Walking into an art exhibition is like walking into the artist's mind. Some are easy to read and understand; others, such as Janet Boulton's exhibition of paintings and reliefs, are so amazingly complex that it's impossible to absorb the qualities in one swift superficial glance. Too much is going on.

The collection of 34 pictures, most of which are in very simple (but effective) wooden frames, fall into five categories: Cosin Ward, Gardens, Mirrors, Shelf, Glass and Check.

Cosin Ward consists of seven watercolours painted while Janet was artist in residence at the Radcliffe Infirmary between 1986 and 1988. They are small in comparison to the other works, but nonetheless evocative. With the lightest of touches, she has captured the pain and the loneliness of the patients she observed. Waiting for Lunch, for example, gives us two people sitting side by side looking out into the middle distance. Not at us, not at each other, but what must surely be a private world into which lunch will arrive eventually.

The eight works in the Gardens series include two extremely powerful watercolours of Apollo and Daphne. The absorbing way the garden connects with the figures playfully romping through the trees and climbing down from a plinth is superb. Such delicate colours - yet such powerful images.

Mirrors brings together five large paintings, including studies of Irish Murdoch, Elizabeth Jennings and Elizabeth James. The haunting picture of Iris Murdoch is instantly recognisable, yet there are no facial details to inform the viewer. Despite that, she has captured the very essence of this great Oxford philosopher with a paint brush.

Shelf, Glass and Check are mostly paper reliefs. Janet was introduced to this technique, created from cotton linters, during a paper-making workshop at Oxford Brookes University. The relief, which simply sings with colour, is named after the greyish-white metallic element Manganese, which I suspect may have been mixed with the paint to add a certain intensity to this work. An unrivalled collection.

This exhibition continues until November 29.