The interior of the Dome plays optical tricks with you. The roof recedes and dematerialises, giving the eye none of the usual reference points of right angles and hard edges. You get no overwhelming sensation of bigness, such as you get in one of the much smaller Victorian railway stations. The yellow steel tripods on which the circle of suspension masts sit again seem nothing much - until you come across a red double-decker bus inside, placed there to give a sense of scale, which fits under those tripods like a Dinky Toy. Even so, it is not until you climb up as high as you can on the various buildings that everything suddenly snaps into focus - and it is enormous, a complete township spread out before you.

This strange intangibility of the space has something to do with the fact that the Dome is not in itself a building at all - merely an enclosure for other structures and activities. As these are finished and populated, they will start to give more of a sense of distance and perspective - as will the Mark Fisher-designed central show, which will somehow fill the entire space up to the apex. But I think the very size and whiteness of the Dome will continue to remove it from our consciousness: its underside becomes the sky, not a roof, and in that sense is too big for dramatic architectural effect.

Did I say "whiteness"? Off-whiteness is more like it, for the dome's fabric lining has become very dirty with all the dust and smoke of the building work going on inside. This poses a problem for the organisers: do they take it down and wash it, thus running up the world's largest laundry bill? Do they take it down and replace it, just before opening, at still greater cost? Or do they decide to leave it dirty, and risk the displeasure of the paying public?

Whatever they finally decide to do, and however people finally take to the Year 2000 show inside the Dome, one clear winner to emerge from the whole Millennium extravaganza here is Will Alsop, who had nothing officially to do with it. The Dome and its surroundings are real enough, but they do not yet feel like part of a living city: it is all a bit disjointed, a touch surreal, round here, rather as the South Bank must have been in 1950, just before the Festival of Britain. But the real city, with its complex architecture, extends invisibly along a tunnel to meet you. Descend the North Greenwich escalators into that dark blue underworld, and you know you are, after all, in London.