It's typically a lot of fun to look at optical illusions. The interesting aspect comes from the fact that we see something different from what we expect. Often, illusions are created on some sort of 2D plane, some ink (or pixels) and are easy to make out. However, this wasn't the case at first with the Magic Eye (TM) books that Dr. Stonedahl brought in. The books were filled with images that were supposed to play with one's depth perception. The problem was that I wasn't able to view the images in the right way to see this.
We soon found through the reading that these books were an example of stereoscopy. Stereoscopy is the name of the technique used for creating and changing the illusion of depth. This is done by placing two offset images so that each is viewed in the left and right eyes of a person, respectively. Both images are then combined in the brain and depth is perceived.
The Magic Eye (TM) books are more specific in that they are examples of autostereograms, which utilizes stereoscopy but does not require special lenses or devices to see the illusion. But because these aids are missing, it is often more difficult to see the 'depth' without practice. Usually, one must try and look 'past' the image so that the brain can combine both images as stated above.
Overall, I enjoyed learning about these techniques. Anything that makes a 2D image look like 3D is pretty cool in my book. Yet I also found some of the applications of these interesting as well. For instance, some head-mounted displays utilize stereoscopy to create a virtual display for the wearer. Besides obvious uses like gaming or movies, head-mounted displays are increasingly being used to create see-through "augmented realities" for certain tasks. For technicians, this can mean a sort of 'x-ray vision' for seeing objects normally hidden from view. Another important use is in surgery. With augmented stereoscopic vision, data from CAT and MRI scans can be combined with the surgeon's vision. It's all very cool and it will be amazing what will happen with these techniques in a few years time.