on a piece of paper no bigger than a postcard.

Looking at these watercolours, particularly at a view of Durham Cathedral, one could be forgiven for thinking that the man so often billed as our nation's greatest painter couldn't paint people. Actually he couldn't, or at least not very well, but in his more successful works (such as Loch Coruisk) it doesn't matter. The people aren't the point. If they are there at all, it is just to give a sense of scale or increase the drama - people, his pictures tell us, are small, nature is very, very big.

There's not enough of this sort of thing in the Vaughan Bequest for my liking, not enough of Turner the explorer and experimenter with colour and light, above all light. None the less, in his quest for a complete collection Vaughan gathered some marvellous things, including a representative group of pictures from Turner's three Venetian tours. The greatest of these is not one of his depictions of familiar architectural sights, grand though these may be, or the daringly empty Sun of Venice, in which the white of the page is left to do the work of both sea and sky, but a little sketch of Venice from the Laguna, dating from his last trip there in 1840. In this the artist looks the other way, not to the glories of Venice, but out to a green blue sea and a smudge of sooty smoke from a passing steamer. As ever, Turner was at his most masterful, most evocative, when he took himself to the edge. It's in works like this that the future of modern art lay.

`Turner Watercolours: The Vaughan Bequest' is at the National Gallery of Scotland, the Mound, Edinburgh (0131-624 6200) to 31 January.