In order to view stereo images properly, the left eye must be focused on the left image, while the right eye independently focuses on the right image. Thus, both eyes must be focused on different things! When this happens, the brain will process this information to provide the perception of 3-dimensionality.

1. Look at the stereo image from a very close distance, so that your eyes cannot focus on either image. Notice that now there appears to be a third image between the left and the right images.

 
2. Concentrate on the middle image, and slowly draw back from the screen. When the center image comes into focus, it will appear to have 3-dimensional perspective.