It is surely one of the most brazen evasions of reality ever painted. John Constable's The Cornfield,completed in 1826, and now hanging in the National Gallery's new exhibition Paradise, evokes, at thevery height of the enclosure movement, a flawless rural harmony. Just as the commoners were being dragged fromtheir land, their crops destroyed, their houses razed, the dissenters transported or hanged, Constableconjures the definitive English Arcadia. A dog walks a herd of sheep into the deep shade of an August day. Aruddy farm boy drinks from a glittering stream, his donkeys browsing quietly behind him. In the background,framed by great elms, men in hats and neckerchiefs work a field of wheat. Beyond them a river shimmers throughwatermeadows. A church emerges from the trees to bless the happy natives and their other Eden.