 The copy gives a better indication of what the portrait looked like at the time, as the varnish on the original has become cracked and yellowed with age.German imaging researchers Claus-Christian Carbon of the University of Bamberg and Vera Hesslinger of the University of Mainz performed further analysis of the Museo del Prado version, comparing it to Da Vinci's "Mona Lisa", and in May 2014 speculated that, based on perspective analysis of key features in the images, the two images were painted at the same time from slightly different viewpoints.They further proposed that two images may therefore form a stereoscopic pair, creating the illusion of 3-dimensional depth, when viewed side by side.