This optical illusion is based on the Poggendorff Illusion. In that illusion we saw that diagonal lines passing through a rectangle could be deceptive. Below we have horizontal bars passing through diagonal spaces. While viewing this illusion it is easy to be drawn into the perception that each set of lines is a little lower each time they pass through the blue gaps.
You and I know the truth though. All we have here are horizontal lines that run straight and true across the entire illusion. Excellent work Herman.
Herman has a large collection of optical illusions that he has created over time. He has generously allowed me to feature a few of his pieces here. This is the seventh (check the sitemap for his others) of many. His illusions run the gambit and I will be showing you more of his excellent Trick-Art in the future.