I had first seen this illustration many years ago, and ran across it again today used in a Scientific American article about making a better light bulb. Without diving into the article, I tried to remember what the picture demonstrated, and came up with this: Two squares are labeled A and B. Which is darker? Well obviously A is darker than B. Duh.
Wrongo! They’re the same brightness! To prove it, put the fingers of both hands together at the tips, index to index, middle to middle, and ring to ring. Leave little diamond shaped holes between the fingers and superimpose them onto the A and B squares, masking the surrounding squares. Surprise! A and B are identically bright.
The whole idea of the illusion is to demonstrate context. You can try another experiment yourself that involves taste, not vision. A lot of people don’t like the taste of broccoli, for example. That’s because they are probably eating broccoli alongside things they do like such as mashed potatoes and gravy and fried chicken. But try this: Take a swig of a mixture of paint thinner, mashed ants and cat pee. Swish it around for a few seconds then spit it out. Then take a bite of broccoli. Wow! It’s love at first bite!