The Ashmolean doesn't officially do' photography, which came rather late to the world as an art form. So we learnt from its director Christopher Brown, deputising with unruffled urbanity for novelist Jeanette Winterson marooned on the M40.

He was opening Oxford at Night the first exhibition of modern photography at the museum, product of three years nocturnal peregrination by the distinguished photographer Norman McBeath during his 14 years residence here (he's now returned to his native Scotland). There are 25 pieces on view only a fraction of the total and the fruit of much refining and editing. I'm reluctant just to call them black and white'; the artful disposition of light sources, angles, reflections and, of course, the weather gives an infinitely wider range of tone and intensity.

The stone, the light and the shadows speak or are made to speak for themselves. Interestingly, this is a boundary to McBeath's work. It is, perforce, limited to the traffic-free central area; however late one wanders, other thoroughfares are never empty.

So we drift round Radcliffe Square and the Bodlean, along the Broad and over to Merton and Oriel Square, across cobbles glistening after rain, lit by small shafts of light from single streetlamps or illuminated by rows of bright windows from (we like to think) hallowed halls of dedicated learning, excluded by formidable barred railings or, more subtly, by wrought iron in sculptured patterns.

There are no visible occupants. Only their traces. Bicycles. Piled up in heaps, propped vertically agaInst walls, or most disturbingly in solitary ones and twos. The only other signs of human habitation are an unlovely wheelie-bin at the Examination Schools, and a tatty notice-board seen through the arch of Blackfriars. Among the august university and college edifices is only one domestic building, looking very desirable in Merton Street, and that's actually part of the college holdings.

Jeanette Winterson, in her introduction, makes great play with the mystery and hidden life, the revelatory quality of these images. Many will agree. I don't really, perhaps because my path has taken me so often and so long through Oxford by day and night. I find comfort (of a sort) in sensing life behind the stone, centuries old and ongoing.

The exhibition is in the Eldon Gallery till July 23.