Forty years ago, in the April 1971 issue of New Scientist magazine, a new type of optical illusion was described: Neil Illusions, named for the man who discovered them, Allan Neil, of the Institute of Behavioral Research at Texas Christian University.

Here’s how Allan Neil described this new category of illusions in the article:

These new illusions, in sharp contrast to those of the 19th century, do not violate the invariance of parity, charge conjugation or time reversal. Full scale research has not yet begun on the information-processing mechanisms which respond to the subtle factors in these illusions, but the preliminary studies have not overestimated their importance.

Here are the examples he gave:

Amazing. You can read the entire article as it appeared in the original issue online at Google Books. (The issue also has an advertisement for the Beckman DB-GT Spectrophotometer, which has the slogan, BECKMAN CAN HELP — with spectroscopy.)

I think more research needs to be done in this field. I’m sure there are other undiscovered Neil Illusions out there. Every year the Neural Correlate Society holds a contest to find the Best New Optical Illusions. I think they should add a category for the best Neil Illusion.