Entering the Louvre: a Greenhouse Effect. The pyramid-entrance achieves a reasonably subtle balance between lightness and monumentality

THERE aren't many museums in the world that you enter through the roof. But thanks to the Chinese-American architect I.M. Pei - not to mention the ambitious notions of President Fran,cois Mitterrand of France - the Louvre, or ``Le Grand Louvre,'' as it is now being proudly called, is one.

As most of the world must now be aware, the new main entrance to Paris's Louvre is in a glass pyramid set up in the middle of the museum's Cour Napol'eon. And this pyramid-entrance, designed by Mr. Pei, is really a large skylight, a glass roof.

Yann Weymouth, architect in charge of the project for I.M. Pei and Partners, has pointed out: ``Pei wanted our design of the surface to be landscape, not architecture, in the great tradition of Andr'e Le N`otre'' - and thus the pools and fountains. And in fact the pyramid does have, to coin a phrase, a kind of greenhouse effect: diamond-lozenge panes and an interior spiderweb of a stainless-steel tension structure `a la Buckminster Fuller, with three baby pyramids, like ducklings in tow, marking the three wings of the museum.

There is a slight feeling of a garden center about it all. The only thing that's missing - which, seriously, is rather a pity - are plants or trees. But aesthetics instead of botanics - it's a fair-enough exchange.

The pyramid has had its share of doubters in the five years from conception to completion. A few still speak out loudly - like Derek C. Pey, writing earlier this month to the International Herald Tribune:

``...Many American shopping centers display more original architecture, lighter construction and more luminosity. But of course they lack the titillating affront to the old monuments surrounding it.'' French visitors are heard muttering about Mr. Mitterrand's audacity in having this monument to his reign built.

But in fact the decorative ``old monuments'' - never, surely, the world's most thrilling architecture, anyway - survive the ``affront'' pretty well. The pyramid achieves a reasonably subtle balance between lightness and monumentality, between cheek and modesty, between drawing attention to itself and deferring to the old buildings.

It is only as you approach the new entrance from the M'etro through the now publicly accessible Richelieu Passage that this glass structure seems momentarily too large: It blocks the view through the passage's end-arch.

As for its luminosity, it could, presumably, have been even more transparent than it is, allowing even clearer views of the old buildings, and a thin layer of urban dust was coating it during my visit in spite of the promise of frequent cleaning. At night, though, it is brilliantly lighted from inside, and the warm effect then is simply magical, a real enhancement.

Going inside the pyramid is also quite an experience. Indeed, it is something from which designers of entrances to subways or metros could learn. You enter at street level, though it is not literally a street you have come from, but the Cour Napol'eon. Once inside, you cross the platform and then down you plunge, by escalator, spiral staircase, or open-topped elevator (for the disabled), to the depths.

The depths, however, turn out to be a vast accueil, or gathering place, which is brilliantly lit by the ``skylight,'' and there's not the slightest sense of claustrophobia or subterranean dinginess.

It's a heroically scaled, splendidly spacious, stone-clad hall, and it seems perfectly able to accommodate the mega-crowds that - out of touristic duty or genuine affection - increasingly invade this sacred old temple of art in search of the ``Mona Lisa,'' the ``Venus de Milo,'' the ``Winged Victory,'' and other favorite symbols of the Greatness of Art.

I say ``seems'' because there remains a mystery. One of the clear purposes of the Grand New Louvre's grand new entrance is to make public access easier. It brings with it a welcome variety of modern facilities hitherto unknown in this palace-turned-museum, things like computerized information services and video monitors and a post office and a place to change the baby's diapers.

Yet, ironically, the lines are running longer than ever. They wind round the pyramid and its reflecting pools and fountains in the courtyard, and they move forward with irregular slowness. No doubt there are many more people being drawn to the Louvre by the sheer architectural novelty of the pyramid entrance. And it is true that Parisians love fervently to queue for their culture, bringing to the practice a willful determination impervious to all weather (they're queuing endlessly for the Gauguin exhibition at the Grand Palais, just along the Seine, with exactly the same devotion).

Perhaps the whole thing is a psychological ploy: What you've had to wait for you appreciate all the more. You don't encounter the ticket booth until you are downstairs in the accueil - entrance that far is free. So why don't the French authorities just open the doors and let in all comers? The Louvre, both in its new and old parts, is surely perfectly able to cope now. And there was always a seething mass around the ``Mona Lisa,'' anyway.

A further minor but significant puzzle is that, although the new museum bookshop is outstandingly well stocked and spacious, the post-card shop, which everyone wants to visit, is ridiculously small and crammed. Today museums need enormous post-card shops. This is one silly mistake at the Grand Louvre.

In fact, the pyramid is by no means the end of the story. And until the far-reaching redistribution of the Louvre's gigantic collection is completed in 1993 - and this includes taking over the palace's third wing, the Richelieu Wing, just about to be vacated by the French Ministry of Finance - the museum is bound to seem, as it still emphatically does now, to be in a state of transition.

But it is already somewhat easier to orient yourself. New signs help to convince the visitor that finding his way around is as easy as grasping the fact that there are only the three main wings, ``Denon,'' ``Sully,'' and (eventually) ``Richelieu.''

In practice, however, I'm not sure that getting lost isn't somehow an ineradicable characteristic of the Louvre experience. I went in search of the beautifully re-lit and re-arranged ``Salle des Cariatides,'' with its stunning array of antique sculpture, Roman copies of Greek originals.

On the way I stumbled into such delightful distractions as the underground display of the Louvre's medieval foundations, saw numerous enormous French 19th-century paintings, smiled at the ``Mona Lisa,'' sighted in the distance the still armless ``Venus de Milo,'' and staggered up and down the grandiose stairway that sports the ``Winged Victory'' in all her grace and glory. She is not to be moved, after all, to the pyramid, as once suggested: Her present, old location was specially designed for her, and there she is to remain.

After cross-questioning various guards (they do seem more willing to assist than they used to be), I did eventually find the Caryatids Room. But no plaque or notice told me I was there, and I had to work it out for myself.

So it might arguably be a pity if all the hard work of exploring the old Louvre were abolished. There is a certain triumph in eventually finding what you are looking for.

On the other hand, welcome improvements do include an excellent permanent exhibition illustrating the Louvre's own long and changing history.

There is also a large temporary exhibition space under the pyramid (though not lit by damaging daylight). Here a thoroughly enjoyable opening exhibition (through Aug. 21) presents 350 works from the museum's seven departments.

``Les Donateurs du Louvre'' pays grateful homage to the 2,700 people who have, since the palace became a museum nearly two centuries ago, given paintings or sculptures, drawings, objets d'art, or Oriental, Egyptian, Greek, or Roman antiquities to this venerable collection.

Such generosity deserves this nicely presented tribute. And it reminds one that, in spite of its monumental self-importance, in spite of its royal connections and palatial housing, the Louvre is the chief museum of a republican country. It's the people's museum.

Clearly real efforts are now dramatically under way to make it more appealing and more accessible to the people. The pyramid - the Pei-ramid - is the effective prow of that praise-worthy boat.