Every term when I’m preparing my 101 notes on illusion, I’m amazed by just how good the “checkershadow illusion”:http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html produced by “Edward Adelson”:http://web.mit.edu/persci/people/adelson/index.html is. I use this as a way to get the students to feel the force of Descartes’ worries about the reliability of sense perception. Here is the illusion.

The point, as many of you will know, is that A and B are the same shade on the screen. Seeing this is, to say the least, non-trivial. I’ve made a small “powerpoint demonstration”:http://brian.weatherson.org/Adelson.ppt of it, which I’ll be using in class.

The reason for making this presentation was that I wasn’t convinced by Adelson’s own demonstration. Here is the picture he uses to show that A and B are the same shade.

To me, A and B still look different. (I’d be interested in knowing whether other people agree.) So in my “powerpoint”:http://brian.weatherson.org/Adelson.ppt, I’ve slowly covered up everything except A and B to make the point more vivid.

In my presentation the space between A and B is covered up slowly, and I think the phenomenology of this process is quite interesting. When I watch the last space between A and B get covered up, it doesn’t look like A gets lighter, or B gets darker, but it does look like they are getting more similar in shade. It is really quite mysterious, and I really can’t put into words the feeling it induces.

It might be interesting to change around which parts get covered up in which order to see what is necessary for A and B to appear to be the same. Anyone who wants to do so should feel free to modify my slides and distribute them. (Note that the original pictures are not copyrighted, and indeed Professor Adelson has gone out of his way to make the pictures easy to distribute.)

UPDATE: As Michael Smith pointed out to me, someone who was really doubtful about the claim that A and B are the same shade could simply copy the graphic above onto their own computer and then copy and paste out the squares to see. (I was enough of a sceptic that I once did this.) This works, indeed here is Michael’s version of what you end up with.

But I do like having a dynamic visual demonstration as well, particularly for 101 lectures!