Legal precedent.

Post-war Cambridge has built in haste, and repented at leisure. There is perhaps no better setting in which to examine the follies, extravagances and occasional triumphs of contemporary British architecture. Cambridge can make - or unmake - an architect's reputation. But it is the city and the university that have had to put up with the long term consequences of a prolonged and enthusiastic flirtation with architectural superstars. Considered en masse, the results can be disheartening. That, at any rate, was the conclusion reached by Philip Booth and Nicholas Taylor in their Guide to Cambridge New Architecture (1972). Even today, after the healing interval of 20 years, it is hard to disagree with their conclusion that the huge influx of post-war talent has led' not to an enriched environment but to conclusion and overstatement'.

There are few better places in Cambridge to study this confusion and overstatement than the Sidgwick Avenue site. Long ago, Nikolaus Pevsner argued that Cambridge's future lay beyond the Cam 'in a campus more beautiful than any in America, a precinct much larger than the precincts of the individual colleges, yet a precinct all the same'. Sidgwick Avenue was to become Cambridge's intellectual theme park, with the individual arts faculties mimicking in their layout the format of the colleges - and by implication vying with them as a focus of learning.

Hugh Casson's masterplan and finished buildings, though much derided at the time for their picturesque obsessions with cobbles and bollards, and finicky fenestration, were true to the spirit of such a collegiate campus. And although the gale-swept 'cloister' running beneath the raised faculty blocks is no place to linger on one of Cambridge's more Siberian winter afternoons, this is an honourable and largely successful piece of townscape in the Cambridge idiom of turf and paving, open and enclosed space. One can have too much of a good thing however, and it is perhaps fortunate that the Casson Conder scheme was not allowed to extend all the way to West Road. The rivalries and trade-offs between dons and departments on one hand, and colleges who own most of the land, on the other, have always put paid to comprehensive thinking on this scale. In consequence, many contemporary buildings at Cambridge stand shoulder to shoulder and ill at ease, like VIPs at a cocktail party who have not been introduced to one another. At Sidgwick Avenue, James Stirling's angry red History Faculty was clearly not on speaking terms with anybody; and so it has fallen to Norman Foster on the adjoining site to effect a few introductions.

Given Cambridge's long-term fixation with glamorous architects, it was obvious that sooner or later Foster would land a commission here. The question to be asked is, has yet another intervention from a brilliantly talented outsider merely added to the confusion and overstatement which is already rife at Cambridge, or has there been a genuine gain? The first thing that needs to be said of the new Law Faculty is that quite apart from its merits as a building - which are considerable - it is a tactful insertion into a very difficult site. To build opposite the History Faculty must be the architect's equivalent of donning the shirt of Nessus. In form, materials and orientation, Stirling,s glowering masterpiece is a calculated snub to the Casson Conder block next to it and a threat to anything which might be placed in the immediate vicinity. One of Foster's achievements has been to establish, or in some cases, reinstate, a coherent north-south axis through the Sidgwick site; and by splaying the form of his own building 45 degrees to match the History Faculty and avoid a few listed trees, he has created a breathing space - not quite courtyard, not quite square - in which all the buildings can coexist in something reasonably close to harmony. Part of this sensitivity to site may be due to Foster's Cambridge-educated partner in charge, Spencer de Grey.

The Casson Conder raised faculty block is a particular beneficiary. Foster and de Grey have restored the stepped plinth on the western side and clad the Law Faculty's southern elevation in reconstituted stone and Cambridge blue panels of opaque glass with narrow clear glass vision strips. The two blocks are in dialogue, and will get on even better when the trees between them have a chance to mature. It is obvious already from this that the Law Faculty is a contextual building whose forms and materials have been influenced by its neighbours. The flat southern facade and the splayed western wall are the direct response to context. Only on the north elevation where the view consists of mature trees and (eventually) a lot more Foster buildings, has Foster had a chance to flex his muscles.

The result is one of those tours-de-force in steel and glass which is neither window nor wall nor roof but all three. Moreover its abrupt termination along the southern facade gives a hint of what might have been - an all-embracing steel and glass enclosure vaulting effortlessly over the bookstacks within.

As built, perhaps the most delicate design problem has been to bring the disjunctive north and south facades together. This has been done by creating a prow at the western end of the building with an array of millions stepping up to meet the curved edge of the steel truss. The wedge-shaped glazed atrium thus formed acts as the entrance, meeting place and collecting point of the whole building.

Entrances have not always been Foster's strong point. One recalls the hard-to-find, almost apologetic hole in the wall to the Sainsbury Centre, when the natural point of entry - since recognised by Foster himself - was surely the glazed southern end. At the Law Faculty, the entrance is certainly at the logical point on the campus - any feng shui spin doctor would have agreed on that. The problem is that it is also marked externally by an unexpected collision of forms and materials. There is a distinctly un-Fosterlike grinding of gears as steel roof truss, stone elevation and glazed wall all come together. This entrance is celebrated - if that is the word - by a slender steel column whose significance is obscure. Is it depending from the eaves as a tie, or is it lending structural support? The answer is neither. Apparently the sole justification for this insouciant appendage is that the entrance looks somehow incomplete without it.

Internally, the entrance foyer/atrium is certainly going to be seen as one of Cambridge's big interior set pieces. But because part of it is sunk below ground level, the visitor disconcertingly finds himself staring into a kind of trench for which, as yet, no satisfactory use has been found. The problem is one of conflicting functions. The faculty combines a library with teaching spaces. Undergraduates collecting outside the basement lecture halls, or milling around the entrance are apt to be noisy, whereas libraries traditionally demand silence. Any attempt at enlivening this void - by, for instance, filling it with paintings and sculpture, or a full-blooded canteen - would simply exacerbate what the faculty at present regard as a noise problem. Their fears may be exaggerated, however. On a recent visit, I found a lively seminar in progress with both doors open to the library spaces beyond. Nobody seemed to object to this, indeed it is possible that a certain amount of white noise may be beneficial, and that the gradual transfer from books to terminals may generate new patterns of study and new tolerance of extraneous sound.

The library is the most satisfying part of the building, although the volume devoted to book storage and book study probably amounts to less than a third of the total. As always in a Foster building, there is much sensuous pleasure to be had from the way things have been put together. The continuous reading tables are closely based on the desks in Foster's own office. A shelving system in solid birch (manufactured in italy and due to go into mass production soon) alternates with vaulted corridors where the lighting levels obligingly increase as the reader enters the bookstacks. Most remarkable of all, is the beauty and uniformity of the concrete, achieved by steel shuttering and a bulk purchase of Scottish granite aggregate.

Characteristically the concrete performs more than one task: as well as holding up the library, it carries communications, fire alarm, and security system points cast within it, while its heat retentive mass plays a key part in controlling the effects of solar irradiation.

In general this building inspires feelings of modified rapture. It is a confident, sweetly functioning work of architecture, which without being in the least self-effacing is a surprisingly tactful addition to the Sidgwick Avenue campus. But something is missing. There is a lack of fluency, a lack of elan and a disturbing muscularity in some of its components that seem uncharacteristic. It will be fascinating to see how Foster manages to knit together his Law Faculty with the next phase - a more restrained group of barrel-vaulted faculty buildings for English and Criminology. These will bring the Sidgwick Avenue complex right up to West Road, and reinforce the north-south axis of a campus which, one day, will surely run all the way up to the University Library itself.