Last year the Philadelphia Museum of Art purchased Dorothea Tanning’s Birthday, 1942, an early self-portrait in which the bare-breasted, bramble-skirted heroine, accompanied by an apparently benevolent minidragon (first of the animal demiurges so often inhabiting the artist's future paintings), stands with her hand on the knob of a white door in an infinite regress of half-open portals. This acquisition has now been celebrated by curator Ann Temkin with a small show of paintings, objects, and drawings from Tanning’s long career, “a hidden treasure of modern art,” concluding with one of the dozen