Chartres Cathedral; A window on the Middle Ages

It has never been necessary for the traveler to Chartres to ask directions to the cathedral. From almost anywhere in the town, and farther on clear days, its spires are visible: the older octagonal, rising to a smooth point, the younger taller and intricate - assertions of a spirit above mere symmetry.

Between 1170 and 1270, 80 cathedrals and 500 churches of the cathedral class were built in France, all in the new Gothic style which, as 19th-century critic Henry Adams put it, had ''swept Europe off its feet with delight.'' Mr. Adams compared the expense of this building boom to that of waging a war.

Today the cathedral at Chartres is one of the most-visited monuments in Europe - partly, perhaps, because it's an easy day-trip from Paris, but mostly because many wars and changes of taste have somehow spared its glorious windows.

Within the cathedral, a dim and echoing cavern hung with panels of blue and red light, on almost any afternoon you will see tourists gathering next to a sign that says ''Lectures in English, 12:15 and 2:45.''

Naturally enough, lectures in English are not really a common commodity in non-Anglophone countries. Frequently the tourist is left to distinguish the early baroque altarpiece from the flamboyant Gothic font with the aid of printed material. But in any case, lectures like those Malcolm Miller, guide to Chartres for the last 23 years, gives here are not very common anywhere.

On the day I visited Chartres, Mr. Miller, after ushering his group to a row of hard little chairs in the back of the church, began, ''Ten thousand hours is not enough to see Chartres.This is a library of stained glass and sculpture. . . . It's rather sad coming to Chartres because it is through Chartres that we understand what once existed elsewhere.''

Mr. Miller, a tall slender British gentleman with a commanding manner, then launched into a brisk history of the site. Chartres was a major city in Roman times - and the Roman roads that led to it ensured that this importance would last through the medieval period. But in the disorder following the collapse of the Roman Empire the city was twice sacked by the Vikings, losing a church on the Chartres site each time. On the first of these occasions, the Viking leaders had asked to enter the gates of the town to be baptised. This request granted, they burnt the church to the ground: Nothing survived. ''This could be the history of any European city,'' said Mr. Miller, adding dryly, ''You wouldn't have enjoyed this period. . . .''

The second Viking raid spared only the crypt, still located under the altar, said Mr. Miller, waving a long arm toward an energetic marble altarpiece.

Chartres, now just a small country town with a famous church, became, after the arrival of a scholar named Fulbert in 990, an outstanding neo-Platonic center of learning. Before universities were founded, education was in the hands of the churches, said Mr. Miller. ''Chartres was a part of the medieval 'Ivy League,' because of the great people who taught here.''

The width of the present church - the fifth known to have been built upon this site - was dictated by the wide crypt of the third (''we know this is so, because we've had the paving up''), a Romanesque affair with low wooden vaultings. ''At Chartres, the solution was found to put a stone arch across the widest church in France,'' said Mr. Miller.

The fourth contributed its facade. ''Had you come here in 1181,'' said our guide, ''you would have come through the same door you came through today.'' The fact that the present church was built in less than 30 years explains the unity of the building.

The Gothic style of architecture (the term Gothic, by the way, is a 17 th-century word meaning vulgar and barbaric) reduced walls to a degree never attempted before. It is hard - for those familiar with the flexibility of modern steel-frame construction - to comprehend the daring that produced these soaring arches. But, remember what the previous structure must have been like - for surely it had the low, heavy, rather cellarlike quality of a Romanesque church; it must have seemed to the residents of Chartres that the architect had obtained a special exemption from the law of gravity. At Chartres, you can understand the construction almost better from the outside; where you can see that between the flying buttresses, the wall is almost entirely window.

Chartres was a highly prestigious international effort - ''though people didn't think in terms of countries in those days,'' commented Mr. Miller. ''The medieval concept was allegiance to a person. In the 12th century the Capets ruled over less of France than the Plantagenets, causing the Hundred Years' War.''

Lords and queens contributed to Chartres, but merchants and craftsmen - the rising new middle class - contributed more windows (43) than any other social group. You can tell which windows were given by guilds: While the main part of these naturally concern biblical stories and figures, yet they have their small butcher or baker in the bottom corner, representing the guild that paid for the window; the shoemakers - not surprisingly for a pilgrimage church, Mr. Miller points out - have two.

''Chartres was built during a century of change - when history speeds up; a time when the feudal system collapsed and the modern political era began.''

Mr. Miller then ushered his listeners over to the front of the church. This is where pilgrims slept, fairly uncomfortably, one assumes; the floor is slanted toward the doors so that it could be sluiced more easily. The windows had been newly cleaned. ''We're probably looking at them as John of Salisbury (bishop of Chartres, 1176) saw them,'' said Mr. Miller. The glass was cut and assembled on the site - a necessity, as there were no standard measurements in those days.

''No one person can understand Chartres. It's like the world's largest jigsaw puzzle. The medieval artists who designed it intended that the cathedral embody the history of the world as they understood it, from Paradise Lost to Paradise Regained, with Christ in the middle of time - we make non-Christians accept that idea.''

The windows of the facade illustrate the geneology and events in the life of Jesus (''Though the Council of Trent (1545-63) purged the Scriptures; there's a lot here you won't recognize,'' said Mr. Miller.)

The three Magi represent the three continents - ''All peoples will adore him.'' At the entry to Jerusalem, Mr. Miller points out, the crowds are waving palm branches - the customary greeting for a hero; the white mule symbolized royalty. (By the way, you really need binoculars to see these details.)

The windows inside are linked to the sculptures outside. ''In the medieval mind, it was the symbol that was important,'' said Mr. Miller, leading us out of the giant doors into the chill gray air to have a look at the South Porch. ''It was believed,'' he explained, pointing to a figure of Jesus surrounded by his disciples high above the central doorway, ''that the 12 apostles would assist on the Day of Judgment - we still have 12 people on a jury.'' All the figures would have been painted in brilliant colors - ''These would have glittered,'' he said.

''The great cathedrals were largely inspired by the Book of Revelation. The church was a symbol of the heavenly Jerusalem, and 'The walls of heavenly Jerusalem are furnished with - no, not stained glass - but precious stones. . . .'

''Seeing Chartres may change your life entirely,'' Mr. Miller remarked to me, between lectures. ''One January, when there weren't many people in the cathedal, a man came over to me and said, 'I just had to tell someone that this is the most beautiful thing I have ever seen in my life.' Some people are indifferent to it; they walk through the cathedral in bathing costumes eating ice cream. . . .

''I don't think you can be confronted with beauty of any kind and not be moved by it, whether it's a Schubert string quartet or the Grand Canyon or the Acropolis.

''Each season has its attractions. I think, on the whole, the most brilliant time of year is October. The windows sparkle. In summer the sun's too strong, really, for looking at the windows. . . .

''People think a guide is just a parrot who speaks the same thing over and over. But really, it's like being a concert pianist who does his best to communicate something he loves to other people. It's different each time.

''I hope that people will go away dissatisfied, so that they will come back for more.''

Practical details: Chartres is about an hour by train from Paris; leave from Gare Montparnasse. All of February and March, Mr. Miller ''takes Chartres on tour'' - which means, of course, that he won't be at Chartres; he might well be in your hometown, however, giving a lecture on the cathedral in an art gallery or university.

There are several pleasant cafes next to the cathedral, where you can get a sandwich or a light meal; if your taste runs more to Pate en Croute or Brochette de Saint-Jacques Grillees aux Herbes, the Henri IV restaurant, which has a Michelin star, is a block or so away.