Summary: Author's update
The Thatcher illusion 28 years on ...
In 1980 my university, the University of York, was establishing an Electronics Department.
In its first few years they had rather too many students and too few staff and the
Department of Psychology kindly put on a course for their students. I had to give a
couple of sessions for them; one was a simple signal-detection practical and another
tried to show them that the visual system could be regarded as a crude Fourier Analyser.
In the course of this latter enterprise I had tried to explain to them the `missing
fundamental' illusion (Campbell et al 1971)öas you move away from a square-wave
grating with a missing fundamental, you should get to the point where the only visible
component of the grating is the third harmonic, and then the grating should appear to
have three times as many bars as it did when other harmonics were visible.
My daily walk from the car park (this was before I became a born-again cyclist)
to my office required me to approach the window of a boiler-room on the ground floor
of our building. The glass in this room was of a `bathroom' variety, with vertical stripe
to distort the view behind. For some reason the boilerman had pinned a page from
a magazine onto this window. I don't know what was on the boiler-room side of the
page; but on the reverse, facing outwards through the window, was the image of a
scantily clad beauty. I happened to notice that, when viewed close to, even with one's
nose pressed against the glass, the image was grotesquely distorted, but viewed from