Two faces of Mary Queen of Scots

This curious portrait is in the National Portrait Gallery in Edinburgh. It's an early form of hologram - which face you see depends on the angle you look at the portrait from. I've put them side by side, but in real life there are slats at 45 degrees to the plane of the frame which show one face on one side and the other face on the other - hope that makes sense. It seems that this portrait was painted after Mary's death, so the painter knew she had come to a bad end.