I learned the cross your eyes method in structural geology and map interpretaion. you have to allow your vision to sort of blur and then cross a bit and then relax a bit.You also have to be at the right distance from the photos. I can do it still but it's easier when you're younger - took me about 15 seconds at three feet from my monitor on this one. it takes about five or ten minutes to do the first time. it works best when there is a small difference (< 15 degrees) between the pictures. there are viewers for this sort of thing but I imagine that they're expensive. - TJ