Viewing a SIRDS involves deceiving the eye-and-brain mechanism. The scheme does not work for everyone. Deliberate defocusing can cause eye strain in some people. The trick is to position the SIRDS or SIRTS 12 to 18 inches (30 to 45 centimeters) away from your eyes, look momentarily at something far away, and then look back at the image again. The image can be moved alternately closer to, and farther from, your eyes, until adjacent sets of dots or characters overlap in the lines of sight.
I find that I get headaches if I look at too many pictures and the definition of too many will vary for each person.
Once you get the hang of seeing the 3D images it will only take a couple of seconds to see them (that's all it takes for me now) and to see some of these 3d images is quite amazing and well worth the effort.
This is one of the easier pictures to view and is really quite amazing, click here to view the image in a popup window.
There are 100 SIRDS images included here, just click on a link and they will each open in a new window.
If you would like to try making some yourself, you can download some grayscale images (used for the 3D image) here and some backgrounds here.
You can google "free sirds programs" to find a number of free programs or you can download a trial version of 3DMiracle here http://www.ixtlan.ru.