ARCHITECTURE / Ark de triomphe: Fitzwilliam is not the most beautiful college. But its new chapel is a revelation. Naomi Stungo takes a pew

IF THERE was a prize for the ugliest college at Cambridge, Fitzwilliam would have to be a contender. It was designed in the early Sixties by Denys Lasdun, and commands even less affection than his more famous work on the South Bank. It stands on Huntingdon Road, just north of the city centre: a bank of deep purple brick, relieved only by ribbons of stone that form pale snakes round the courts. On a winter afternoon the college has a dour, oppressive air, relieved only by the brilliance of the gardens, with their modern, abstract shapes and surprising colours.

Now the gardens have some competition. For the first time, Fitzwilliam has a chapel, and, in a neat twist on the usual role of the ecclesiastical building, it subverts the solemnity around it. It is not a dramatic building: its effect comes not from a brilliant eclat but from a subtlety of form and materials. Its rounded bulk unfurls gently at the end of a severe block of student rooms, like a pair of arms, gently offering something good.

Approaching it through the garden, you see that it is both like the rest of the college and unlike it. Its architect is Richard MacCormac, president of the Royal Institute of British Architects. Like Lasdun, MacCormac has used purple brick and stone. But his bricks are packed together with a darker mortar, and his stone bands become four narrower strings which wrap the chapel and further emphasise its generous rotundity. A cornice of stone adds a sense of certainty and completion that is lacking in the rest of the college.

Inside, the chapel discloses itself slowly. Spiral staircases peel off to the left and right as you enter the small entrance hall, doors ahead of you lead into the crypt, light filters down from above. The hall itself makes little impact, but it draws you in, and its oriental-looking doors set an exotic tone that is picked up as you climb the stairs to the main body of the chapel on the first floor.

A massive concrete canopy (its underside scored with a cross) is suspended above four double columns in the middle of the chapel like a baldachino or a tent. Above this, you catch glimpses of the highly crafted wooden roof of the chapel, which fans out like a primitive tepee. Something seems to happen on every surface.

Richard MacCormac, who is 54, has advised the Prince of Wales on his new Institute of Architecture. But Fitzwilliam Chapel is anything but institutional. It spurns the traditional hierarchy of altar, choir and nave in favour of a provocative form that conjures up unexpected ideas and associations; new ways, perhaps, of thinking about religion. The oak floor of the chapel sits like a raft on top of the crypt below, but protected by rails that lean out over the gap left between the exterior walls by the stairwells. Angled like railings on a liner, they make the chapel feel like a boat or ark.

The image is not an accident. 'The ark,' MacCormac has written, 'represents the idea of passage and of protection, an archaic metaphor which recurs at conscious and unconscious levels - the 'nave' of a church, the 'night sea journey' of myth and of religious and human experience - Jonah and the Whale, Noah's Ark, the womb, the cradle and the coffin. In the Christian symbolism of medieval manuscripts, the ship signifies the way of salvation.'

And not just in Christian symbolism. The image of the ship has long been recognised by pagans too - dead Viking heroes cremated on their longboats, ancient Egyptians who saw the afterlife as a voyage along a river. The chapel draws its power from these archetypal images, which are intended not to have a single meaning, easily understood and just as easily forgotten, but several.

The most obvious of these, and the idea that seems to have generated much of the form of the chapel, is the huge plane tree that it looks onto. The chapel's roundness is interrupted at the east end by a glass window so big that the interior is dominated by its views of the gardens and the tree. The tree is the real focus of the chapel, its visual power detracting even from the altar that stands in front of it. The tree of life? The Garden of Eden? The tree the Buddha gained enlightenment under? It is all of them.

What the chapel really celebrates is man's creativity. Instead of pointing to the heavens, it hugs the ground. Its highly wrought finish emphasises the role of the human hand in building a chapel to God. The experience it offers is not transcendental. For some worshippers, the hand of the architect may be a little too evident, the effect too busy. But Fitzwilliam Chapel is an outstanding building, full of generosity. The thinking behind it is deep, and broad, but never overwhelming. Its human form echoes an extremely humane feeling.

Fitzwilliam Chapel can be seen by appointment (0223 332000). It is also featured in a current exhibition on sacred architecture at the Venice Biennale.