Mirrors, invisible wires, false bottoms, lighting: the most effective illusions often turn out to have been achieved by the most elementary means. Yet there is little satisfaction in discovering this, no sense of having acquired some valuable piece of knowledge. On the contrary, one has a feeling of irritation and disappointment, as though one had stumbled across something demeaning. By reverse magic, the great illusion is transformed into vulgar trickery.
Of course, if we are over 10 years old, we know that David Copperfield can't fly; but how much good comes of knowing the precise means by which he doesn't do it? "We don't want to see Byron on his chamber pot," Pushkin said - though a part of us wants just that, as almost any modern biography will illustrate. Even if we have an unwritten contract with the illusionist, an agreement that we will allow him to trick us, for our entertainment, there is a nagging urge to get hold of his little secret, to steal a march on him, even at the cost of enjoyment. Magic's greatest secrets are probably best left unrevealed (especially by magicians: it seems that another kind of contract has been taken out on the man who told all to Breaking the Magician's Code).
The tacit understanding is paramount. Quite rightly, there is indignation when a documentary film-maker fakes interviews or sets up a particular scenario, then presents it as unrehearsed spontaneous action: something is being passed off as "reality" when it isn't. But, while the terms of the contract in Panorama or World in Action are usually clear, this is often not the case elsewhere. What about the trout in a natural history documentary, filmed in a studio fish-tank instead of at the bottom of a river? Is that cheating? Special effects are now so good that it is hard to tell where "authentic" film ends and fakery begins. No one is deceived by the pterodactyls that fly around in the first part of David Attenborough's new series for the BBC, Birds, but the scenes are so well done that the prehistoric creatures look as real as the real ones. And how dishonest is it to mix newsreel footage with extracts from feature films in a documentary about, say, British life in the 1950s? Or should one scrupulously identify each clip, at the risk of - the worst sin - boring the viewer?
Film is the great deceiver. The evidence of our eyes comes with a greater weight of truth than any other. It is easy to lie on radio. In the Fifties, one of the most improbable radio personalities was the ventriloquist Peter Brough; he even had his own show, Educating Archie, "Archie" being his dummy. Peter and Archie would chat away, in their different voices, just as they would on stage, except that now they were invisible. The question is: did Peter move his lips when he was being Archie? Probably not. The unwritten contract with the listener forbade it; and there is something to be said for a scrupulous, even a ridiculously over-scrupulous approach in such matters.
The problem is that it is so much harder to lie on film. It requires considerable art, an art that has been constantly perfected since Georges Melies first realised that objects on a screen can be made to look as large or as small as you want them, provided everything was made to scale.
The appearance of reality was what mattered. Now, with computer-generated effects, anything is possible - which makes it all the more important to draw the line; only, with film, some lines between fact and fiction are impossible to draw. The medium, as I said, is inherently deceptive, if only because the narrator, the camera, is never part of the action it shows; and because the action takes place in a continual present. It is no accident that television documentaries love the historic present: "Hitler is massing his troops on the Polish border," the voice tells us, in menacing tones, over pictures of jackboots and stukas; lucky we know that Hitler is fifty years dead, or we might be worried.
In some of the earliest films of the Lumiere Brothers, the people who are being filmed suddenly realise what is going on, stop and point at the camera. Their gestures are touching: here are people, now long dead, pointing across a century of time at us, the unseen spectators of the future. But that is nonsense. The people on the film were pointing at an unusual object, and doing so because they had not yet learned how to be filmed. No one points nowadays. Instead, if we get in the way of the lens, we pretend not to notice, we start to act, we tell the camera a little lie about ourselves and collude in giving the illusion, to those future spectators, that there is no intermediary between them and our ever-present lives.
It is precisely when the camera seeks to be most honest that it lies most blatantly. Think of Wilderness Walks, where two hardy hikers set off up the Pyrenees or across some other remote tract of the earth's surface, noticing everything along the way - except, apparently, the camera crew trudging up behind them. No wonder the fly-on-the-wall documentary is becoming a discredited form. But the contract is being subtly rewritten. We know, don't we, that television is not a window on the world, it can only mediate reality. And we are clever enough now, aren't we, to understand what is going on. So, a truly post-modern television doesn't even pretend any longer to offer truth, only a more or less pleasurable suspension of disbelief. Let us amuse you.