Optical Illusion

Sometimes a phenomenon is called an illusion when it really is not, but is simply a true picture of an unexpected observation. An example is the searchlight illusion described by Luckeish. The beam of a bright searchlight is visible because of scattering by dust and fog in its path, so that it seems practically a physical object. When the beam is projected up into the sky, it seems to vanish abruptly while still in full glory. When you look at this apparent end of the beam, you are looking in the direction in which the beam is pointed. If the beam were parallel (as your mind expects) it would, by perspective, narrow to a point. However, a searchlight beam is actually more or less divergent, fooling this expectation. It is only one's mental interpretation that is an illusion in this case, not the observation. Stars can be pointed out to others by means of a strong laser using this effect. If you view the searchlight beam from a distance, you see it diverge and become attenuated, and perhaps penetrating the layer of dusty air.

Other Illusions

Illusions can also arise from contrast of brightness, as the perception strives to maintain line and shade. The well-known illusion shown at the right is an example. There are gray patches at every crossing, except for the one you are looking at directly. This effect is something to avoid when designing linoleum.

Illusions of motion and color are difficult to illustrate in text, and are so extensive as to require individual study. Color can be perceived in a rotating black-and-white disc of suitable pattern, which is probably due to different fatigue characteristics of the color-sensitive proteins in the cone cells of the retina. Many color illusions are due to physical causes, because of the poor spectral resolution of the eye, and differences in illuminants and pigments. Adaptation, where the ambient illumination comes to appear as white as possible, and color constancy, where colors are interpreted similiarly under different conditions of illumination, are fundamental and useful properties of the color sense, not illusions.

Camouflage

Tricking the eye into recognizing one thing while observing another is often very useful to living things. There are three different ways to do this. First, one might mimic something dangerous or nasty-tasting, as does the fly who resembles a wasp, a brightly-colored butterfly, or an armed, uniformed policeman. Another way is to merge with the background, as do moths, stick insects, tabby cats, or wealthy people wearing old clothes in the street. An interesting way to do this is to break up a familiar outline by a contrasting pattern. Warships were painted in bold, zig-zag patterns in the First World War for this purpose. The patterns did indeed break up the outline when you were close enough to see that they were ships, but at large distances aerial perspective (blue haze) smoothed the pattern, revealing again that they were ships. The third way is to look like something else. Cylindrical snakes and lizards are dark on top and light on the bottom, contrary to the normal modelling of a cylinder, so they resemble flat objects containing no meat.

Deliberate Illusions

A picture drawn on a flat background is an attempt to trick the eye into perceiving a three-dimensional scene. This is very effective, since the eye must do something similar in its normal functioning, because the retina is two- dimensional. The skill of perceiving depth and perspective in a painting is learned, not innate. In moving pictures, the mind interprets the succession of static frames as continuous motion, again something it must do in its normal functioning. There must be a temporal element in sensing a changing world, which is revealed by the flicker frequency, the rate above which continuous motion is perceived instead of jumps, of about 20 to 30 Hz. We are very thankful for these illusions (if we realize what they are) and are glad to have them.

Conjurors, three-card monte men, swindlers, mediums, priests, and others interested in influencing people sometimes make effective use of visual (and other) illusion. Stage magicians who are only concerned with entertainment call themselves illusionists to make it clear what they do, and to distinguish themselves from those who ascribe their wonders to spirits or chemicals. Illusionists, and the the other sorts of entrepreneurs, mainly use other kinds of illusions, but optical illusions are not ruled out. These procedures have been perfected through centuries and even millenia of profitable use, and remain evergreen owing to the continuous copious production of fools.

Final Remarks

A curious illusion shows how the mind does its best to interpret its data. Fixate on a finger resting on a book at normal reading distance from your eyes. Now move the finger toward your eyes, keeping your fixatin on the page of the book. As soon as your finger is far enough from the page that what is obscured from the right eye is seen by the left, and vice-versa, it will become transparent, and you will see the book unobstructed by the finger. The finger is surely there, right in the way, but it is suppressed, probably because your fixation shows you are looking at the book, not the finger.

Illusions show that visual perception is much more complicated than was ever imagined in primitive views of it. One early view interpreted sight as touching by visual rays from the eye in the presence of activating rays from the source of light. More recently, the eye was perceived as a camera making a picture that was viewed somehow by the brain. The most interesting aspects of vision are, however, yet unexplained.