Eileen Agar: An Eye for Collage, Pallant House, Chichester

Collages rarely seen before shed light on a male-dominated art movement

Over the new Pallant House show, Eileen Agar: An Eye for Collage, hangs a brooding cloud of injustice. Agar, who died in her nineties in 1991, is commonly seen as the victim of a double conspiracy, having been both a woman and a surrealist.

Female painters have always had a hard time of it, while Surrealism with a capital S went into steep decline after the 1940s. The Surrealists were a gentlemen's club to which ladies – Gala Dali, Nusch Eluard, Lee Miller – might be admitted as muses and helpmeets, but never as members. This has been the view long peddled by feminist critics such as Germaine Greer and Whitney Chadwick, and it has stuck.

For what it's worth, Surrealism was kinder to women artists than any other mid-20th-century movement I can think of – certainly than the macho wrist-flicking of Abstract Expressionism. True, most female surrealists have been sidelined by history, but then so have most surrealists. (Does Roland Penrose rank high on anyone's list of greats these days?) Compounding the supposed injustice suffered by Agar is the belief that she was never a Surrealist at all until those beastly men, Penrose and Herbert Read, told her she was; that hers was additionally a case of mistaken identity. To which revisionism one can only say hmm, and look at Agar's work.

Which is wonderful. If this show tends to historical gallantry, its choice of pictures, many from private collections and gallery store rooms, is immaculate and informed. As its title suggests, Agar is approached via her collages, these being a mainstay of surrealism. The surrealists shared Freud's taste for parapraxes – puns, jokes, uncanny juxtapositions – reasoning that they revealed the unconscious mind. Sticking something over something else offered all kinds of opportunities for happy accident, even if the coincidences were artfully contrived.

For collage to work, the collagiste needs a keen eye for irony, and Agar had this in spades. One work, Precious Stones (1936), superimposes the cut-out profile of a classical head over a page from a catalogue of antique jewellery, so that the rings illustrated – cameos, intaglios, seals – show through. Beneath this page again is another of red paper, one corner of which the artist has folded back to reveal her signature on a piece of grey paper below, which in turn overlies others of parchment and marbling. The work trompes the oeil variously and archaeologically, excavating both ancient history and its own, layer by paper layer, meditating on platonic beauty while at the same time being beautiful. Precious Stones is, in a number of senses, gemlike, a little piece of lapidary perfection.

Surprising for a show of collages, many of the works in Pallant House are oil paintings on canvas and involve no cutting and sticking at all. The rationale for their inclusion is that Agar's predilection for paper and scissors was so pronounced that it spilt over into non-collage works such as A Sea Change (1959). If you want to pick at art-historical nits, then this is a good place to start. The dancing figures of A Sea Change arguably owe as much to Matisse as they do the Surrealist preachings of André Breton, their undulating shapes bearing a strong resemblance to the papier-coupé forms with which the Frenchman had begun to experiment a few years earlier.

Matisse had joyfully described his cut-outs as "a simplification", and you feel that the same thought may have struck Agar. Apart from the broad joke of looking as if they have been stuck on to the blue field behind, the figures of A Sea Change have little to do with accident and a great deal to do with rhythm and colour – qualities that more properly belong to the realm of 1950s abstraction than they do to 1930s surrealism.

What they suggest is that Agar was struggling to escape from Surrealism's once warm embrace into less deathly arms – the vaguely Picasso-esque forms of Untitled (1974), the sort-of-Pop joshing of her Letraset drawings. As a result, a lot of her later work feels unsure, as though she is trying to sign up to a way of being rather than being herself. When she is, the results are wonderful: the wrinkled, scabby picture of her mother is one of the great British portraits of the 20th century. It, and the extraordinary Autobiography of an Embryo, show how things might have been had the young Eileen Agar not fallen among thieves.

Pallant House, Chichester. West Sussex (01243 774557) to 15 Mar 2009