Danish psychologist Edgar Rubin is best known for the Rubin Vase -- an optical illusion that looks like either two faces or a vase. Here's a real vase that was created for Queen Elizabeth II's Silver Jubilee in 1977. When positioned just right, one side shows the Queen and the other the Prince Consort. You can find it at the Camera Obscura and World of Illusions museum in Edinburgh.
Link -via My Modern Met | Photo: Camera Obscura and World of Illusions