Take a good look at this image. In fact, stare at it, dead center. There's no trick to it; just look straight ahead, relax, and watch as something you know is there vanishes into nothingness.
Got it? Good. Here's how it works.
What you just experienced is what's known as Troxler's Fading, which simply holds that when you stare at a fixed point for long enough, everything in your peripheral vision eventually disappears. That's why our eyes are constantly, imperceptibly moving. In the same way you feel your t-shirt when you put it on but don't notice it after several minutes of sitting still, neurons in your visual system tend to ignore an unvarying stimulus. Ergo: fade to white.
And if you're wondering why all the soft colors and blurriness? A more defined image presents your eye with too much stimulus; it's too hard to keep them locked into one spot long enough (about 20 seconds) for the effect to take place. Still can't get it to work? You may be missing M or L cones, according to one informed Redditor, who also provides alternatives that might work for you.
This is what Magic Eye should have been all along: not the creation of mermaids, but the blurry evaporation of cable bills and ex-girlfriends and runny grits. [Reddit]