Though this isn't directly about photography I think there are important lessons here for photographers. I'm not quite sure what they are, but I'm sure there must be some! I suppose it's a lesson not to always trust your eyes when it comes to colors and intensities, especially when doing digital editing.
Below you'll find an image called the Checker Shadow Illusion. It's reproduced here with permission from Dr. Edward H. Adelson of MIT's Dept. of Brain and Cognitive Sciences and Artificial Intelligence Lab, who first described this illusion.
Look at the squares labeled "A" and "B". A is obviously a dark square and "B" is obviously a light square, right?
Wrong. "A and "B" are exactly the SAME shade of grey. Now I don't expect you to believe this. In fact I expect you'll think I've mislabeled the squares or I haven't really understood the illusion. You can squint at the image, turn it upside down, block off parts with your hand and try as hard as you can to see those two square as identical intensities, but you'll fail. However the fact remains that they are the same. They are both color #787878 or in decimal RGB terms, 120, 120, 120.
I know you still think I'm wrong but I'm not. The figure below is the same except that a grey line has been drawn though both images. This line is the same RGB 120:120:120 as I said the squares were. As you can see it matches both squares - but it looks like it varies in darkness along the length. Again, it doesn't. It's all the same shade of grey. Honestly!
The only way you're going to believe me is to copy the images into your image editor and use the color picker to check the RGB values of the two squares. You'll find they are the same. You won't believe it, but image editors don't lie.
So the next time you say you think your images looks a little dark in some areas or light in others, you're right. Perception rules - even when it's wrong! The squares DO look different, so for all visual purposes they ARE different, even if, in fact, they are the same...
If you want an explanation of what's really going on, you can see these comments by Dr. Adelson