It demonstrates that /perceived/ brightness is different from
photometric brightness ... different in a big way. One can
monkey with the hue so as to produce the perception of a
bright ray-like feature, even when there is no such feature
in the actual photometric brightness.
Making a hue-circle with constant perceived brightness is not
an easy task.
If anybody knows of any relevant references, and/or knows the
name of this illusion, please let me know.