Think of London. What do you see? The Palace of Westminster? Tower Bridge? A Barbican show will make you think again.

IT'S almost axiomatic: If an Englishman gets lost in London, his best bet is to ask a visitor from overseas the way. By the same token the most telling images of London have been made over the centuries by painters and draftsmen from abroad - or at least that is the contention of a fascinating exhibition at the Barbican Art Gallery. Called ``The Image of London,'' the show's subtitle is ``Views by Travellers and Emigr'es 1550-1920.''

The artists range surprisingly from Rembrandt to Whistler, from Canaletto to Monet, from Gustave Dor'e to Andr'e Derain. Many of the artists represented are little known, and some of them never even visited London - like Utagawa Yoshitora, a Japanese 19th-century printmaker. His naive woodblock color prints of the distant ``barbarian'' capital of England is described in the show's informative catalog as based on ``a little research among Western engravings and a great deal of imagination.''

Even more curious is the case of Rembrandt. His two known sepia ink drawings of London in about 1640 (prior to the Great Fire of 1666 that changed the face of the city beyond recognition) look like quick sketches on the spot, until some of their oddities are pointed out. He shows the tower of Old St Paul's Cathedral, for example, swiveled so that it rests diagonally on the body of the building. Rembrandt almost certainly worked from available prints, unaware he was being inaccurate.

What emerges frequently here is that although accuracy did concern the host of foreign artists who depicted London, of far more importance to the most interesting ones was its image in the mind's eye. The way they imagined this giant metropolis radically affected the way they painted it.

The Great Fire, for instance, was seen from the Continent as divine vengeance wrought on a den of iniquity. Equally, however, the rapidity of its rebuilding - and its consequent modernity and subsequent expansion - astounded people from other countries. London was the modern city of the world, a fact it is hard for us to realize today. Eighteenth-century artists started to see it as a new Rome - classical, enlightened, and ordered.

In the 19th century, its reputation as the center of a fast-industrializing nation led to an artistic fascination with it as a place of extremes, mainly grim and unnatural.

Beyond these general themes, artists brought their own individual visions and preferences to bear on London. Venetian painters like Canaletto and Joli could see in it little but its similarities to Venice. The 19th-century French painter and book-illustrator Gustave Dor'e - whose almost apocalyptic visions of London are among the most oppressive and expressive reactions to the city in the exhibition - thought nothing of making topographical errors or exaggerations for effect. He particularly liked overcrowding his scenes with swarms of humanity. A simplified Dickensian atmosphere hangs over his back streets. If he saw Londoners as either appallingly poor or prettily rich and noticed little of the social strata in between, it did not detract from his work: The impact of his London paintings were still indelibly persuasive and potent. His London could be called a fiction: But then ideally art looks for the underlying truth, the imaginative feel of a place, rather than its surface appearance.

Whistler's etchings of wharves, ship's masts, and ramshackle buildings capture another facet of Victorian London. But it was his love of Thames mists in the falling evenings that most impressed other artists.

Fog had been associated with London since early in the 19th century: It was the dark measure of its funereal, anti-human conditions. But Whistler the aesthete perceived only poetry in it. In the evening mist ``the whole city hangs in the heavens,'' he waxed lyrically, ``and fairyland is before us.'' From such dreaminess came his painted visions of London as ``Nocturnes'' in blue, gray, and gold.

As might almost have been expected, the atmospheric envelope that enshrouded London in the imagination of foreigners appealed productively to that arch-painter of atmosphere, Monet, particularly late in his career. The Houses of Parliament became a repeated motif, but his real subject matter was the continually changing weather of the place. It is intriguing that even for him, mere observation was not the sole aim: He was still in pursuit of an idea of London. As marvellous examples in the exhibition demonstrate, it was the mystery that appealed to him.

The fog, he said ``gives [London] its marvellous breadth. Its regular, massive blocks become grandiose in that mysterious cloak.''

Compared with the final paintings in the show, however, it clearly did matter to Monet that it was London itself he was observing: The Houses of Parliament were not just another Rouen Cathedral or wheat stack. The specific atmosphere of London gave him as much as his paintings have given back to our accumulated image of London.

It is the London paintings of the French Fauve Andr'e Derain, mostly painted in 1906, that this show presents as the signal of the 20th-century breakdown of artistic interest in the city as a subject. A deliberate follow-on from Monet, Derain's paintings are as dazzling as anything he and Matisse painted in the fierce bright sunshine of southern France. But London's landmarks have become no more than that: recognizable shapes around which the excited painter has exploded his sunbursts of exhilarating color.

London, by then, had become an artists' clich'e.

(Exhibition runs through Oct. 18.)