As soon as you enter the new exhibition at the Grand Palais in Paris, you have a strange sense of the "jamais vu". Many of the works on display are bizarre but the real oddity is the behaviour of the visitors.
Art exhibitions in Paris are generally consumed in a strained, self-conscious intellectual silence. Here the visitors chatter to each other, pull puzzled faces, laugh out loud. In short, they have fun and are not afraid to admit it.
The show is called Une Image Peut en Cacher une Autre or "one image can conceal another". The title is a tribute to a poetic noticeboard found beside every railway level crossing in France ("Un train peut en cacher un autre").
The exhibition, which lasts until 6 July, is the largest ever single gathering of "trick" art: art which makes use of optical illusion; or changing viewpoints; or double take; or visual double entendres; or trompe l'oeil; or the surreal blending, or yoking together, of disparate images. The exhibits range from the medieval to the contemporary; from the religious to the erotic; from the Western to the Middle Eastern and oriental; from the obscure to the famous (Hockney, Picasso, Dali, Giacometti, Ernst, Magritte, Gauguin, Archimboldo, Dürer).
All visual art is a form of trompe l'oeil. The Mona Lisa is just a thin piece of wood daubed (rather cleverly) with 500-year-old oil paint. However, some artists, from the earliest times, have enjoyed playing with, or drawing attention to, their own trickery.
The emblematic image in the exhibition, which turns up more than once, is a maddening, visual puzzle of the kind that you see in children's comics. Look at the sketch and it's a duck; look at it again and it's a rabbit; look at it for too long and you go mentally cross-eyed. (The original was devised by an anonymous German in the 19th century.)
Like the duck-rabbit or rabbit-duck, the Grand Palais exhibition asks you to consider, or reconsider, the wiring between your eyes and your mind. The curator of the exhibition, Jean-Hubert Martin, says that the information gathered by the eyes passes through a "grill" or "filter" of cultural expectations and experiences before it is assembled and interpreted by the brain. The first instinct of the mind, like an internet search engine, is to return to what it knows, to project standard images that it has seen before.
"It is not difficult to trick us by sending the eye on false trails," explains Martin.
The exhibition includes a section of "anamorphoses" or works which change completely, showing a different face or a different landscape, depending on which side of them you stand. This kind of experimentation, going back 500 years, led eventually to lantern shows and the ultimate exploitation of the image filtering system in our brains, the cinema.
Sometimes, artists have messed with our mental filters for allegorical reasons (as in the tedious medieval paintings assembled here in which the rocks or clouds have demonic or saintly faces). Artists, as far back as Dürer, have delighted in erotic double entendres. The exhibition includes a Dürer wood engraving from 1496 of men bathing. Their private parts are covered but also revealed by a centrally placed and suggestively shaped water tap.
There are also private jokes, such as the paintings by Paul Gauguin and his Pont-Aven school which include hidden silhouettes of the painter and his friends. There is a beautifully sensuous painting by Edgar Degas of seaside cliffs ("Côte Escarpée") which includes, if you crane your head at just the right angle, a concealed image of a reclining nude woman.
Is this a wry comment on the prudish 1890s disapproval of the erotic side of the Impressionist movement? Is it an allegory of the age-old personification of woman as Mother Earth? Neither, it appears. According to the catalogue, Degas set out to paint a nude woman but was unhappy with his work and turned it into a landscape.
As the exhibits become more modern, the visual ambiguities are no longer concealed. They get right into your face. Instead of adding an extra layer of meaning or amusement, they become the whole point of the work.
You could argue that Magritte has no place here. There is nothing "hidden" in his image of boots with toes or in his erotically bold painting of a face which is made into a naked, female torso.
Actually, the great Belgian fits in perfectly. His originality and wit are illuminated by the work that goes before. The earlier trompe l'oeil artists were exploiting, or having fun with, the wiring system of our brains. Magritte was directly challenging our mental image filter, offending it or scrambling it.
Martin says the "double image", although avoided by most artists, has been around from the dawn of creation. There is, and always has been, a "temptation" for the artist to show off his artistry by "playing with the real, with conventions, with appearances and disappearances".
Sometimes, he says, the artist's intention is merely playful. "Trick" art can brings us close to "mere amusement" or "cabinets of curiosities". However, the distinction between art and amusement was not always rigid. Leonardo da Vinci was an occasional painter but a frequent designer of 16th-century special effects, such as a clockwork, walking, full-size lion.
On other occasions, Martin says, the artist's trickery is deadly serious or playfully serious, a means of matching the shifting viewpoints found in other forms of art, such as the works of great writers. "Ambiguity is not an optional extra in artistic creation," he says. "It is its very foundation."
First and foremost, however, as the constant murmuring and giggling of the stream of visitors suggests, the exhibition is great fun. The French artist Jean-Jacques Lebel, who has one of his own collages in the exhibition, told Le Monde: "I left the Grand Palais in a state of exquisite jubilation and I danced all the way home."Reuse content