Bridget Riley throws some curves

"Surprise" is a word British artist Bridget Riley has used when talking about her paintings.

The word might itself seem a surprise for an artist whose work is so evidently calculated. Her paintings are composed of systematic progressions of clearly delineated components placed and spaced with rigorous precision. Nothing, it would appear, is left to chance.

Even when her early black-and-white paintings gave way to paintings that additionally explored subtle grades of gray (as in "Pause"), which in turn gave way to color, every aspect of her paintings was selected, considered, and consciously finalized. Assistants could then actually paint the paintings.

An absence of personal touch might seem to bring her work dangerously close to graphic design. Yet these are self-sufficient and intentional paintings. She has been making them for more than 40 years, with a scrupulous consistency of vision, and she shows them in art galleries.

The paradox is that Riley's paintings defy predictability. "Pause" demonstrates this remarkably. Its units and organization are apparent. But what happens unexpectedly in Riley's paintings seems independent from the evident logic of their elements. The actually stationary elements are read as movements, not unlike single notes becoming music. In "Pause," the beat of the discs proceeds from full circle to attenuated ellipse. They also proceed from full black to faint gray. All this occurs on a flat plane.

But as the flat circles seem to turn into ellipses and the blacks into grays, they become less visible as well as less frontal. The surface of the painting then seems drawn into an illusion of depth that is strangely immeasurable. The unmistakable has become the elusive. And along with this comes the biggest surprise: The straight horizontals and verticals are taken over by insistent, sweeping curves and twists and almost tangible and sensuous rounded forms.

• The exhibit 'Bridget Riley' is at the Tate Gallery, London, until Sept. 28.