These early works are the paintings that made her name and, at the time, her images were appropriated by the worlds of fashion and television and translated into fancy dresses and studio sets for pop programmes. Yet for all this devaluing of their visual currency they have worn well. Nearly 40 years on, they still look unquestionably of their time but not in the least bit dated.
It started with a Kiss - a black canvas, 4ft square, bisected two thirds of the way down by a narrow segment of white, the two black sections not quite meeting, not quite kissing at the thinnest point of the dipping white line. It is a simple picture, but effective: an uninterrupted movement of white across black from the point of entry on the left to the exit, rising on the right. The sensation of movement is crucial to these early paintings, though the more complex they become the harder it is to pin down exactly what the movement is. They play complicated, disorientating tricks on the eye: rhythms speeding up and slowing down across the picture plane, coming and going between foreground and background. The contradiction, as Riley has described it, is between stability and disruption, but there's another contradiction at work: an odd tension between seduction - one feels drawn to look, almost at times drawn into them - and repulsion; the more you look, the more you want to turn away.
One or two of them (the densely grouped waves of Crest or Current, both of 1964, are the most pronounced) are actually painful to look at. From the side these pictures look three-dimensional, swelling and buckling at the point where the lines are most tightly grouped. Viewed head on they produce an odd, not to say unpleasant, sensation: at first a dazzling surprise, like staring at the sun, followed by a kind of pressure spreading inwards from the temples to a point just beneath the eye on either side of the nose.
Colour creeps into these early works slowly, through shades of grey and what Riley calls "coloured greys" softening the hard edges of black and white, to the candy- coloured stripes of paintings like Byzantium and Zing, and with the colour comes an almost electrical charge that makes the canvas hum from end to end.This - the almost physical power of a purely visual sensation - is what all Riley's paintings are about. These pictures are not about mood, or nuance, the mainstays of so much abstract art, but about the basic nature of sight and the simple act of looking. The more recent work takes a subtler approach to the same end: some, particularly those that are made of softer, gently coloured curves (like Aurulum and Andante of 1981) are genuinely beautiful and altogether easier on the eye, but they too are built around the uneasy relationship between the retina and the brain.
Tiny, subtle shifts of width and colour move the rhythms up and down and left to right, turning in arcs and half spirals: it's a very meticulous process. Not surprisingly, Riley doesn't paint them herself - she'd need a 12ft brush to get the necessarily long view and clearly this wouldn't work, but under her eye and instruction a team of assistants inch their way across the canvas, leaving as little as possible evidence of the picture's making.
The same can't be said for Sean Scully, whose recent work went on show at the South London Art Gallery last week. There is a huge physical presence in his paintings: the hand of the man undisguised in the sweep of a big brush loaded with paint. Physicality aside, the ingredients of Scully's and Riley's work are similar: an emphasis on order, structure and method, a basic language of stripes and colour, but the effect of their work is very different.
The last show of Scully's paintings in this country, in 1997, was held in a rather cramped municipal building in Manchester where his huge paintings didn't quite have the space they need to breathe. Here, happily, we have the opposite scenario with just 11 paintings (seven large, four small) elegantly hung in a beautifully proportioned room. The large paintings, I'd guess, were made specifically with the space in mind and the result is a stunning and surprisingly beautiful exhibition.
I say surprisingly because beauty isn't what Scully's paintings usually suggest. They are generally impressive and almost always moving in a sombre sort of way, but not quite beautiful, at least not in the manner of Riley's arrangements of pale pinks and blues and mustards. Of the seven large pictures, four are ranged down a single wall: Four Large Mirrors, each one made of two 9ft columns of not quite aligned stripes. It's a very simple format and immensely powerful: each mirror with its own certain mood and each of these contributing to that of the whole. Scully has written eloquently of the melancholy beauty in Mark Rothko's work: "Nothing in a Rothko is hard, nothing is secure and nothing is definite. He works sadly and constantly against the dying of his own light." And though his temperament is very different there's something of the same to be found in Scully's own paintings. Layers of colour are layered over colour: greys and blues and browns offset by mustard orange and red and, as with Rothko, the effect of these nuances of colour and tone is an ache of longing for something unspecified and not quite grasped. In a sense, though the components are similar to Riley's, the effect could hardly be more different. Riley's are pictures which exist on the surface - the eye is moved on before it can ever come to rest. Scully's hold still and beg to be stared into forever.
Bridget Riley: Paintings from the 1960s & 70s, Serpentine Gallery, London W2 (0171-402 6075), until 18 June. Sean Scully, South London Art Gallery, 65 Peckham Road, (0171-703 6120), until 1 August