Martin Droeshout's 1623 Engraving Of William Shakespeare.The purpose of any portrait is to capture the essence of the subject. To somehow convey in a single image not just the outward appearance of the sitter, but his soul. But if the subject is a great writer, does that task become impossible? Poet Ben Jonson thought so, and maybe the curators at Princeton University's Firestone Library do, too.Those curators have just opened a new exhibit of 100 paintings, drawings, prints, photographs, marble sculptures, and plaster death masks, depicting literary giants. The title of the gallery show is: The Author's Portrait. But the...