RAVENNA IS A PLEASANT NORTHERN ITALIAN TOWN practically on the Adriatic coast. The city and its surroundings are flat, which makes for easy walking and accounts for the city's bicycle cult. Bicycles are parked thickly, leaving the streets comparatively free of standing cars.

I live in Tuscany and Rome and am used to the southern and central Italian cities that teem with expansive human life and self-conscious beauty. But Ravenna gives a clean, rational, hard-working impression. Since the city was bombarded during World War II, most of the buildings are modern or reconstructed, but fortunately still not tall enough to dwarf such landmarks of antiquity as a leaning tower (preserved under the tutelage of the local Lions Club), the churches, baptisteries, the Cathedral.

In Ravenna is the tomb of the exiled Dante Alighieri, who finished the ''Divine Comedy'' there shortly before his death in 1321. The tomb is a quaint edifice of the 18th century, altogether inadequate to the grandeur of its purpose. Byron lodged in Ravenna during his courtship of the Countess Teresa Guiccioli and was involved in the city's violent politics; the site of his lodgings is commemorated by a plaque. But Ravenna is not essentially a city of exteriors. Its truly great marvels are the mosaics on the interior walls of its early medieval (fifth to seventh century) monuments.

The city was from the earliest times on the Eastern trade route. The important period of its history began in the fifth century. Rome was on the wane, her troops were evacuating Britain and Gaul to defend Italy. Ravenna, now a few miles inland, was then on the seacoast, a famous Roman port that had been been built by the Emperor Augustus. It was busy with exotic traffic. One can imagine the poet John Masefield's ''Quinquireme of Nineveh from distant coming into port with its ''cargo of ivory / And apes and peacocks, / Sandalwood, cedarwood, and sweet white wine.''

The art of mosaic mural designs and representations constructed of small colored stones and glass, often on gold foundations and embedded with mother-of-pearl, had already appeared in Italy by the end of the Roman Empire. By the classic process of invasions, battles, murders and usurpations, Ravenna fell within the power of the Ostrogoth king Theodoric, largely to whom and to whose Byzantine successor we owe the marvels of cultural heritage preserved in Ravenna today. No photography can really convey the effect of these mosaics. The nature of mosaics is based on prismatic reflections. Picture postcards and photographs are very good for depicting the close-up details, but the total impression can only be got by being there, inside those buildings, and walking around. From every point of contemplation a new aspect emerges.

Mosaics are more durable than other pictorial arts; they are less likely to be badly restored or faked. The work of restoration and preservation of Ravenna's mosaic treasures in recent years has been particularly fine and diligent, involving centimeter-by-centimeter attention and almost microscopic treatment of the original cementing.

It helps to know that in those early days, and to those artists and their patrons, theology was politics, and that many of these mosaic murals were inspired by the Arians, a heretical, powerful and, in many ways, noble branch of Christianity. The Arians acknowledged the human supremacy of Jesus but denied His godhead. It is also useful to know that the monuments belong to three periods: the Imperial, fifth century, influenced by the adventurous Catholic Roman Galla Placidia and her brother Honorius; the period of the Arian Theodoric, in the fifth and sixth centuries, and the Catholic Byzantine period dominated by the Emperor Justinian and his wife, Theodora, who was attached to the Monophysite heresy, which held that in Christ there existed the divine nature alone.

There is always a connection between an art form and the thought process of the people to whom it spoke. Just as Renaissance art, with its defiant portraiture, its flowing robes and hair, its fluid religious figurations, addressed itself to a new humanistic spirit, bursting with adventure and bold concepts of good and evil, so do the mosaics of the early Middle Ages, still and tranquil, gleaming in their own spiritual light, reflect the encroaching Oriental sensibility.

TO KNOW SOMETHING of the history of these records of faith, triumph and civilization adds to their appreciation. But if the nuances of Western history between the late fifth and seventh centuries are not your vital passion, you can, with equal and marvelous profit, simply plunge into the glory of color and light that livens the walls and domed ceilings of Ravenna's monuments -as did Henry James, lamenting ''the thinness of [ his ] saturation with Gibbon and the other sources of legend.'' The sites of interest fall within a rectangle and are of easy access. Most of the interiors are lighted by means of a coin machine, so it is advisable to take a supply of 100- and 200-lire coins. Any itinerary is equally dramatic in its effect, but it is perhaps more logical to start with one of the oldest examples. This is the small but sumptuous Mausoleum of Galla Placidia, built in the form of a cross, an oratory bejeweled with symbols. A semicircular cupola represents a deep blue, gold-starred sky surrounded by the Lion of St. Mark, the Bull of St. Luke, the Eagle of St. John and the Angel of St. Matthew. Each alcove is decorated with lively animal symbols mingled with more stylized abstract designs. The predominant color is greenish blue. Above the entrance shines a picture of the Good Shepherd and six mystic yet realistic sheep. Here, too, are the famous drinking doves, perched on the edge of their vases. IN ALL THE MOSAICS OF