Many books call describe the figure below as a Mach Band. However, the phenomenon might be better called the Chevreul Illusion after the scientist who first described it. M. E. Chevreul was a very influential figure for color theory and his work influenced many artists. His writings are well worth reading.

 

 

The effect also works for irregular boarders. The figure looks like mountains in the distance.

 

 

 

The Chevreul effect happens in the environment, The different ranges appeared darker at the top and brighter near the horizon. And, the effect was even stronger than the photo shows. However measurements show that the intensity was the same for each range.

 

 

 

Notice that the effect does not occur when different hues are used.

 

 

 

But the effect will occur if a Gestalt version of the stepwise figure is prepared. This version raises questions about the typical, simple explanation found in many texts.