It all happened so quickly...

How do artists capture a fleeting image or show the passage of time in a picture? Two new exhibitions at the National Gallery examine the solutions.

Curators have their work cut out just convincing people that Impressionists aren't way past their sell-by date. Renoir, Monet, Degas and Co have for so long adorned coasters, calendars, posters and T-shirts that they almost seem unfit for academic scrutiny. Exhibitions devoted to single artists, genres or eras just aren't enough. Nowadays the gallery must invest the familiar with fresh ideological approach, to give us something new to think about. So the National Gallery has come up with the theme of speed, or as they put it, "Painting Quickly...". Hardly going to get them queuing round the block, is it?

Curators have their work cut out just convincing people that Impressionists aren't way past their sell-by date. Renoir, Monet, Degas and Co have for so long adorned coasters, calendars, posters and T-shirts that they almost seem unfit for academic scrutiny. Exhibitions devoted to single artists, genres or eras just aren't enough. Nowadays the gallery must invest the familiar with fresh ideological approach, to give us something new to think about. So the National Gallery has come up with the theme of speed, or as they put it, "Painting Quickly...". Hardly going to get them queuing round the block, is it?

It's not as if the Impressionists tried to mask their painting practice - they were proud of their neglect of detail and they poo-pooed the pristine finish of their forebears. For them painting was an outdoor pursuit - you can imagine great rows of bearded bohemians setting up their easels in fields and parks, in restaurants and on pavements, scratching frantically at their canvases and leering at passers-by. The show encourages the viewer to disregard the contents and focus purely on the style.

But looking at painting this way can be disappointing. The glittering silvery surface of Monet's Towing A Boat, Honfleur (1864) is, on closer inspection, made up of unappealing swipes of pink, grey and white. Figures in Manet's and Degas's canvases can appear grotesque with their absence of fully formed noses, ears or eyes. However, it can also be a revelation, such as in the work of Berthe Morisot. The smallest strokes create the greatest effects. Figures are merely hinted at, boats are rendered in thin streaks of paint, the sea is just a series of zig-zags. Similarly, Monet's Marine near Étretat reveals a subtle distinction between the sea and the sky - a buoy, perhaps or a boat - is all that separates them.

Several of these pictures are sketches in the conventional sense, though. Renoir's rough draft for Ball At The Moulin De La Galette is far more appealing than the finished work. Autumnal colours supplant his usual sickly pastel palette; his dappled style is temporarily abolished in favour of boldly harsh strokes.

The experimental nature of these paintings is certainly evident. Objects are seen through the light that falls on them; textures aren't real, they are just, well, impressions. As the exhibition's curator, Richard R Brettell, points out, these artists were simply transcribing their visual sensations on to canvas. But this much we already know. What else can we learn from this exhibition? To be honest, not a huge amount. As the title suggests, speed was of the essence to this lot. They painted hastily in order to cope with the changes in their environment and, we are told, had a job convincing people that what they produced were in fact finished works.

It may seem odd that Manet should be included in this catalogue of rush-jobs - he was after all considered to be a draughtsman. But, as this exhibition delights in telling us, he was also a cheat. His Races At Longchamps (1866) may create a vivid sense that you are about to be trampled, but Manet spent hours in the studio trying to create this effect. Every single stroke - and there aren't very many - was the result of solicitous analysis.

Perhaps more unconvincing as a quick performer is Van Gogh whose paintings are relegated to a room simply marked "Post-Impressionism". Yes, his brush strokes were brutal and unrefined but, as we know, this was not all to do with capturing a moment in time. In fact, he came back to his paintings time and time again; how exactly does this square with the idea of painting quickly?

Telling Time is more clear-cut in its objectives. This exhibition brings together a small selection of paintings and photographs to show how artists have depicted the passage of time. The prevailing theme is how things haven't changed that much. At the start, the show sets a Tintin cartoon next to an illuminated 12th-century hymnal. Both present a sequence of images that are to be read frame by frame.

What's more startling is the foresight shown by early painters. At first glance, Giovanni di Paolo's Saint John The Baptist retiring to the Desert (c1453) seems sweetly naive. Saint John appears both at the gateway of the city and halfway up the mountain path. But then look at Etienne-Jules Marey's "chronophotograph" Jump of 1882. This revolutionary image combined different phases of a movement on a single plate. The technology may be different, but both di Paolo and Marey are working under the same principal. The same idea was later picked up by the Italian futurist, Gino Severini. In Dynamism of a Dancer a series of three-dimensional forms sliced into one another, thus creating the effect of a moving body. His visual language may have been inspired by photography but if di Paolo had been around to see it he would have laughed his socks off.

Artists' attempts to cram a whole story into a single painting allow us to read the series of events without question. In his famous Saint George and the Dragon (c1460) Uccello depicts two separate incidents in one scene: the wounding of the dragon and its subsequent humiliation (being tied to the princess's girdle and led back to the city). In fact, the princess already has the dragon on a leash and stands there, apparently oblivious to the danger, while St George gallantly stabs it in the eye. It's a truly spectacular image - who cares that it's really quite absurd?

Impression: Painting Quickly In France, to 28 Jan. Telling Time, to 14 Jan. National Gallery, London WC2 (020-7747 2885). Tom Lubbock will return in the new year