mering on in the desert. In general, visual illusions are mirages (Ramon, 2002). It is generally believed that visual illusions are simply curiosities (Robinson, 1972). Nevertheless, by studying illusions, it is possible to come to an understanding of how normal vision works - and how it can be fooled by unexpected cues, such as those provided in many ads containing semi-subliminal material. Furthermore, understanding visual illusion shows how knowledge of such rules can be put to use in constructing semi-subliminal and manipulative adverts. Vision is our richest sense, and is the most studied (Bruce, Green & Georgeson, 1996). Consequently, the study of illusions may provide trace to the more general mechanisms and principles of space perception (Schiffman, 2000). In this essay, the four principal kinds of visual illusions: ambiguities, distortions, paradoxes and fictions are precisely discussed (Gregory, 1997).

Probably the most famous and studied distortion illusion was created by German psychiatrist Franz Müller-Lyer in 1889 (Schiffman, 2000, p267). The identical segments of two vertical lines appear in different length (Schiffman, 2000, p267). Although our visual system tells us that the left one is longer, a ruler would confirm that they are equal in length. The most popular explanation of the Müller-Lyer illusion is that our brain makes assumptions about the relative depths of the two lines (Hundert, 1995). We are used to seeing outside corners of buildings with lines sloping towards the two distant perspective points on the horizon. The brain knows that the vertical line forming the outside corner is the closest part of the building. The brain realises that this line is not really longer than the other corner lines of the building. We are also used to seeing the inside corners of rooms with the lines of the roof and floor sloping outward away from them. The brain realises that this line is really longer than it appears when compared to the rest of the room. Therefore, the brain's visual system makes compensations for lines enclosed by slanting lines that could be interpreted as horizontal lines seen in perspective (Hundert, 1995). When enclosed by lines slanting towards the horizon, it is assumed to be a little shorter, but when enclosed by lines slanting away from the horizon, it is assumed to be a little longer (compared to other nearby vertical lines).

R. L. Gregory (1973) discusses a similar case with the impossible triangle, in his essay, "The Confounded Eye." The impossible triangle is a physical object which appears to be impossible. The impossibility arises from the assumption that the triangle lies in a plane. In fact, one side projects out at an angle from the other two. If the object is viewed from the correct angle, the images of the sides overlap in such a way that it appears to be a closed, planar triangle, although twisted in

Some topics in this essay:
Coren Girgus, Franz Müller-Lyer, Confounded Eye, Eye Brain, Sckuler Blake, Green Georgeson, Gaetano Kanizsa, Illusions Various, Bem Hoeksema, Impossible Triangle, visual illusions, schiffman 2000, robinson 1972, visual system, gregory 1997, coren girgus, visual illusion, impossible triangle, coren girgus 1978, girgus 1978, transport accident investigation, necker cube, accident investigation commission, brain realises line, horizon assumed little,