Gallery is still one of Oxford’s best kept secrets

"Secreted behind an ordinary doorway within the Georgian façade of the Canterbury Quad and half-submerged beneath the Dean's garden, the Picture Gallery is still part of unseen Oxford: beauties and treasures accessible to those who know where to look."

The words are Joanna Woodall's, the first woman curator at Christ Church Picture Gallery, in her essay in the very readable catalogue produced to accompany the gallery's 40th anniversary celebrations. It's hard to better her description: you do have to know where to look for this gem of a gallery. Tucked away in a sleek modernist building within the college, it is intentionally invisible', made to blend in without disturbing the lines, light or views of the beautiful buildings around it. Nonetheless, it provides space for a rich collection of art, predominantly Italian of the 16th to 18th centuries, and regular exhibitions.

To celebrate its 40-year history the Picture Gallery offers two exhibitions: Treasured Drawings, a delightful selection of Old Master drawings personally chosen by all nine previous and present-day curators, and a second exhibition documenting the history of Powell and Moya's prize-winning building.

The curators had to choose four from almost 2,000 drawings in today's collection (Christ Church's art collection also holds 300 paintings). Not an easy task, if undeniably enviable. But a quick glance around the drawings on show in the exhibition had me smiling straight away. Treasures indeed: seldom have such absolute highlights of the drawings collection been on display together.

It starts with Leonardo da Vinci's Grotesque Head, a drawing chosen by Christopher Lloyd, their first curator, at the gallery when it was opened by The Queen in 1968. Then, one recently described as the most beautiful object in the possession of Christ Church, Verrocchio's Head of a Woman, which, combining intense luminosity with beguiling pose and reflective expression, is "the quintessence of Verrocchio's type of beauty," states Lloyd. Claude Gellée's (Claude Lorraine) superb depiction of the flooding of the Forum in 1637 and an elaborate work by Hugo van de Goes come next.

I have to keep to only a few examples, of course, but you get the picture. This is a quality show.

I loved the two selected by the current curator Jacqueline Thalmann (2003 - ): the demure yet contradictory female figure in Personification of Silence or Night by a follower of Giulio Romano, and the recently acquired Giaquinto, a red chalk drawing of Saint Joachim and Saint Anne. Though highly finished, it was a preparatory study for the background figures of his monumental painting, the Birth of the Virgin in Pisa cathedral. A small oil, perhaps the presentation modello for the Pisa painting, is in the paintings gallery.

Above all, and the one I'd take away with me, is Tintoretto's black chalk Head of Giuliano de'Medici, after Michelangelo. Sometimes it is beyond comprehension how so few lines, contrasting tone, such simple shading, can convey so much.

The pictures are displayed in order of each curator's tenure. After being at the Picture Gallery, three went on to work at Oxford's Ashmolean Museum: Christopher Lloyd (at the Picture Gallery, 1967-1968) to the Ashmolean's Print Room before becoming Surveyor of The Queen's Pictures; Jon Whiteley (1975-1978) and Catherine Whistler (1985-1988), both Senior Assistant Keepers at the Ashmolean.

Other highlights include The Archer by an anonymous French artist of around 1400 (one of Jon Whiteley's choices), Bernini's Head of a Young Man in a Cap (Joanna Woodall's), Raphael's Seven Putti Playing (Catherine Whistler's), and Pontormo's Deposition.

The second exhibition illustrates the story of architects Powell and Moya's design for the gallery constructed in 1968, recognised as an outstanding piece of modern architecture, and given Grade II listing in 1998.

In 1765 Christ Church received General John Guise's bequest to his former college of his collection of more than 200 paintings and almost 2,000 drawings. The collection, supplemented by later gifts, was housed in the college library. Other paintings were distributed around the college, some in places unimaginable today. The best example of this is Annibale Carracci's enormous genre painting, The Butcher Shop, perhaps the most spectacular work in the gallery today, formerly hung in the college's Tudor kitchen.

Having made the decision to build a purpose-built gallery, a site had to be found and the Dean's garden was decided upon. It was by no means an easy commission. The architects faced severe restrictions, historic trees on the site, for example, including Lewis Carroll's Cheshire Cat tree. In short, the architects had to design an invisible' building, there but not there, tucked in a corner of a garden and somehow connected to the world outside.

The resulting semi-subterranean Picture Gallery was considered by the architects their most successful building. Architectural historian Sir Nikolaus Pevsner described it "as a subtle and attractively detailed building which, while being entirely in the Modern idiom, fits with great sensitivity into its important historical setting".

It's much-loved by visitors, too, 25,000 making their way down the steps in the gallery's ingenious entrance in the past year. Inside they find an almost cloistered calm, a secret garden, and six or seven temporary exhibitions of drawings and contemporary art a year, plus wonderful paintings on permanent display. In the future they may even be lucky enough to see all 20 pieces of the Lives of the Hermits reunited: one of the gallery's aims.

A final word about the exhibition catalogue, which I think you'll have to get if you're to enjoy it all as fully as I have. Unique among such catalogues, apart from insights into the chosen drawings by art historians united through the Picture Gallery, it contains the curators' reminiscences about their early careers there.

I enjoyed reading these very human essays, Jon Whiteley's post-interview thoughts, for example, that they were looking not for an art historian but a plumber with accountancy skills (leaks were a constant it seems) and making icons' (to help the budget along). Also, Jacqueline having to "learn a new vocabulary" (Christ Church-speak), and her - reassuring to the rest of us - error in artist identification, exclaiming "Oh, Mantegna!" when shown a Bellini.

Both exhibitions run until October 29. For more information, see chch.ox.ac.uk/gallery website. To visit the Picture Gallery without visiting the rest of Christ Church, enter the college through Canterbury Gate, off Oriel Square, only a couple of minutes' walk from the High Street.