All sorts of analogies to be made here.

A co-worker did a low-tech comparison. He printed out the image and snipped out Square A and Square B. Same color, you betcha.

Playing around with PhotoEditor, I find that the secret is that the squares diagonally adjoining A and B change shade much more gradually than the fuzzy shadow edge suggests. Even quite close to the corners they're a markedly different shade from the AB color. This is more strikingly seen in the part of the "light" square just to the upper-left of B.

As an experiment, I tried turning my back on it for a minute or two, then suddenly spun around to confront it. Not only did I discover that it was in the process of quietly changing its hue, I also caught it trying to hack into my personal bank account!

This breakthrough led me to try various methods of ascertaining its intentions. It finally broke and admitted that it was placed here surreptitiously by agents of a foreign power (an evil one whose name you would recognize instantly) to undermine our very American way of life!

I disposed of it properly, and wiped all records of it from my computer, and urge you to do the same. It seems to me that your web page uses what looks suspiciously like the same shade in one or two key places. I think this also explains the mysterious Grays (and their cognate fellow travelers, the Greys) of Roswell, NM.

Don't let emotion sway you in this matter. The truth, objectively viewed, is black and white.

Alas, now I have no choice, but to disbelieve my eyes at all times. This is going to make driving a lot more difficult.

Also, if you've ever been to a "Mystery Spot" they do the same thing with visual orientation -- build a house where all the visual cues for vertical and horizontal are actually tilted. Even though you *know* that's what's going on, it's nearly impossible not to see those cues as closer to vertical/horizontal than they are, with the result that it feels like gravity is tilted the opposite way, pendulums appear to hang at an angle, etc. (The DC Metro system has a number of escalators that are pretty good unintentional Mystery Spots -- look across at the opposite escalator and your visual system tries to make the escalator horizontal, with all the people leaning over at a steep angle.)

Ditto the Colorado Rockies. There were several times where Mike Pins and I were convinced that we were either driving on a flat, or a downhill slope -- but the GPS and Altimeter clearly showed us ascending a pretty decent grade.

http://www-bcs.mit.edu/gaz/demos/craik.html

It is *not* a true illusion, because although they have taken squares out of the same region, the region itself has a light-to-dark *gradient* variance so the left side of each portion of the square is indeed a bit lighter and the right is darker. Think of it as every pixel to the left being lighter than every pixel to the right by an infinitesimally increasing degree.

When juxtaposed to "itself" in the exact same orientation, the right side is touching the left side, so of course they are different. However if you were to flip *one* of the clone squares 180 degrees in either direction so that sides of the same gradient touch, you will see either a concave or a convex strip.

Makes sense?

:-)

Vera

---L.