Tara Donovan’s new exhibition, ‘Stratagems,’ gives new use to a forgotten medium. At New York’s Pace Gallery, a series of 11 iridescent green sculptures flicker in the sun. The structures tower over viewers, ranging from seven to 11 feet tall, and spiral upward in intricate patterns. They’re so detailed, in fact, that it requires a double take to realize they’re entirely made out of CDs.