This beautiful theorem involving angles of lines to a conic crystalized from experimental observations in 1986 by Dick Nickalls (retired) from the Dept. of Anaesthesia, Nottingham University Hospitals, UK in the area of vision physiology - a rich source of mathematical problems. It relates to the so-called Pulfrich effect, an optical illusion in which an object which is moving in a plane parallel to the viewer's forehead seems to move out of that plane and to approach (or recede from) the viewer.