If Oxford's Examination Schools building is thought of today, it is generally as a backdrop to Trinity Term celebrations, when undergraduates complete their finals and stream out into the sunlight. Champagne and confetti will greet them in the High. Who could blame them for wanting to get out of the Schools as fast as they can?

And yet the building at 75-81 High Street is of great historic interest. It was the first major work of Thomas Graham Jackson, one of the foremost English architects of his time, and so prolific in building for the university that he was dubbed Oxford Jackson'.

Besides the Examination Schools, TG Jackson worked at a dozen colleges including Brasenose, Trinity, Lincoln and Hertford where he designed the iconic Bridge of Sighs'.

Among other enterprises he repaired the Bodleian Library, restored Carfax Tower and built the city's two high schools, for boys and for girls.

Nor were his efforts confined to Oxford alone. Jackson worked for many of England's major public schools, for the University of Cambridge, and elsewhere.

Jackson was a pioneer of eclecticism in architecture, delighting in historical references and decorative detail; here inspired by a Queen Anne effect, there by a French chateau.

An influential writer, friend of William Morris and pioneer of the Arts and Crafts movement, he was hugely significant in his day. And yet, soon after the fulsome obituaries appeared on his death in 1924, his name sank into obscurity.

The reason can be found in a ferocious reaction against Victorian taste as Modernism took the initiative. Even in Jackson's lifetime, Chicago architects were using industrial concrete to develop a new functional aesthetic that simplified form and eliminated ornament.

By the 1920s, the movement was international, with Le Corbusier working in France, and the Bauhaus group in Germany. In the face of their austere geometries, Jackson and his like were relegated to what one critic has called, the late Victorian orgy of bric-a-brac'.

Decades of living with the avant garde's more brutal creations in concrete, steel and glass have prompted a reaction. Today's Post-Modernist designers permit a return to ornament, a sensitivity to surrounding buildings, and to historical references.

In this context, T.G. Jackson receives a welcome re-appraisal in a recent biography, Oxford Jackson, by William Whyte, Fellow at St John's College.

William Whyte was an undergraduate at Jackson's own college, Wadham, and told me that he was drawn to the architect by accident.

"While a student at Wadham, I had to come up with an undergraduate thesis, and I just couldn't think of anything very much.

"One evening at dinner I realised that for two of my three years there I'd been sitting under the portrait of someone I didn't know anything about. It was the portrait of Jackson. I went away and read his memoirs and thought, well, that will fill a summer quite happily, I'll get my thesis out of that'."

In fact, William Whyte so warmed to his subject that Jackson became an abiding interest.

"One of the things that interested me was the fact that people either ignored his work, or thought it was pretty hideous. But, increasingly, I came to understand what he was trying to do."

Whyte's biography gives lively insights into the politics of Victorian building. Jackson started his career during the Gothic Revival, at the London office of Gilbert Scott, its supremo. Scott was responsible for the Martyr's Memorial in Oxford (1841), and later for countless other pinnacled Victorian public buildings nationwide. The style had immense impact on Oxford. For decades, just about every new building in the city was done in the Gothic manner.

Jackson came to doubt the stylistic preferences of his master and the ideology which it expressed.

Mid-Victorian Oxford was home to reactionary dons who lived in monastic celibacy and looked back nostalgically on the mediaeval order.

"By the 1870s, Oxford had been for two generations a Gothic town," William said. "And that Gothic had become increasingly associated with conservatism and exclusive Anglicanism, and the idea of Oxford as an elite institution that basically churned out parsons."

Built by Jackson between 1876 and 1881, the Examination Schools were a manifesto in stone - the first Oxford building for decades to break the Gothic mould.

Jackson was associated with the university's progressive thinkers, reforming dons who would win the right to marry, throw open doors to a wider range of undergraduates, and broaden the fields of study. As an architect, Jackson drew his inspiration more from the Renaissance than the Middle Ages, and the Renaissance had liberating associations.

