If you really want to know, look in the mirror

Funny things, mirrors. But what's really interesting about them is how they reveal human drama.

Jonathan Miller has done it with mirrors before. Last year, for instance, there was an opera programme on television, and he was rehearsing the closing love duet from The Coronation of Poppea. He directed the singers to sing it face to face, palms pressed to palms - as though they were each other's mirror images, either side of the glass.

I imagine that the idea was prompted partly by the duet's echoing cadences, partly by a sort of pun on its first line, "Pur ti miro" ("I gaze on you"). And the implied thought, that Nero and Poppea's serene ecstasy is really a mutual narcissism, was perfectly apt. Note that in this brilliant bit of staging no actual mirrors were involved.

There are plenty to be seen in Mirror Image, a concept-show devised and curated by Dr Miller at the National Gallery. Mirrors in pictures is the theme, plus reflective surfaces generally. It is rather a Science Museum affair. As well as a very choice assembly of paintings and prints, there are large explanatory wall-boards, and large colour copies of relevant but unobtainable works (such as a full-size repro of Velazquez's Las Meninas).

There are diagrams and demo-models, mug's guides to the physics of light and the psychology of perception. And there is fun stuff, like a two-way mirror and a mirror that pretends to be a through-doorway. A lecture by Ernst Gombrich with interventions from Harry Worth, that is the tone.

Now it seems to me that a person can be too interested in pictures with mirrors in them. At least, a view of art which is liable to make MCEscher seem as valuable as Velazquez must be a limited one. On the other hand, it may be that our artists have more in common with the world of Escher than I would like to admit. And obviously Western art, so obsessed with the look of the world and the act of looking, is bound to be drawn to looking glasses, because they are such visually weird things - in effect invisible objects, only shown by what you can see in them, and what you can see in them does not exist.

Mirrors are in some ways like pictures, in others not, but pictures tend to level all differences: put on canvas, a face and the reflection of a face may look indistinguishable, are equally intangible and equally real.

I paraphrase Miller's own reflections on the subject. Most of what his captions say is true and interesting.

Looking at Van Eyck's "Arnolfini Portrait", he points out that, if you concentrate on the image reflected in that famous round convex mirror, you no longer see the mirror as shiny - even though it is hard for us to shed the idea that the shininess of shiny things is a property independent of what they reflect. And he makes you very conscious of all those pairs of tiny white squares that occur in so many paintings - usually called highlights, but indicating reflected windows, windows that may lie well outside the picture's view. Reflections are often used to give a glimpse of a picture's "off-stage"; the wider world around it.

So Miller uses pictures to demonstrate perception, and perception as a clue to pictures. Very good. He displays some beautiful and curious instances, like Johann Erdman Hummel's two pictures of a vast and highly polished granite bowl. But this approach risks being rather literal-minded about depiction, by presuming that pictures are normally records of something seen, painted on site. The truth is, pictures are often extremely cavalier about this. They put up a show of visual probity, then do something outrageous - but you are not really meant to notice.

Almost all pictures that show an object, a mirror and a reflection of that object, get the optical relations wrong. Ingres's Madame Moitessier is a spectacular example: no way could the glass behind her reflect back her profile. Yet to see the image as impossible or paradoxical is to miss its point. Ingres wants to compose a two-fold image of his subject. Optics are just tools for use. Likewise, in the section devoted to self-portraits, though all the artists probably used a mirror in the process, one must recognise that while some are true to or curious about this fact, others are just not.

Well, perception is interesting and paradoxes are boring (yes, even those of Las Meninas, I am afraid). What one really wants to see is mirroring used, as Miller used it with that Monteverdi opera, to do human drama. There are excellent examples here. A Man with a Mirror (after Ribera) shows a man staring into a square mirror that he holds in front of him with both hands. It seems as if he is holding himself in his hands.

In a lovely little Lucian Freud, Small Interior, a big free-standing mirror occupies almost the whole image, the artist standing reflected in it, and the mirror is as much a character as the man. Narcissus, by a follower of Leonardo, wins by concentrating on the young man's gazing face, showing only the very edge of the pool and none of his reflection - good to leave it to the imagination, because in the myth it is his first time with a mirror and he does not know it is him.

And Gustave Caillebotte - what an intelligent artist! (He is not at all unknown, of course, this fringe Impressionist, but the news of just how good he was is still coming in.) In a Cafe does the psychology of isolation simply and superbly and does it with a compendium of mirror effects. A man stands alone, a large cafe glass behind him; the two people chatting in the cafe, who he is looking at, only appear as reflections in it, an "off-stage" insert, made small and extra remote; so he seems to turn his back on them too and also upon his own reflection which shares a space with them; thus he is split from his social self. This splitting or doubling is one of the best ways pictures use mirrors.

But you may notice that all those are pictures of men, and it is a general truth. Women and mirrors usually do not get interesting pictorial treatment. Female vanity or male voyeurism is the rule, though it must be said that for cunning titillation, Christoffer Wilhelm Eckersberg's Woman Standing in front of a Mirror is a neat piece of work. It uses a doubling effect to show the woman twice - naked but with her back to us, and her reflection facing us but cropped just above the nipples by the mirror's frame. A strip-tease, precisely.

The other big lesson here, not explicitly drawn, is oddly enough about artistic style. Dwelling on all these highlights and reflective surfaces makes you aware that shininess - how glossy a painter makes things look generally, or on the other hand how matt - is one of the great stylistic axes. Some artists create wet/polished worlds, others make dry/rough ones, and which you prefer is a very basic point of taste.

But no doubt there is much more to learn, and any viewer can think up further examples. Here is an oblique one, in the National Gallery itself: Jacob Jordaens' The Holy Family. The virgin holds up the baby, staring out earnestly, while his gaze drifts to the side. The scene (I interpret) is in front of a mirror, and seen from the mirror's point of view. She is saying "Look, it's you!"

Mirror Image - Jonathan Miller on Reflection: National Gallery, London; until 13 December; Admission pounds 5.50, Concs pounds 3.50