What to do when your photographs depicting the Thames have already been exhibited internationally, including in a room at Tate Modern overlooking the river itself? Roni Horn, for this exhibition in the Art Institute's Focus series, foregrounded context, spreading “Some Thames,” 1999-, through roughly two dozen galleries dedicated to art from the past hundred years. Finding her works, without wall labels, where other objects from the permanent collection would normally hang offers both the pleasure of an Easter-egg hunt and the more serious reward of canny juxtaposition. To take two examples: The brackish green water of one photo finds its corollary in the sickly sky of a Roberto Matta painting; in the next room, inky black water, likely photographed at night, seems to suggest that the surface ripples of Clyfford Still's monumental all-black Monochrome, 1951-52, are little crests one could easily sail across. This is a more ambitious—and more successful—variation on her recent project for the 2004 Whitney Biennial, which used the same tactic of stealth installation. Horn's playful investigation of context is a natural outgrowth of her interest in the contingency of language and identity, and this exhibition is the sign of an artist running on all cylinders.