ART / Diamonds are a girl's best friend: Post-Op, Bridget Riley has taken to making lattices of jolly colour. Tom Lubbock tries to make sense of them

LIKE Juan Gris (previous page), Bridget Riley is another artist who has acquired a scientific reputation, but this, she always believed, was a terrible mistake. It was a mistake that was perhaps easy to make in the early Sixties. The pulsating swimming surfaces of her famous black-and-white paintings were assumed to declare a technical interest in optics. And a school of painting, Op Art, was then christened, making a happy rhyme with Pop. As abstract paintings, they had a kind of Poppish appeal: you could immediately get something out of them. This is still what most people associate with her name. I don't myself have a very clear idea of her work through the late Sixties and the Seventies. But the Hayward Gallery provides a more recent update with a selection of paintings from the last decade.

The painting is now in full colour, and if the retina is played on, it is much more subtly. The decade begins with a series of almost square pictures, where thin, parallel, vertical stripes fill the canvas, like multi-coloured bar-codes. Riley's colours here, and in the later works, are not immediately appealing; it is a palette that looks bright, fun and rather tasteless, like liquorice allsorts. But it is what is done with their combinations and juxtapositions that counts. The bar-code paintings - not very exciting in themselves - give a foretaste of things to come: the way the same colour can jump out or fall back depending on its neighbours, the changing groupings that the lines fall into.

In 1986, Riley introduced diagonals cutting across the uprights, and things got going more. The pictures are now organised round with a grid of vertical and diagonal lines - more broadly spaced - and become lattices of coloured diamonds and lozenges of different shapes and sizes. The slant is always from bottom left to top right, at 45 degrees, and for us left-to-right readers this is a rising movement, giving the structure an immediate up beat. (View them in a mirror, on the other hand, and it looks like rain, a downer.) But these are pictures that repay a longer look, and actually respond to a certain pedantry.

Try counting (at a distance) the number of different colours used in any one painting. I think you anticipate an economy of means, and expect to be surprised at how few there will be. The surprise is that there are so many - between 15 and 20 per canvas. In the course of this enumeration, you become involved in other things, a complex game of 'seeing as', as the paintings offer up a range of evolving focuses and formations. What looks at first sight, and certainly in reproduction, like an intense sort of fabric design, comes into life.

Flat patterns, deeper spaces, rhythms and counter-rhythms appear and give way. It's a pretty spaced-out vision, once you get into it. A vision of what, though? The broad format of the pictures may suggest landscapes, but don't bother seeking out a view. The language of pure painting is always a teaser (which is why it's comforting to have something definite like optics to talk about). Riley herself speaks of 'sensation' - she seems to mean something between the physical and the emotional. Her titles - High Sky, Set Fair, Certain Day - tend to be moody and meteorological. But I doubt whether any clearer names could be put to whatever feelings are being called upon. Her art seems to be moving in roughly the same territory as the music of Philip Glass, without being quite so annoying.

'According to Sensation: Paintings 1982-92' is at the Hayward, SE1 (071-928 8800), to 6 Dec.

(Photograph omitted)