These stereo pairs are displayed with the left and right images switched for "crossed-eyed" viewing.  With practice this allows for a 3-D illusion without the need for a separate viewing device.  I like how this underscores how any viewing and subsequent understanding of an image requires a set of learned skills, that the act of viewing an image is an interweaving of imagination and neural flexibility.  To learn more about how to view stereo pairs with crossed eyes click here..