One of the most enigmatic of Vermeer paintings. It's called Woman with a Balance or Woman Weighing Pearls or Woman Weighing Gold: there are pearls on a table in front of her, and a strongbox, and she's delicately holding and carefully examining a little balance: but what is she weighing? I read about this point in a review in advance of the great show that came from Holland to London and Washington, and when I saw it for myself close up I saw that what I'd read was right: she's not weighing anything. Or nothing you can see. Light; air; an invisible feather; a breath; a spot of paint marking nothing but a scintillation of the dim light off the brass pan.

The room is dark. As with all of Vermeer's paintings there's a strong sideways light from a window beyond one edge. But this is almost entirely shuttered in: a tiny splash of daylight high in the uppermost left. It gives a modest illumination to the centre, to her shining, peaceful face, to the snowyness of the edging of her robe, and to the front of the table.

Her further hand holds up the little pair of scales. Oddly, that hand is rather shadowed. Attention is taken away from it. The light's not directly on the pan of the scales, either. The scales are so thin you can barely see the hanging parts; and only that almost imperceptible touch of sienna glow in the bottom of the pans gives the least prominence to it: except that it's in the centre. And she's looking at it. So intently.

So very intently. Yet distantly, detached. Almost... amused? "She" is Vermeer's usual model, presumably his wife Catharina, very pretty and pregnant, in a blue cloak edged in white fur, a white hood, all very domestic and probably unremarkably middle-class for 1660s Delft; except that, and this can't be coincidental, I'm sure; those colours and the stillness and innocence and purity and understanding wonder of her gaze make her look like some kind of incarnation of the Virgin Mary. And that weighing of nothingness, of light, of air, of a pearl too ethereal to register on our gross senses, that becomes like some spiritual operation. There's something metaphysical going on.

Behind Catharina, in the dark but illuminated somewhat by her bright white headdress, is a grimy and rather fearsome religious painting on the wall. It looks like the spindly, tormented figures of El Greco perhaps: it's some kind of Last Judgement that experts have suggested various minor Dutch artists for, perhaps even Vermeer's own invention in a completely different style. There's an allegorical touch to this, some comparison of being weighed, but it's far from clear what.

Oil on canvas, 42 cm x 35.5 cm, painted circa 1662, not signed (but absolutely unquestionable). Now in the National Gallery of Art, Washington. Probably the one sold in a 1696 auction for 155 florins. Sold for 235 florins in 1777, 2410 francs in 1830, £141 in 1848, and passed to the Widener family of Philadelphia in the early 1900s, and from them to the NGA.

Provenance from Bianconi, The Complete Paintings of Vermeer, Rizzoli 1967 (Penguin 1987)