This station for modern masters in their milieu

A slightly rueful Charles Saumarez Smith conducts us on a guided tour of France's monumental temple of 19th-century art

Incredible though it may now seem, permission was granted in 1970 to demolish the Gare d'Orsay, the monumental railway station designed by Victor Laloux and opened in 1900 to take passengers from the centre of Paris to the Gare d'Austerlitz and beyond. It was proposed that the site should be occupied by a luxury hotel, designed in the most contemporary style, and Le Corbusier himself submitted proposals for a huge rectilinear block that would have disfigured the whole of the centre of Paris south of the Tuileries.

Fortunately, Parisians were sickened by the 1971 demolition of Les Halles and, in 1973, the Gare d'Orsay was listed as a historic monument. The question then was: how should its great open spaces, which had lain more or less empty since the outbreak of the Second World War, be used?

A plan was put forward by the Réunion des Musées Nationaux for a museum that would bridge the gap between the Louvre and the Centre Pompidou, showing the history of Western European art (predominantly French) from 1848 to 1914. It was a grand scheme, perhaps possible only in France, which had an integrated structure for the management of the national museums and which allowed the combination of the great collection of Impressionist paintings - previously exhibited in the Jeu de Paume - with collections of 19th-century academic painting, sculpture, photography and decorative arts.

The Italian architect Gae Aulenti was commissioned in July 1981 to convert the old station into a working museum. She retained the overarching central structure and the atmosphere of a late 19th-century exhibition hall - it had, after all, been described as having the appearance of an art gallery when it first opened in May 1900 - but, at the same time, created a subsidiary, slightly Egyptian structure of internal pavilions built out of a pale pink Buxy limestone.

Not everyone likes it. It provides a slightly unorthodox space for the display of art - with paintings hung on steel rods attached to holes drilled into the limestone. But it does succeed in creating an agreeable, complex, differentiated set of gallery spaces, many of which are filled by natural daylight and punctuated by plenty of spaces in which to sit, reflect and read.

It is hard now to recollect the impact of the opening of the new Musee d'Orsay in 1986: the opulence of its installation; Aulenti's ostentatiously monumental postmodernism; the way in which the Impressionists had been wrested from their discrete and comparatively intimate display in the Jeu de Paume and replanted in the midst of the 19th-century academic tradition.

Instead of works of art displayed on their own in the expectation that they should be judged aesthetically, the implied thesis of the Musee d'Orsay was that works of art were better understood in the context of their time, and that the work of Monet and Manet would benefit from being shown alongside the work of Thomas Couture and Jean-Baptiste Carpeaux.

It is, in truth, an astonishing collection, as is brilliantly shown in the new illustrated catalogue, which has been published in this country by Thames and Hudson and edited by Serge Lemoine, the museum's scholarly director.

The collection begins with the extraordinarily supercilious picture by Ingres of La Vierge à la Hostie , in which the Madonna looks down from above at the erect host. Delacroix is represented by some of his vigorous oil sketches of horses - the Chasse aux lions and Chasse au tigre , both of 1854, and Chevaux Arabes se battant dans une ecurie of 1860.

There are rich holdings of works by both Corot and Millet, including portraits by both artists, and by Courbet, including Un enterrement a Ornans , which caused such a stir at the 1850-51 Salon.

What the book demonstrates, as does the museum, is that the work of these well-remembered masters can best be understood alongside the work of those painters now much less well regarded but who were the artistic stars at the time - fêted at the Exposition Universelle - such as Paul Huet.

The ground floor of the Orsay is dominated by Couture's Des Romains de la decadence , the star of the Salon of 1847 and the only painting that, because of its size, is not hung in one of the pavilions.

Not surprisingly, it makes much less impact in the book, and, instead, one is struck by works such as Jean-Léon Gérôme's Jeunes Grecs faisant batter des coqs as representative of this period of polished, but slightly decadent, late neo-classicism, so much admired by Théophile Gautier.

After the insipid, languid maidens of Pierre Puvis de Chavannes and the glorious, glittering, overheated imagination of Gustave Moreau, one of the great masterpieces of the collection is Degas's La famille Bellelli . Each person is caught isolated in a separate interior life, but between them is an insistent rhythm of inter-relationship: the mother proud and aloof; the father looking sadly at his younger daughter; a clock on the mantelpiece and a double view in the mirror of a window beyond. It is an enigmatic domestic psychodrama, as powerful as anything he painted subsequently.

Henri Fantin-Latour depicts a group of artists standing alongside a heroic self-portrait of Delacroix. It is a reminder of the close interconnectedness of the artists in Paris in the 1860s, Whistler standing beside Fantin-Latour (co-founders, along with Alphonse Legros, of the Society of Three) and Manet next to Baudelaire, who had written a long homage to Delacroix (who died the year before the picture was painted).

In the context of the Musée d'Orsay, it is possible to reconstruct the shock of Manet's Dejeuner sur l'herbe , so freshly painted, so unconventional in the way that it poses a naked female in the company of the two bourgeois males, overdressed for the countryside and ostensibly oblivious to her, the only person paying attention to the artist. As Baudelaire wrote: "This man will be the painter, the true painter, who will make us see how grand and poetic we are in our ties and patent leather booties."

But the freshness of the painting and its evident references to Titian's Fête Champetre did not prevent the jury of the 1863 Salon from rejecting it, which led to the establishment of the Salon des Refuses.

Manet's Olympia , exhibited two years later, was regarded as even more shocking, perhaps partly because it, too, is based so obviously on Titian's Venus of Urbino , but instead of a classical Venus it depicts a prostitute with large breasts and rather short legs, and a cat at the end of the bed arching its back in defiance. As Manet himself wrote: "I render the things that I see as simply as possible." But he must have known that it was an act of provocation.

Upstairs are the Impressionists, whom the Japanese tourists come to visit, as far away from the entrance as it is possible for them to be, which is perhaps a high-minded museological tease, like the determination of the Louvre to ignore the fact that it has become an international tourist destination.

By now, one has already seen the equivalent of at least three blockbuster exhibitions, including, most of all, extraordinary holdings of the works of Manet, Monet and Degas, each one of which would, on its own, justify a pilgrimage across the world.

The justification for a book such as this is that it makes it possible to study the works before a visit and to remind oneself of the holdings on one's return. The quality of reproduction is good and the texts helpful, but necessarily rather in the background, as few people are really likely to read the texts, but more to leaf through the book in admiration of the wealth of the Musee d'Orsay's holdings, mingled, if one works in a museum, with not a little jealousy at the extraordinary generosity of both private individuals and the state in France.

As Lemoine says in a nice piece of understatement in his introduction: "The collection has profited from the exceptional and beneficent national system of dations , tax-offsetting bequests". As a result, the Musee d'Orsay is graced with, for example, Maurice Denis's Les arbres verts , Monet's Effet de vent, serie des peupliers and Courbet's still startlingly graphic portrait of a female nude, L'origine du monde.

Is it imaginable that, in a similar vein, Gordon Brown might help the National Gallery to obtain a full-frontal view of a vulva?

Charles Saumarez Smith is director of the National Gallery.