"The reformers associated Renaissance architecture with a flowering of free thought, knowledge and science, all the things that the conservative dons were frightened of. They associated it with a progressive view of Englishness - an Englishness open to the Continent and open to change."

It seems almost comic today that a chronological move from retro-mediaeval to retro-Renaissance should have passed for modernity' in architecture. But in fact, Jackson's real argument was for a judicious eclecticism in building. He regarded any style or decorative motif as fair game for the modern architect. Jackson's real hostility was to rules' - the slavish obedience to any architectural formulae.

His own influences were cheerfully random, and often drawn from buildings seen on foreign holidays. His Bridge of Sighs' over New College Lane was one such flight of fancy, modelled on the Rialto in Venice.

"It is a silly building in a way," said William. "It was created in order to bring the two parts of Hertford College together. The reality was that it was very much easier for the undergraduates just to walk along from one gate to another. The good thing was that Hertford needed to make an impression on the university, because it was such a new college. The Bridge of Sighs did that. It must be one of the most photographed buildings in Oxford."

Jackson was extraordinarily well connected and his university buildings won him many lucrative commissions from public schools keen to insinuate through architecture a relationship with Oxbridge.

One such was Cranbrook School in Kent. Royal Academician John Calcott Horsley was a governor there and commissioned Jackson to build Big School'. I am delighted to have been introduced to this engaging character through William Whyte's biography.

Horsley, it appears, was briefly famous for waging a bitter campaign in the Times against nude modelling; his efforts won him his nickname of Clothes' Horsley.

At Giggleswick School in North Yorkshire, Jackson built a richly decorated, domed chapel commissioned by a millionaire philanthropist, with no expense to be spared. William believes this is Jackson's most striking work.

"It is just the most amazing space and in the most amazing place," he said. "Jackson's most impressive buildings tend to be the ones on which most money was spent.

"In Oxford, the interior of the Examination School is very, very impressive; but the exterior isn't as good. That is partly because it was his first big project and he didn't really know what he's doing; also, he ran out of money so that some of the carving is unfinished - it was never quite as he wanted it'.

Jackson believed that an architect was essentially an artist, and the ornamental carvings which decorate his best buildings were all done to his own designs.

"He wanted to have total control over every aspect of the building. If you go through the papers you find that he designed the locks and keys to the doors, the windows and the window frames - if allowed to, he'll design the cutlery," William said.

"The buildings I don't like are where he did things on the cheap, or in a rush. Lack of money led to lack of effect."

Whatever budget was on offer, commissions continued to roll in for the cultured, likeable Jackson.

"The secret of his success was that university men thought of him as "one of us". As a Fellow of Wadham he was an academic in his own right, he was an influential and important architectural historian. Also, he was the quintessential, late-Victorian clubbable man. He was a member of the right clubs, knew the right people, talked their language, and as a result fitted in perfectly at Oxford and Cambridge and the big public schools".

By an irony, his mastery of university and school building adversely affected his chances on the grand national stage. What was termed the Anglo-Jackson' style became synonymous with educational architecture. He entered - and failed in - several important building competitions in London: the Victoria Memorial, for example and London County Hall.

His proposals were not deemed bad, simply inappropriate; his Imperial Institute design looked like a college'. In short, he was a victim of his own success.

Still, on his home patch Jackson reigned supreme, a man responsible for more buildings in Oxford than any other architect in the city's long history. His impact was acknowledged in his own day when the Oxford Magazine drolly suggested that the city should be renamed Jacksonville.

William Whyte's scholarly, fascinating biography reveals just how much he expressed in stone the ideals of a reforming generation, and advertised them for all to see. "In a symbiotic relationship," the author writes, "Oxford made his career, as he remade Oxford."

William Whyte's Oxford Jackson, Architecture, Education, Status and Style 1835-1924 is published by Oxford University Press, www.oup.co.uk