Holusion Art, aka: Sterograms
Stereographic vision allows us to perceive the world as 3d - objects look like they are in front of other objects. What allows us to see in 3d is the fact that our eyes each see a slightly different image.

The REAL SECRET is to NOT focus your eyes on the picture itself, but 'look through' the picture and thus focus your eyes behind the picture. Your eyes therefor will see two different parts of the picture, which can be seen in the illustration below. Your brain will 'merge' these two parts (see different views by the two eyes on black block on top) to one logical virtual (3D) image (see dashed/gray box in the middle)

A simple test to see if you will be able to see the images in the stereograms is to hold a finger up in front of you and look at something behind it. Without shifting focus to your finger, how many fingers do you see? If you see 2 ghosted images of your finger, this is the first step toward being able to see our 3d images. 
Methods to try
There are a few different methods you can use to see the images. It may be easier to use a printout of an image than to see the image on the screen. With practice, though, you should be able to see the images on a screen, also.
•	Bring the image up close to your face without attempting to focus. Slowly move away from the image and keep your eyes relaxed. Before you reach arms length, a picture should jump out at you. 
•	Hold the image about arms length in front of you but focus on something behind it. Shift your field of vision to the image without shifting focus. The image should be apparent if your focus is correct. 
•	Several of our pictures have 2 black dots near the bottom. If you stare at them and relax your eyes, you should see 4 dots. If you can bring the 2 middle dots together so you see 3 dots, the image will be in focus. 
•	Put reflective glass in front of the image and focus on your reflection. At the proper distance, this should allow you to see the 3d object. 
Some people see the 3d picture very quickly, others may need to spend awhile practicing before seeing their first stereogram. Still others never see the image. If you are having difficulties, remember to be patient. If you know of someone who can see the images, ask for advice. There are a lot of different tricks people use to see the images. 
Seeing the 3d object for the first time can be surprising. You don't expect to see something stand out from the page.