Murillo, Bartolomé Esteban: Two Women at a Window (c1655)

The Theatre gives us representations of things, but these are transient' whereas painting remains, is always at hand." Jonathan Richardson, 18th-century art critic, makes an important point. A painting, unlike a play, is always on. Plays and concerts have beginnings and ends. Audiences attend - turn up for the start, leave at the close - according to an allotted programme. Time wise, the work makes the going.

The Theatre gives us representations of things, but these are transient' whereas painting remains, is always at hand." Jonathan Richardson, 18th-century art critic, makes an important point. A painting, unlike a play, is always on. Plays and concerts have beginnings and ends. Audiences attend - turn up for the start, leave at the close - according to an allotted programme. Time wise, the work makes the going.

But painting remains. Pictures are continually open for viewing (even if the buildings they're in shut). They wait there, wallflowers, for someone to arrive and take an interest. With a picture, it's the viewer who makes the going: initiating the relationship, deciding how long it will be, whether to return. There are exceptions to this custom. Icons may be exposed only at certain times. Hinged altarpieces are opened and closed. Some paintings hang behind screens or curtains, to be only temporarily revealed. But most pictures never close. It's you, the itinerant viewer, who sets the duration. You arrive and you stand and you leave. Indeed, one distinguishing feature of the visual arts is that they're normally experienced on foot.

Can pictures take account of your comings and goings? Mostly, they don't. They presume a viewer who is already in place, who is simply present and looking. Even when (as often happens) a figure in the picture turns to you, reacts to your presence, they react to a viewer who is assumed to be just there. They suppose a constant, timeless viewer - a viewer, in that respect, quite unlike a real one.

But there are pictures that do reckon with your timing. They play upon your actual arrival or departure. How? Their figures are shown reacting to a viewer whose position is more contingent - who has just turned up, or looks like leaving, or won't go, or dares to turn away. They address an inconstant viewer, whose presence and attention are provisional and variable.

Take this one. It's a street scene, though no street appears. It shows two women at an open window, looking out. The street in front of them is implied. The window is a house window, the room behind them is in cool darkness, and the light that falls on them is the high light of day. This indoor scene is predicated on the outdoor world before it. The women's looks can't be understood without imagining what they're looking at: the passing street-life in a Spanish town.

Equally, the scene is a direct interface with the viewer. The window frame corresponds closely to the edge of the picture. The women gaze out of their window straight into the gallery. The viewer's space is equated with the street. You are the passing street-life that the women have come to look at. You are the person, the man in the street, who's walking along and catching the young woman's eye.

It's a scene that reckons with an ambulant viewer. It doesn't assume that you're already there, looking. It's on the look out for you. It's lying in wait. It plays on your arrival. The young woman leans on the sill, bare-shouldered, head in hand, half-smiling, facing and staring ahead. And her gaze can be read, not as actual eye contact, but as something prior. She's watching out. She's watching someone who isn't yet looking at her. She's steadily giving the eye. And when you do approach this picture, and catch this gaze, you see it as a gaze that's been waiting for you, tracking you, inviting you, all along, before you noticed. And now it has got you.

There's a second gaze in this scene, of course. The older woman - the duenna? - standing half-behind the shutter, peeping from behind her veil, is in possible eye contact, too. Or she is, if you look at her. But in this picture, it's clear that the claims of the two gazes are unequal. The younger woman, bolder, more attractive, has priority. The older woman, poking into view, is a secondary figure. You're aware of her, in the corner of your eye, as another gaze, also aimed at you, which you could meet. But her role in the scene is specifically as an interference. She's keeping an eye on you. Her intruding, monitoring presence intensifies the intimate exchange between the younger woman and you, the man in the street.

Except that the twist is, it never gets that intimate. The expected pay-off is frustrated. You catch the young woman's gaze, and approach. From afar it seemed to offer reciprocal eye contact. But when you get close, and try to make these eyes meet yours, they won't. Close up, you encounter an inherently distant gaze. It looked promising, but you're getting nowhere.

There's a trick in the way that Murillo has painted it. The focus of the woman's two eyes won't quite resolve. There's a slight but decisive disco-ordination of the pupils. She's looking straight in your direction, but her look won't fix yours (though it always feels it might). You can see her as in a reverie, or pretended reverie. She's blurring or blanking you out. Staring through you. Waiting for your next move. She's already lost interest and is waiting for the next guy. She doesn't even see you.

The effect emphasises again the timing of your viewing, the duration of this encounter. You can see her as someone who hasn't yet noticed you, or who's thinking you away, or staring on after you've departed. The painting reminds you that your position in front of it is transient. It points to before, and after, your brief appearance. The woman remains. Her neutralised gaze allows you, as you stand before her, to imagine that you're nowhere in her sight, to imagine your own absence, as she stares out, dreamily, on to the empty street.

THE ARTIST

Bartolomé Esteban Murillo (1618-1682) has become synonymous with the word "urchin", due to his paintings of mischievous, pitiable, lovable poor children. He was once among the most esteemed Spanish artists. His pious religious images were popular, prints of them in every parlour. His misty-eyed manner had its own brand-name, estilo vaporoso, the vaporous style. But then, in the 19th century, he became a byword for sentimentality and vanished from consideration. And yet, there are some cooler, sharper pictures by Murillo, such as the above