The eyes really see two images and the brain integrates them into one. WHen everything is working, they overlap and the brain fills in the 'blindspot'.
When you look at a thetagraph, also called a sterogram, your two eyes see one image.
But! If you begin to 'let go' of the one image, you soon see a wholly different 'something'.
This happens when you let your eyes perceive the different images, and you can begin to see 'between the worlds', or what is really hidden beneath/between what your brain is showing us.
It's a great exercise to play with, and begins to teach your brain to perceive in a new way.
Here are a few to play with:
EYETRICKS.COM
Some say to stare intently, some say to stare slightly above the image itself. I tend to have more success by going 'through' the image. In all cases, you are playing with letting your eyes NOTdeliver one image to you.
Hint: the less you try, the better you do. Isn't that the truth!