“Like most museums in the United States, the Art Institute of Chicago owns nothing by Leonardo, but that doesn’t mean its permanent collection isn’t studded with artworks that harbor secrets of their own. In some cases — particularly in works from the Renaissance, the golden age of “coded” art — the images originally were meant to be mysterious. To demonstrate their erudition, the artists packed their paintings with so many symbols, allegories and allusions that viewers were forced to resort to reference books in which the most popular emblems of the day were unpacked and explicated.”