de Zurbaran, Francisco: 'Still Life with Pottery Jars' (c1635)

Because, of course, the milk in the glass may be spiked. And you can see why a spotlight wouldn't do. The glass is being carried upstairs on a tray. It might be hard to track it steadily. More than that, it would be hard for a spot to pick out the glass alone, without shedding light and casting shadow around it, and making it clear that lighting was being used deliberately to direct our attention.

The emphasis needed to be subliminal. We mustn't feel too nudged. Suspicion is all about not being sure. We should feel that it's the inherent suspiciousness of this glass of milk, the possibility that it's doped, that makes it so interesting to us. And if the glass's luminosity is itself a little weird, a little mysterious, that only strengthens the effect.

Long afterward, when it had done its work, Hitchcock was happy to make all clear. And once you know, what strikes you is the outrageous artifice of this solution, its head-on defiance of naturalism. Need to make something conspicuous? Put a light bulb in it!

Things that count as special effects in cinema are normal business in paintings. If a painting wants to give particular emphasis to something, no problem, it has numerous means at its disposal. It can do what it likes with lighting and luminosity. It doesn't have to come up with brilliant solutions because, unlike the photographic arts, it doesn't have to deal with the resistance of the real world. Painting remakes its world from scratch. All the same, some painting puts a premium on having an appearance of high realism. In which case, its tricks may need to get a bit clever.

Francisco de Zurbaran painted very few still lifes. Their subjects are simple and few " pieces of fruit, baskets, vessels, plates, a flower. Their scenarios are simple, too, a narrow shelf with objects laid along it, lit by a shaft of strong light coming almost directly from the left. This Still Life with Pottery Jars is one of them. It has no organic matter, only man-made objects. The treatment is highly realistic. The painting does all it can to make us believe in these things, in their shaded, rounded volumes, their hard, gleaming surfaces. It makes them tangibly solid. It makes them audibly solid. It makes you imagine the sound they'd make, when the clay pot is put down on the shelf, when the brass cup is set in the thin silver platter. It's a level of realism you might well call magical.

But these objects are magical in another way. They have a heightened presence, which realism alone doesn't account for, which realism actually makes mysterious. Somehow, these ordinary material things feel mystical, numinous, holy. The jars are utterly real and utterly supernatural. That's the power of the image, this ambiguous balance. It's also a puzzle. How does Zurbaran get the visionary effect with such seemingly plain ingredients? There has to be a trick.

There is. It's partly in the layout and partly in the lighting. The vessels appear before us in a solemn procession across the picture-stage. But they're not paraded overtly, like soldiers. They're not set on a single line; some are nearer than others. They're not evenly spaced either. But the picture adjusts the differing shapes, heights, widths, brightness, colours of the four vessels (and two plates) into a visual equilibrium. Result: there is no protagonist among them. Each is very different. All have equal weight. The row looks casual, but holds a perfect, subliminal order.

That seems a respectable artistic effect. When it comes to the lighting, the tricks get tricksier. The depicted facts say one thing, that these objects are illuminated, lit by a light source from the left. The impression they give is something else: not illuminated, but luminous. Like the glass of dodgy milk in Suspicion, they glow. What's going on?

Look at the strength and at the direction of the falling light. There are discrepancies both ways. Clearly, the light hits the vessels very strongly " but then doesn't hit the surface of the shelf nearly so strongly. Strangely, the level of illumination has dimmed by several degrees between pots and shelf-top. The vessels stand out from their setting, a row of lights in the darkness.

And then, observe the shadows they cast. They do cast shadows, which correspond to the direction of the light. They cast them on the surface of the shelf. But they don't cast them on the other vessels. No vessel interrupts the lighting of its neighbour. It's as if, although lined-up and side-lit, each vessel had a special light source all of its own.

Of course, it can be hard to say precisely where the shadows ought to fall. But certainly, the tall white vase, left centre, should block the light from the red, high-necked vase beside it. But it doesn't. The white vase's shadow falls around the red vase's base, but the red vase itself stays inexplicably, miraculously illuminated. It's a trick of which Hitchcock might have been proud " even though now, the name of the movie is Sanctification.

THE ARTIST

Francisco de Zurbaran (1598-1664) painted the light of God. The Spanish artist worked mainly for churches and monasteries, doing images of saints and martyrs, visions and crucifixions. Inspired, like so many of his 17th-century contemporaries, by the high-contrast lighting of Caravaggio, he devised a series of minimal but stunning dramas of visibility. A typical image: a standing figure, often in beautifully folding monastic robes, stands against a dark background, in a narrow sentry-box frame " caught in this frame and caught in a shaft of light, held visible to the Christian viewer and to the eye of God, bearing witness. Still-life elements are sometimes emphasised, too, a book, a loaf, a basket. Just occasionally, they have a picture all to themselves, and are then imbued with a sacramental feeling. Everything feels seen.