“Painting,” Rembrandt once said, “is the grandchild of nature.” He proved his point by painting landscapes, genre scenes, animal studies, and biblical stories. And yet his greatest creative triumphs were the portraits, which he executed with great dramatic flair. Spirit and emotion are sculpted into his subject’s faces through subtle chiaroscuro. For this exhibition, loans from around the world have come to Vienna, in honor of the 17th-century Dutch master. —Elena Clavarino